-ODs OF MEASURING ELECTRICAL CRARACTERISnCS OF ULTRASONIC DELAY LINES

A H Meitzler Bell Telephone Laboratories Inc

Mhippq Neu Jersey

-- This paper i s concerned with methods of masuring useful electrical amparacteriStiCS of ultrasonic delay lines emplqing piezoelectric transducers m e impedance characteristics of delay lines are discussed by means of an equiva-lent circuit representation the advantage being that it presents the essential impedance variations of the l i n e i n terms of quantities that are direct ly measur- able by an admittancebridge Measurements of insertion loss and the determina-tion of band-pass characterlstics are carried out by the w e of a conventional loss-measuring circuit In regard to t h i s type of measurement the recentdevel-o p e n t and use of short delay lines (delay t h e 8 of 50psec o r l e s s ) with ceramic transducers requires the consideration of certain measuring problems resulting f r o m the low loss i n the l i ne and the high electromechanical coupling of these transducers It is shown that the measurement of m a t e d signals a r i a from the sonic pulse making multiple path traversals requires special care because the I

inputaswell as theoutput terminationaffects the measurement Sample results I

obtained for various types of measurements are given using data from experimental delay lines h a h g ceramic transducers bonded to fused quartz However all the jmeasuring techniques discussed may be applied t0 l ines using quartz crystal o r qother piezoelectric transducers

( INTRDDUCTION

As most frequently used delay lines consist of an input transducer a delaymedium and an outputtransducer The physical sizes and shzpes of delay medium and transducers and the mode of motion used in the transducers vary depending upon the delay time and other considerations but usually the arrangement of transducers i s symmetricalrdquo Whether or not the arrangement of transducers i s m e t r i c a l a delay line having separate input and output transducers is from the circuit point of view a four-terminal network Associated with sucha net-work are several important electrical quantities whfch are measurable for anyParticular delay line andwhich may be used to characterize i t s lsquoperformance nese are (1) input and output impedances (2) insertion loss and (3) the level Of unwanted signals relative to the main dewed signal a t the output terminals

of these three types of quantities is measurable as a function of the car-d e r frequency of an input rf pulse III any specific appxcation of an ult ra-OMC delay line these quantities provide a circuit designer with the informa-tion he needs t o know i n order that both the l l ne and i t s associated circuitry

be used under optimum e lec t r ica l 0peratjng conditions

frequency dependence of the impedance and loss characterist ics for the four-tenninal network repre enting a delay l ine may be given i n tems of an equiv-uent c i rcu i t W p Mason9 has derived an equivalent network Of fixed lumped

elementsrepresentingtransducersApplications of t h i s network to the

1

analysis of band-pass characterist ics of delay lines using quartz crystal and ceramictransducershavebeen made by several authors2-b The elements used i n the equivalent circuits depend on the elastic piezoelectric and d ie lec t r ic con- stants of the transducer and bond materials the mode of motion and the e l a s t i c properties of the delay medium mile t h i s type of equivalent circuit is useful i n theoretical analyses usually not a l l the pertinent constants will be known i n the case of a specificdelaylineconsequently it is desirable t o have an equivalent circuit involving only experimentally determinable quantities

In most applications ultrasonic delay lines are used with pulses short enough so that the operation of one transducer i s not affected by the way i n which the othertransducer is terminated Hence each transducercanbeconsid-ered by itself a s a two-terminaldevice fo r which a t any arbi t rary frequencythe impedance o r admittance m a y bemeasured by various means Because e i ther impedance o r admittance is a cmmplex quantity a complete specification involves two quantities i e either a magnitudeand a phaseangle o r a res i s t ive and a reactive component For this reasonanexperimentallydeterminableequivalent c i r cu i t involves basically two c i r cu i t elements which have frequency dependentvalues

The equivaar i t c i rcu i t for a receiving transducer ctFffers f r o m tha t of a transmitting transducer i n t h a t an ideal voltage source the magnitude o f which is frequencydependent is required in addi t ion t o the two frequency-dependent c i r cu i t elements A n example of such an equivalent circuit is shown i n Fig 1 For a symmetricaldelay line the input and theoutput transducers are represented by the same functions Rz(f) and CE(f) The two c i r cu i t s d i f f e r on ly in that f o r the output transducer an ideal voltage source has been inserted i n s e r i e s with the paral le l combination of RE(f) and CE (f) The output circuit is the equivalent of thereceivingtransduceraccording t o Thevenins theoremEoc(f) is the open circuit voltage a t the outpat terminals and i n general it is a quantity that will depend on the circuit to which the input transducer i s connected as well as the inherent properties of a particular delay l ine (e g length of path nature of bonding between transducers and delay medium backing of transducers) ThoughEoc(f) could be measuredand plotted graphically for any given line it will not be of par t icular interest to the circuit designer who i s primarilyconcerned with the impedance and insertion-loss characterist ics of a particular delay-linedesign

The form of the equivalent c i rcui t for a delay line as given i n Fig 1is a reduction t o two variable ele ents of the more general equivalent circuits dis-cussed by Mason and In representing a delayline by the reduced fom of equivalentcircuittheprimaryadvantagesgained are (1) the impedance characteristics of a specific delay-line design are presented by two functional quantit ies which may be experimentally determined and (2) the information on impedancs variations with frequency is presented i n a form more directly useful to thecircuitdesigner

IMPEDANCE MEAS-NTS

A Generd Xethod

RE and CE may be determined by using an rf admittance bridge which a t an arbitraryfrequencydetermines the electr ical chmacter is t ics of a transducer i n

2

c

TRANSDUCERS

LAYEROFBONDING MATERIAL FLECTRICAL EQUJVALENT CIRCUITE3 LAYER OF TRANSDUCER MATERIAL

P H Y S I C A L ESSENTIALS OF DELAY LINE

Fig l - Suggested equivalent circuit representation of a symmetricaldelay l ine

PULSENG GAT1 PULSESYNC

v 4 v 4

GATEDRF GEN Inrsquo PULSE r __ AM - GEN

IVf GATED SINE WAVE

-TRANSDUCER UNDER TEST

Fig 2 Block diagramof apparatusused d t h bridge

d

e

-terms of a paral le l combination of resistance and capacitance When measured a t a rnunber of frequencies the values obtained fo r these quantities can be graphedand theresultingcurvesdefine Rx(f) and CE(f) For a symmetrical delay l i ne the average curves based on the data from both transducers may be used as the RE(f) and CE(f) characterizingthe line It may be noted tha t the resu l t s for these tw6 quantit ies depend not only on the transducer material but a l s o on lsquothe delay medium and on the layers of electrode and bonding materials

Because ref lected e las t ic waves returning to a transducer being measured a l t e r i t s admittance a measuring circuit of the type shown i n Fig 2 is required

3

3

RF

I F f

Q DETECTOR TO

CE -COMBINATION OF VAR AND FIXED CAPACITORS RE -VAR RESISTOR RANGE SELECTEDTOSUIT CO-5 TO 50 p p f VARIABLE Cl - 5 0 P P f c2 - IOPPf c 3 - OOlPf A 0 - 5000 VAR RESISTOR RaRb-750a 12w TI -INPUT TRANSFORMER 5 TURNS PRI IO TURNS SEC

WOUND ON FERRAMIC a TOROID

Yd= ADMITTANCE OF TRANSDUCER

_- 1 1

The c i rcu i t i s designed 80 that the input pulses are long enough t o permit the admittance to be measured under conditions of stea+state vlbration but short enough 80 that reflected pulses do not interfere The minimum length of pulserequired will depend on the type of tranactucers and t he i r loading For examplei n the case of ceramlc transducers bonded fa hzsed quartz and operating at 15 mc a pulse width of 1tn 3 sec i s long enough Tbe exact fonn of the bridge will depend on the type of transamp~cers measured and the precision desired A bridge used to measure ceramic t a m d u c e r s in the range of 10 t o 30 mc is shown in Fig 3

Typical hta and Interpretation c

me forms of the functions CE(f) and RE(f2 fo r the specific case of a delayline using barim t i t ana te ceramic transducers are shown in Fig 4 The curves

Throughout this paper the words barium titanate ceramic are used t o designate a

material which on a weight basis is 8 0 Barn0 12 PbTiO3 and 8CaTiO3The transducers referred to were disk-shaped haampa diameter of 0 -250 inch The upper m f a c e was partiallp covered a silver electrode 0218 inch i n diameter the louer surface was completely covered The completelyelectrodedsurface of each transducer was bonded fo thesilveredsurface of a fused quartz a low-melting-point -solder

4

analysis of band-pass characterist ics of delay lines using quartz crystal and ceramictransducershavebeen made by several authors2-b The elements used i n the equivalent circuits depend on the elastic piezoelectric and d ie lec t r ic con- stants of the transducer and bond materials the mode of motion and the e l a s t i c properties of the delay medium mile t h i s type of equivalent circuit is useful i n theoretical analyses usually not a l l the pertinent constants will be known i n the case of a specificdelaylineconsequently it is desirable t o have an equivalent circuit involving only experimentally determinable quantities

In most applications ultrasonic delay lines are used with pulses short enough so that the operation of one transducer i s not affected by the way i n which the othertransducer is terminated Hence each transducercanbeconsid-ered by itself a s a two-terminaldevice fo r which a t any arbi t rary frequencythe impedance o r admittance m a y bemeasured by various means Because e i ther impedance o r admittance is a cmmplex quantity a complete specification involves two quantities i e either a magnitudeand a phaseangle o r a res i s t ive and a reactive component For this reasonanexperimentallydeterminableequivalent c i r cu i t involves basically two c i r cu i t elements which have frequency dependentvalues

The equivaar i t c i rcu i t for a receiving transducer ctFffers f r o m tha t of a transmitting transducer i n t h a t an ideal voltage source the magnitude o f which is frequencydependent is required in addi t ion t o the two frequency-dependent c i r cu i t elements A n example of such an equivalent circuit is shown i n Fig 1 For a symmetricaldelay line the input and theoutput transducers are represented by the same functions Rz(f) and CE(f) The two c i r cu i t s d i f f e r on ly in that f o r the output transducer an ideal voltage source has been inserted i n s e r i e s with the paral le l combination of RE(f) and CE (f) The output circuit is the equivalent of thereceivingtransduceraccording t o Thevenins theoremEoc(f) is the open circuit voltage a t the outpat terminals and i n general it is a quantity that will depend on the circuit to which the input transducer i s connected as well as the inherent properties of a particular delay l ine (e g length of path nature of bonding between transducers and delay medium backing of transducers) ThoughEoc(f) could be measuredand plotted graphically for any given line it will not be of par t icular interest to the circuit designer who i s primarilyconcerned with the impedance and insertion-loss characterist ics of a particular delay-linedesign

The form of the equivalent c i rcui t for a delay line as given i n Fig 1is a reduction t o two variable ele ents of the more general equivalent circuits dis-cussed by Mason and In representing a delayline by the reduced fom of equivalentcircuittheprimaryadvantagesgained are (1) the impedance characteristics of a specific delay-line design are presented by two functional quantit ies which may be experimentally determined and (2) the information on impedancs variations with frequency is presented i n a form more directly useful to thecircuitdesigner

IMPEDANCE MEAS-NTS

A Generd Xethod

RE and CE may be determined by using an rf admittance bridge which a t an arbitraryfrequencydetermines the electr ical chmacter is t ics of a transducer i n

2

c

TRANSDUCERS

LAYEROFBONDING MATERIAL FLECTRICAL EQUJVALENT CIRCUITE3 LAYER OF TRANSDUCER MATERIAL

P H Y S I C A L ESSENTIALS OF DELAY LINE

Fig l - Suggested equivalent circuit representation of a symmetricaldelay l ine

PULSENG GAT1 PULSESYNC

v 4 v 4

GATEDRF GEN Inrsquo PULSE r __ AM - GEN

IVf GATED SINE WAVE

-TRANSDUCER UNDER TEST

Fig 2 Block diagramof apparatusused d t h bridge

d

e

-terms of a paral le l combination of resistance and capacitance When measured a t a rnunber of frequencies the values obtained fo r these quantities can be graphedand theresultingcurvesdefine Rx(f) and CE(f) For a symmetrical delay l i ne the average curves based on the data from both transducers may be used as the RE(f) and CE(f) characterizingthe line It may be noted tha t the resu l t s for these tw6 quantit ies depend not only on the transducer material but a l s o on lsquothe delay medium and on the layers of electrode and bonding materials

Because ref lected e las t ic waves returning to a transducer being measured a l t e r i t s admittance a measuring circuit of the type shown i n Fig 2 is required

3

3

RF

I F f

Q DETECTOR TO

CE -COMBINATION OF VAR AND FIXED CAPACITORS RE -VAR RESISTOR RANGE SELECTEDTOSUIT CO-5 TO 50 p p f VARIABLE Cl - 5 0 P P f c2 - IOPPf c 3 - OOlPf A 0 - 5000 VAR RESISTOR RaRb-750a 12w TI -INPUT TRANSFORMER 5 TURNS PRI IO TURNS SEC

WOUND ON FERRAMIC a TOROID

Yd= ADMITTANCE OF TRANSDUCER

_- 1 1

The c i rcu i t i s designed 80 that the input pulses are long enough t o permit the admittance to be measured under conditions of stea+state vlbration but short enough 80 that reflected pulses do not interfere The minimum length of pulserequired will depend on the type of tranactucers and t he i r loading For examplei n the case of ceramlc transducers bonded fa hzsed quartz and operating at 15 mc a pulse width of 1tn 3 sec i s long enough Tbe exact fonn of the bridge will depend on the type of transamp~cers measured and the precision desired A bridge used to measure ceramic t a m d u c e r s in the range of 10 t o 30 mc is shown in Fig 3

Typical hta and Interpretation c

me forms of the functions CE(f) and RE(f2 fo r the specific case of a delayline using barim t i t ana te ceramic transducers are shown in Fig 4 The curves

Throughout this paper the words barium titanate ceramic are used t o designate a

material which on a weight basis is 8 0 Barn0 12 PbTiO3 and 8CaTiO3The transducers referred to were disk-shaped haampa diameter of 0 -250 inch The upper m f a c e was partiallp covered a silver electrode 0218 inch i n diameter the louer surface was completely covered The completelyelectrodedsurface of each transducer was bonded fo thesilveredsurface of a fused quartz a low-melting-point -solder

4

c

TRANSDUCERS

LAYEROFBONDING MATERIAL FLECTRICAL EQUJVALENT CIRCUITE3 LAYER OF TRANSDUCER MATERIAL

P H Y S I C A L ESSENTIALS OF DELAY LINE

Fig l - Suggested equivalent circuit representation of a symmetricaldelay l ine

PULSENG GAT1 PULSESYNC

v 4 v 4

GATEDRF GEN Inrsquo PULSE r __ AM - GEN

IVf GATED SINE WAVE

-TRANSDUCER UNDER TEST

Fig 2 Block diagramof apparatusused d t h bridge

d

e

-terms of a paral le l combination of resistance and capacitance When measured a t a rnunber of frequencies the values obtained fo r these quantities can be graphedand theresultingcurvesdefine Rx(f) and CE(f) For a symmetrical delay l i ne the average curves based on the data from both transducers may be used as the RE(f) and CE(f) characterizingthe line It may be noted tha t the resu l t s for these tw6 quantit ies depend not only on the transducer material but a l s o on lsquothe delay medium and on the layers of electrode and bonding materials

Because ref lected e las t ic waves returning to a transducer being measured a l t e r i t s admittance a measuring circuit of the type shown i n Fig 2 is required

3

3

RF

I F f

Q DETECTOR TO

CE -COMBINATION OF VAR AND FIXED CAPACITORS RE -VAR RESISTOR RANGE SELECTEDTOSUIT CO-5 TO 50 p p f VARIABLE Cl - 5 0 P P f c2 - IOPPf c 3 - OOlPf A 0 - 5000 VAR RESISTOR RaRb-750a 12w TI -INPUT TRANSFORMER 5 TURNS PRI IO TURNS SEC

WOUND ON FERRAMIC a TOROID

Yd= ADMITTANCE OF TRANSDUCER

_- 1 1

The c i rcu i t i s designed 80 that the input pulses are long enough t o permit the admittance to be measured under conditions of stea+state vlbration but short enough 80 that reflected pulses do not interfere The minimum length of pulserequired will depend on the type of tranactucers and t he i r loading For examplei n the case of ceramlc transducers bonded fa hzsed quartz and operating at 15 mc a pulse width of 1tn 3 sec i s long enough Tbe exact fonn of the bridge will depend on the type of transamp~cers measured and the precision desired A bridge used to measure ceramic t a m d u c e r s in the range of 10 t o 30 mc is shown in Fig 3

Typical hta and Interpretation c

me forms of the functions CE(f) and RE(f2 fo r the specific case of a delayline using barim t i t ana te ceramic transducers are shown in Fig 4 The curves

Throughout this paper the words barium titanate ceramic are used t o designate a

material which on a weight basis is 8 0 Barn0 12 PbTiO3 and 8CaTiO3The transducers referred to were disk-shaped haampa diameter of 0 -250 inch The upper m f a c e was partiallp covered a silver electrode 0218 inch i n diameter the louer surface was completely covered The completelyelectrodedsurface of each transducer was bonded fo thesilveredsurface of a fused quartz a low-melting-point -solder

4

RF

I F f

Q DETECTOR TO

CE -COMBINATION OF VAR AND FIXED CAPACITORS RE -VAR RESISTOR RANGE SELECTEDTOSUIT CO-5 TO 50 p p f VARIABLE Cl - 5 0 P P f c2 - IOPPf c 3 - OOlPf A 0 - 5000 VAR RESISTOR RaRb-750a 12w TI -INPUT TRANSFORMER 5 TURNS PRI IO TURNS SEC

WOUND ON FERRAMIC a TOROID

Yd= ADMITTANCE OF TRANSDUCER

_- 1 1

The c i rcu i t i s designed 80 that the input pulses are long enough t o permit the admittance to be measured under conditions of stea+state vlbration but short enough 80 that reflected pulses do not interfere The minimum length of pulserequired will depend on the type of tranactucers and t he i r loading For examplei n the case of ceramlc transducers bonded fa hzsed quartz and operating at 15 mc a pulse width of 1tn 3 sec i s long enough Tbe exact fonn of the bridge will depend on the type of transamp~cers measured and the precision desired A bridge used to measure ceramic t a m d u c e r s in the range of 10 t o 30 mc is shown in Fig 3

Typical hta and Interpretation c

me forms of the functions CE(f) and RE(f2 fo r the specific case of a delayline using barim t i t ana te ceramic transducers are shown in Fig 4 The curves

Throughout this paper the words barium titanate ceramic are used t o designate a

material which on a weight basis is 8 0 Barn0 12 PbTiO3 and 8CaTiO3The transducers referred to were disk-shaped haampa diameter of 0 -250 inch The upper m f a c e was partiallp covered a silver electrode 0218 inch i n diameter the louer surface was completely covered The completelyelectrodedsurface of each transducer was bonded fo thesilveredsurface of a fused quartz a low-melting-point -solder

4

FREOUENCY IN PERMEGACYCLES SECOND

Fig 4 - The curves CE(f) and RE(f) f o r a d e w line using barium t i t ana te ceramic transducers

shown i n t h i s f i gu re a re based on the average of the data f r o m two similar trans-ducers on the line The following information is immediately obtainable f r o m an inspection of these curves

1 the mechanical o r series resonant frequency of the bonded transducers is determined ky the minimum of the resistance curve

2 The shunt capacitance of thetransducers is the valueof CE(f) a t the series resonant frequency This is the capacitance which is usuallytuned out under operating conditions by an inductor i n p a r a l l e l with tihe trans-ducer men the shuntcapacitance is antiresonated by a paral le l coi l the input impedance a t the series resonant frequency is given by the mini-mum of the resistance curve (assuming a high Q coi l )

5

A Loss Comparison C i r c u i t

The insertion loss of a d e w line under 8peciiied termination conditions may be conveniently measured by means of a conventional l o s ~camparlson circuit6 of the Qpe shown In Fig 5 The circuit consists of two branches One branch

GATING SYNC PULSE PULSE

DRIVERRF r-- RF PULSEANDOSCILLATOR AMPLIFIER = n- GENERATOR RF GATE

1- I-- -- I I --- --I

CI INPUT- - SYNC LINE

PULSECALIBRATED ATTENUATOR

I I RF AMPLIFIER

- 1 AND DETECTOR

--

--I _

Y-AXIS INPUT IMPEDANCE OF RF AMPLIFIER AND DETECTM gtgt R OR R - SYNC - HOR

--

I I

containsthe network under t e s t and the other branch contalna a step attermator by meana of which a known amount of loss may be switched into the circuit The output signals of the two branches are^ comparsdand tbe 8tbnu8tor 58 a-ted untd1 their amplitudss me the same When t h i s condition l 8 obtained the inser-

6

l

t ion loss of the d e w line operating between impedancesRs will be equal to the 1088 indicated the attenuator if

and

(2) Q the i t e ra t ive fmpeampnce of the atturntor

An important feature of this c i rcu i t l 8 that a8 long a8 conditions (1) and (2) are met the attenuator reads inssrtionloss correctly regardless of the hpctanceof the source drivlng the measuring circai t

A series or parallel c o i l is urntally used i n the input and output c i rcu i t s of a completed l i n e i n order to reS01Utte or antiresonate the shunt capacitance a t the m5ampand frequencies These Coil8 will be considered as part of the delay line assembly The c i rcu i t of Fig 5 perrnifs a dew line aS8embfY la be measured under 8 wide range of bla3inatiOn COnditiOhs but U 0 u d ~the loss peamretnent of moat interest is that 3n which RS and RL correspond to the telminations of Intended use

The measuring procedure for inser t ion loss as discussed i n the precedingparagraphs is based on the use of a steady-state sinusoidal signal input The same procedure i s applicable to ampsed delay linea under the condition that the input pulse be long enough such that the transducers have time to reachacondi-t ion of steady-state vibration If thf8 condition were not imposed one might expect the measured value of the insertion loss of a delay line to depend on the amplitude of the applied signal I n practice signals consisting of rf bursts have been observed to cane uprsquo to the i r steacly-state amplitude Wrthin approximately 2 to 3 cycles when ceramic transducers are operated i n the range of 10 to 20 mc

Figure 6 shows the band-pass characteristics of a short fised Quarts delayline (approximately 2 s e c d e w t h e )using the same ceramic transducers as those from which the data I n Fig 4 were obtained These curves show the results obtained by us parallel coila t o antiresonate the shunt capacitance a t the ser-i e s resonant frequency 155 mc and using sylmretsical resistive terminations of the values indicated on the graph The loss measuring c i r cu i t in Fig 5 was used i n taking these memements

brmnents on Loss Measurements

A counnon practice i n the cam of long dew lines uding quar ts transducers has been to me88ure insertion loss under conditions 6f aero input termination and some nonzero output terrdnation Tbe c i rcu i t shown in Fig 5 could be used i n

7

irsquo

t I

1i 3

l I f

2 1 I I I I I l2 l3 14 15 16 1 7 laFREQUENCY IN MEGACYCLES PER SECOND 19 20 $

1 I

Fig 6 - Band-pass characterist ics of a delay line assembly uaing ceramic 3

transducers $

this way rnaking Q = r 0 and l e t t i ng RL have ariy desired value muamp always beequal t o the i te ra t ive impedance o f the attermator While there is nothingincorrect i n this procedure there are severalundesirable consequences -

(1) Because a delay line in actual use will bedriven by a source having a f i n i t e internal impedance the loss and band-pass characterist ics measured will not be those corresponding to the conditions of use

0 Sometimes the c i rcui t i n Fig 5 is usedwith = 0 and RSA equal t o some highvalue of resistance This is done to increasetheload Lnpedance seen by t h e source driving thecircuit The insertion loss curveobtained i n t h i s manner 1

is the same as t ha t obtained for thecondition of zero input termination and output termimtion equal to RL except t ha t the numericalvalues of t he a t t e rn -tor readings are now decreased by a constant amount N given by

R u 0

A RLA -21N = 10 loglo[ L I

8

(2) Low-loss l i nes measured in this manner may show an insertion gain depend-ing upon how large 9 is relative t o the impedance of the transducer

The latter statement may seem a t first surprising but the theory indicates that f o r a line having ideal transducers and operated a t the midband frequencythe limiting value of $he outputvoltage as becomes large i s twice the value of the voltage across the input transducer 3r F short lines using ceramic trans-ducers of the tspe used for the data of Figs 4 and 6 have shown small insertion gains when measured between zero input and 100 ohms output teminatiom

MEASUREMENT OF UNWANTED SIGNALS

Types of Unwanted Signals

The main pulse out of a deley line is accompanied amp various tSpes of unwanted signals which may arrive aheadof o r behind it These unwanted s igna lsappearing in the output may be classif ied into three types

(1) An -delayed signal resulting from the direct electrical feecE-through of a portion of the input pulse

(2) Signal8 amp v i n g before o r after the main pulse cawed by thespreading out of the sonic beam and consequently portions of it taking pathsthrough the line other than that followed by the main signal

3) Signals resulting f r o m a portion of the main pulse making multiple tra-versals of the path between input and output transducers I n generalthe electrical termination of the output transducer is not such as to result in complete absorption of the energy in the incident sonic pulseConsequently p a r t of the pulse t ravels back and forth u n t i l all the energy is dissipated in the termination of the input and output trans-ducers and i n the delay medium i t s e l f

The level of any ulwanted signal is conveniently measured by the c i r cu i t i n Fig 5 For a part icular unwanted signal the loss of the calibratedattenuator is increased until the pulse amplitude from this branchcorresponds t o the pulseamplitude of the particular unwanted being measured The difference between the level of the main pulse and the final attenuator reading corresponds to the num-ber of db by which the unwanted signal is below the main signal Ordinarily this measurement is made only on the strongest unwanted signal a t each frequency f o r which the loss is measured

Variation i n the Level of Type 3 Unwanted Signals Depending Upon the Effective Input Termination for Reflected Pulses

-r C-1 t

9rsquo

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

A Loss Comparison C i r c u i t

The insertion loss of a d e w line under 8peciiied termination conditions may be conveniently measured by means of a conventional l o s ~camparlson circuit6 of the Qpe shown In Fig 5 The circuit consists of two branches One branch

GATING SYNC PULSE PULSE

DRIVERRF r-- RF PULSEANDOSCILLATOR AMPLIFIER = n- GENERATOR RF GATE

1- I-- -- I I --- --I

CI INPUT- - SYNC LINE

PULSECALIBRATED ATTENUATOR

I I RF AMPLIFIER

- 1 AND DETECTOR

--

--I _

Y-AXIS INPUT IMPEDANCE OF RF AMPLIFIER AND DETECTM gtgt R OR R - SYNC - HOR

--

I I

containsthe network under t e s t and the other branch contalna a step attermator by meana of which a known amount of loss may be switched into the circuit The output signals of the two branches are^ comparsdand tbe 8tbnu8tor 58 a-ted untd1 their amplitudss me the same When t h i s condition l 8 obtained the inser-

6

l

t ion loss of the d e w line operating between impedancesRs will be equal to the 1088 indicated the attenuator if

and

(2) Q the i t e ra t ive fmpeampnce of the atturntor

An important feature of this c i rcu i t l 8 that a8 long a8 conditions (1) and (2) are met the attenuator reads inssrtionloss correctly regardless of the hpctanceof the source drivlng the measuring circai t

A series or parallel c o i l is urntally used i n the input and output c i rcu i t s of a completed l i n e i n order to reS01Utte or antiresonate the shunt capacitance a t the m5ampand frequencies These Coil8 will be considered as part of the delay line assembly The c i rcu i t of Fig 5 perrnifs a dew line aS8embfY la be measured under 8 wide range of bla3inatiOn COnditiOhs but U 0 u d ~the loss peamretnent of moat interest is that 3n which RS and RL correspond to the telminations of Intended use

The measuring procedure for inser t ion loss as discussed i n the precedingparagraphs is based on the use of a steady-state sinusoidal signal input The same procedure i s applicable to ampsed delay linea under the condition that the input pulse be long enough such that the transducers have time to reachacondi-t ion of steady-state vibration If thf8 condition were not imposed one might expect the measured value of the insertion loss of a delay line to depend on the amplitude of the applied signal I n practice signals consisting of rf bursts have been observed to cane uprsquo to the i r steacly-state amplitude Wrthin approximately 2 to 3 cycles when ceramic transducers are operated i n the range of 10 to 20 mc

Figure 6 shows the band-pass characteristics of a short fised Quarts delayline (approximately 2 s e c d e w t h e )using the same ceramic transducers as those from which the data I n Fig 4 were obtained These curves show the results obtained by us parallel coila t o antiresonate the shunt capacitance a t the ser-i e s resonant frequency 155 mc and using sylmretsical resistive terminations of the values indicated on the graph The loss measuring c i r cu i t in Fig 5 was used i n taking these memements

brmnents on Loss Measurements

A counnon practice i n the cam of long dew lines uding quar ts transducers has been to me88ure insertion loss under conditions 6f aero input termination and some nonzero output terrdnation Tbe c i rcu i t shown in Fig 5 could be used i n

7

irsquo

t I

1i 3

l I f

2 1 I I I I I l2 l3 14 15 16 1 7 laFREQUENCY IN MEGACYCLES PER SECOND 19 20 $

1 I

Fig 6 - Band-pass characterist ics of a delay line assembly uaing ceramic 3

transducers $

this way rnaking Q = r 0 and l e t t i ng RL have ariy desired value muamp always beequal t o the i te ra t ive impedance o f the attermator While there is nothingincorrect i n this procedure there are severalundesirable consequences -

(1) Because a delay line in actual use will bedriven by a source having a f i n i t e internal impedance the loss and band-pass characterist ics measured will not be those corresponding to the conditions of use

0 Sometimes the c i rcui t i n Fig 5 is usedwith = 0 and RSA equal t o some highvalue of resistance This is done to increasetheload Lnpedance seen by t h e source driving thecircuit The insertion loss curveobtained i n t h i s manner 1

is the same as t ha t obtained for thecondition of zero input termination and output termimtion equal to RL except t ha t the numericalvalues of t he a t t e rn -tor readings are now decreased by a constant amount N given by

R u 0

A RLA -21N = 10 loglo[ L I

8

(2) Low-loss l i nes measured in this manner may show an insertion gain depend-ing upon how large 9 is relative t o the impedance of the transducer

The latter statement may seem a t first surprising but the theory indicates that f o r a line having ideal transducers and operated a t the midband frequencythe limiting value of $he outputvoltage as becomes large i s twice the value of the voltage across the input transducer 3r F short lines using ceramic trans-ducers of the tspe used for the data of Figs 4 and 6 have shown small insertion gains when measured between zero input and 100 ohms output teminatiom

MEASUREMENT OF UNWANTED SIGNALS

Types of Unwanted Signals

The main pulse out of a deley line is accompanied amp various tSpes of unwanted signals which may arrive aheadof o r behind it These unwanted s igna lsappearing in the output may be classif ied into three types

(1) An -delayed signal resulting from the direct electrical feecE-through of a portion of the input pulse

(2) Signal8 amp v i n g before o r after the main pulse cawed by thespreading out of the sonic beam and consequently portions of it taking pathsthrough the line other than that followed by the main signal

3) Signals resulting f r o m a portion of the main pulse making multiple tra-versals of the path between input and output transducers I n generalthe electrical termination of the output transducer is not such as to result in complete absorption of the energy in the incident sonic pulseConsequently p a r t of the pulse t ravels back and forth u n t i l all the energy is dissipated in the termination of the input and output trans-ducers and i n the delay medium i t s e l f

The level of any ulwanted signal is conveniently measured by the c i r cu i t i n Fig 5 For a part icular unwanted signal the loss of the calibratedattenuator is increased until the pulse amplitude from this branchcorresponds t o the pulseamplitude of the particular unwanted being measured The difference between the level of the main pulse and the final attenuator reading corresponds to the num-ber of db by which the unwanted signal is below the main signal Ordinarily this measurement is made only on the strongest unwanted signal a t each frequency f o r which the loss is measured

Variation i n the Level of Type 3 Unwanted Signals Depending Upon the Effective Input Termination for Reflected Pulses

-r C-1 t

9rsquo

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

t ion loss of the d e w line operating between impedancesRs will be equal to the 1088 indicated the attenuator if

and

(2) Q the i t e ra t ive fmpeampnce of the atturntor

An important feature of this c i rcu i t l 8 that a8 long a8 conditions (1) and (2) are met the attenuator reads inssrtionloss correctly regardless of the hpctanceof the source drivlng the measuring circai t

A series or parallel c o i l is urntally used i n the input and output c i rcu i t s of a completed l i n e i n order to reS01Utte or antiresonate the shunt capacitance a t the m5ampand frequencies These Coil8 will be considered as part of the delay line assembly The c i rcu i t of Fig 5 perrnifs a dew line aS8embfY la be measured under 8 wide range of bla3inatiOn COnditiOhs but U 0 u d ~the loss peamretnent of moat interest is that 3n which RS and RL correspond to the telminations of Intended use

The measuring procedure for inser t ion loss as discussed i n the precedingparagraphs is based on the use of a steady-state sinusoidal signal input The same procedure i s applicable to ampsed delay linea under the condition that the input pulse be long enough such that the transducers have time to reachacondi-t ion of steady-state vibration If thf8 condition were not imposed one might expect the measured value of the insertion loss of a delay line to depend on the amplitude of the applied signal I n practice signals consisting of rf bursts have been observed to cane uprsquo to the i r steacly-state amplitude Wrthin approximately 2 to 3 cycles when ceramic transducers are operated i n the range of 10 to 20 mc

Figure 6 shows the band-pass characteristics of a short fised Quarts delayline (approximately 2 s e c d e w t h e )using the same ceramic transducers as those from which the data I n Fig 4 were obtained These curves show the results obtained by us parallel coila t o antiresonate the shunt capacitance a t the ser-i e s resonant frequency 155 mc and using sylmretsical resistive terminations of the values indicated on the graph The loss measuring c i r cu i t in Fig 5 was used i n taking these memements

brmnents on Loss Measurements

A counnon practice i n the cam of long dew lines uding quar ts transducers has been to me88ure insertion loss under conditions 6f aero input termination and some nonzero output terrdnation Tbe c i rcu i t shown in Fig 5 could be used i n

7

irsquo

t I

1i 3

l I f

2 1 I I I I I l2 l3 14 15 16 1 7 laFREQUENCY IN MEGACYCLES PER SECOND 19 20 $

1 I

Fig 6 - Band-pass characterist ics of a delay line assembly uaing ceramic 3

transducers $

this way rnaking Q = r 0 and l e t t i ng RL have ariy desired value muamp always beequal t o the i te ra t ive impedance o f the attermator While there is nothingincorrect i n this procedure there are severalundesirable consequences -

(1) Because a delay line in actual use will bedriven by a source having a f i n i t e internal impedance the loss and band-pass characterist ics measured will not be those corresponding to the conditions of use

0 Sometimes the c i rcui t i n Fig 5 is usedwith = 0 and RSA equal t o some highvalue of resistance This is done to increasetheload Lnpedance seen by t h e source driving thecircuit The insertion loss curveobtained i n t h i s manner 1

is the same as t ha t obtained for thecondition of zero input termination and output termimtion equal to RL except t ha t the numericalvalues of t he a t t e rn -tor readings are now decreased by a constant amount N given by

R u 0

A RLA -21N = 10 loglo[ L I

8

(2) Low-loss l i nes measured in this manner may show an insertion gain depend-ing upon how large 9 is relative t o the impedance of the transducer

The latter statement may seem a t first surprising but the theory indicates that f o r a line having ideal transducers and operated a t the midband frequencythe limiting value of $he outputvoltage as becomes large i s twice the value of the voltage across the input transducer 3r F short lines using ceramic trans-ducers of the tspe used for the data of Figs 4 and 6 have shown small insertion gains when measured between zero input and 100 ohms output teminatiom

MEASUREMENT OF UNWANTED SIGNALS

Types of Unwanted Signals

The main pulse out of a deley line is accompanied amp various tSpes of unwanted signals which may arrive aheadof o r behind it These unwanted s igna lsappearing in the output may be classif ied into three types

(1) An -delayed signal resulting from the direct electrical feecE-through of a portion of the input pulse

(2) Signal8 amp v i n g before o r after the main pulse cawed by thespreading out of the sonic beam and consequently portions of it taking pathsthrough the line other than that followed by the main signal

3) Signals resulting f r o m a portion of the main pulse making multiple tra-versals of the path between input and output transducers I n generalthe electrical termination of the output transducer is not such as to result in complete absorption of the energy in the incident sonic pulseConsequently p a r t of the pulse t ravels back and forth u n t i l all the energy is dissipated in the termination of the input and output trans-ducers and i n the delay medium i t s e l f

The level of any ulwanted signal is conveniently measured by the c i r cu i t i n Fig 5 For a part icular unwanted signal the loss of the calibratedattenuator is increased until the pulse amplitude from this branchcorresponds t o the pulseamplitude of the particular unwanted being measured The difference between the level of the main pulse and the final attenuator reading corresponds to the num-ber of db by which the unwanted signal is below the main signal Ordinarily this measurement is made only on the strongest unwanted signal a t each frequency f o r which the loss is measured

Variation i n the Level of Type 3 Unwanted Signals Depending Upon the Effective Input Termination for Reflected Pulses

-r C-1 t

9rsquo

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

2 1 I I I I I l2 l3 14 15 16 1 7 laFREQUENCY IN MEGACYCLES PER SECOND 19 20 $

1 I

Fig 6 - Band-pass characterist ics of a delay line assembly uaing ceramic 3

transducers $

this way rnaking Q = r 0 and l e t t i ng RL have ariy desired value muamp always beequal t o the i te ra t ive impedance o f the attermator While there is nothingincorrect i n this procedure there are severalundesirable consequences -

(1) Because a delay line in actual use will bedriven by a source having a f i n i t e internal impedance the loss and band-pass characterist ics measured will not be those corresponding to the conditions of use

0 Sometimes the c i rcui t i n Fig 5 is usedwith = 0 and RSA equal t o some highvalue of resistance This is done to increasetheload Lnpedance seen by t h e source driving thecircuit The insertion loss curveobtained i n t h i s manner 1

is the same as t ha t obtained for thecondition of zero input termination and output termimtion equal to RL except t ha t the numericalvalues of t he a t t e rn -tor readings are now decreased by a constant amount N given by

R u 0

A RLA -21N = 10 loglo[ L I

8

(2) Low-loss l i nes measured in this manner may show an insertion gain depend-ing upon how large 9 is relative t o the impedance of the transducer

The latter statement may seem a t first surprising but the theory indicates that f o r a line having ideal transducers and operated a t the midband frequencythe limiting value of $he outputvoltage as becomes large i s twice the value of the voltage across the input transducer 3r F short lines using ceramic trans-ducers of the tspe used for the data of Figs 4 and 6 have shown small insertion gains when measured between zero input and 100 ohms output teminatiom

MEASUREMENT OF UNWANTED SIGNALS

Types of Unwanted Signals

The main pulse out of a deley line is accompanied amp various tSpes of unwanted signals which may arrive aheadof o r behind it These unwanted s igna lsappearing in the output may be classif ied into three types

(1) An -delayed signal resulting from the direct electrical feecE-through of a portion of the input pulse

(2) Signal8 amp v i n g before o r after the main pulse cawed by thespreading out of the sonic beam and consequently portions of it taking pathsthrough the line other than that followed by the main signal

3) Signals resulting f r o m a portion of the main pulse making multiple tra-versals of the path between input and output transducers I n generalthe electrical termination of the output transducer is not such as to result in complete absorption of the energy in the incident sonic pulseConsequently p a r t of the pulse t ravels back and forth u n t i l all the energy is dissipated in the termination of the input and output trans-ducers and i n the delay medium i t s e l f

The level of any ulwanted signal is conveniently measured by the c i r cu i t i n Fig 5 For a part icular unwanted signal the loss of the calibratedattenuator is increased until the pulse amplitude from this branchcorresponds t o the pulseamplitude of the particular unwanted being measured The difference between the level of the main pulse and the final attenuator reading corresponds to the num-ber of db by which the unwanted signal is below the main signal Ordinarily this measurement is made only on the strongest unwanted signal a t each frequency f o r which the loss is measured

Variation i n the Level of Type 3 Unwanted Signals Depending Upon the Effective Input Termination for Reflected Pulses

-r C-1 t

9rsquo

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

(2) Low-loss l i nes measured in this manner may show an insertion gain depend-ing upon how large 9 is relative t o the impedance of the transducer

The latter statement may seem a t first surprising but the theory indicates that f o r a line having ideal transducers and operated a t the midband frequencythe limiting value of $he outputvoltage as becomes large i s twice the value of the voltage across the input transducer 3r F short lines using ceramic trans-ducers of the tspe used for the data of Figs 4 and 6 have shown small insertion gains when measured between zero input and 100 ohms output teminatiom

MEASUREMENT OF UNWANTED SIGNALS

Types of Unwanted Signals

The main pulse out of a deley line is accompanied amp various tSpes of unwanted signals which may arrive aheadof o r behind it These unwanted s igna lsappearing in the output may be classif ied into three types

(1) An -delayed signal resulting from the direct electrical feecE-through of a portion of the input pulse

(2) Signal8 amp v i n g before o r after the main pulse cawed by thespreading out of the sonic beam and consequently portions of it taking pathsthrough the line other than that followed by the main signal

3) Signals resulting f r o m a portion of the main pulse making multiple tra-versals of the path between input and output transducers I n generalthe electrical termination of the output transducer is not such as to result in complete absorption of the energy in the incident sonic pulseConsequently p a r t of the pulse t ravels back and forth u n t i l all the energy is dissipated in the termination of the input and output trans-ducers and i n the delay medium i t s e l f

The level of any ulwanted signal is conveniently measured by the c i r cu i t i n Fig 5 For a part icular unwanted signal the loss of the calibratedattenuator is increased until the pulse amplitude from this branchcorresponds t o the pulseamplitude of the particular unwanted being measured The difference between the level of the main pulse and the final attenuator reading corresponds to the num-ber of db by which the unwanted signal is below the main signal Ordinarily this measurement is made only on the strongest unwanted signal a t each frequency f o r which the loss is measured

Variation i n the Level of Type 3 Unwanted Signals Depending Upon the Effective Input Termination for Reflected Pulses

-r C-1 t

9rsquo

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

A 2 Opeec d e w linewas connected between terminating resistances of 30 Ohme a8 Shown i n the c i rcui t i n FJgJ The co i l s in the input and output cir-cui ts of the d e w line uere tuned f o r maxim output of a pulse with a 1s mc carrier frequency A resistance Q was connected i n ser ies with the output of the driver etage When RD was varied the insertion l o s s of the l ine remained conatant a t 5 db because the eame reference voltage ER was shared by both branches however the w a n t e d signals varied i n both level and f o r m I n the graph in Fig 7 data for the variation in the level of the first and second unwanted pulses are shown These wan ted s igna le correspond to what are fre-quently called third-time through and fifth-time fhroughn signals The data i n this particular case 8how that f o r a low value of thethird-time through signal was the stronger unwanted As thevalue of vas increased this signalbecame weaker and above a value of 20 ohme the secondunwanbd was the strongerof the two As a result of t h i s tspe of behavior there is generally an optimum termination producing a mininun level f o r the urwanteds when the main unwanted6 are those cawed by multiple path traversals of the Ionic pulse

29

27

W v)-l 3 a

25z UI

4 23

v)J lU S VW Q

21

19 20 40 60 SO I O 0 I20 I40

RD IN OHMS

This was not the same l ine as t ha t which was used fo r the data shown i n Figs L and 6

10

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

Figure 8 l e a photographic record of the form of the output pulses under the indicated test conditionsaeseen on the face of an oscilloscope The signalsshown are the envelopes of the detected rf pulees The vertical deflection l e mportional to the i r amplitude and the horieontal deflection increases linearly d t h time On the records a large division in the horizontal direction corre-sponds t0 SAsec The upper picture shows the output pulse when 0 i n addition the first two unwanted signals are visible In thecase of the middle picture conditions are the eame 88 for the upper except that the signal inb the delay line has been increased making the unwanted signals more visible The increased level caused the main signal to go off scale he lower gicture shows the change i n the level and form of the first w a n t e d pulse when RD was increased t o 200 (The Signal output of the driver was of courseincreased to compensatefo r the drop rermlting f r o m the increased value of RD) It shouldagain be

Fig 8 - Fhotographic records of var ia t ions in the output pulse8 caused by change8

mphasizedat al though the level and form of unwanted signals were al tered by Changes i n the insertion loss f o r the main pulee remained the same

The reason for the variation in the leve l of the unwanted signals can be made more apparent by considering the delay line and rneazmring circuit pictured In 9 The input transducer is excited by means of the rf pulse shown the firno This produces an e l a s t i c wave packet which travels through the delas

with theappropriateelastic wave velocity while the elastic pulse IS lt ldquoavel in~ through the line the input signal drops to zero When the e l a s t i c

11

I

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

pulse arrives a t the receirlng transducer part of its mechanical energy i s con-vertedintoelectrical energy which is diseipated i n the loadresistance P a r t of the elast ic wave motion is reflected back t o the input transducer7partlybecause the receiving transducer does not cover the whole reflecting surface and

ATTENUATORRF INPUTPULSE

I

7

partly because the e l ec t r i ca l t en ina t ion is generally not such as t o producecomplete dissipation i n the e l ec t r i ca l c i r cu i t of the incident mechanical energy) khen the reflected pulae arrives at the input transducer the amount of energyremoved by this transducer w i l l depnd upon how the input electrical termination appears when the reflected elastic pulse is incident A t this time is zero and the input termination of the line is given by

(Input Termination for Reflected pulses)

In this equation Ru is the i terat ive impedanceof the step attenuator

The preceding discussion takes into account the fact that the amplitude o f a pulse reflected from a Surface with a transducer on it depends appreciably on the deCtriCd termination of the tr=ampcer That t h i s is thecasefor short delay line8 using barium titanate transducers i s demonstrated by the data shown i n Fig10 These data were obtained using the barium titanatetransducers (approximatelY 1h inch i n diameter and resonant near 15 mc) on the same short delay line used in obtaining the data shown i n Figs 7 and 8 The line was operated a s a single-ended d e w line i n each case the same transducer was used for transmitting and r e c e i h g The second transducer on the reflecting surface was terminated i n a c o i l (used to antiresonant the shunt capacitance of the transducer at 15 mc) and a variable resistance RT The data show the pwer level of the outputserela-t ive t o the input s igna l as a function of the value of RT for the two different

12

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

Cases The transducers on the delay line were designated A and B and were con-nected as indicated i n the c i r cu i t shown i n Fig 10 I n each casethe termina-tion conditions a t the input c i rcui t were held constant so that the indicated variations depended only on the changes in the terminating circuit of the reflect-ing transducer

The variation i n the form and level of the reflected signal with changes i n thevalueof RT are shown by thephotographicrecords i n Fig 11 The top pic-ture shows the form of the i n p t and reflected pulses observed on the oscillo-scope when RT = 0 The middle record shows the change produced i n t h e received signals changing RT to 75 ohms keepingthe input leve l the sameThe bottom photograph shows the appearanceof the output signal when the input signal was increased enough to make the reflected pulses clearlyvisible The photographs were taken for the case in which transducer B was reflecting For t h i s trans-ducer the data i n Fig 10 indicate that a t 75 ohms the reflected signal i s a minimum The record shows that the first reflected pulse has an initial and trailing transient separated by a m i n i m u m which was found to be extremely sensi-tive to small changes i n RT and LT The cause of these transients i s interpreted t o be the changes i n the effective impedance of the transducer when its vibration is started or stopped

A Modification of theBasic Measurinf Circuit Permitting Complete Specification ermination

Formeasuring purpses it is desirable to have a means of specifying com- pletely the input termination of a delay line In the measuring c i r cu i t of Fig5 the input sermination of a delay line from the point of view of reflected pulses incident upon the input transducer includes the impedance of the drtver Usually this impedance has a complicated dependence upon frequency and may varyconsiderably depending upon whether o r not t h e driver is turned on or off A way of eliminating this complication is to t i e down the impedance to a small enoughvalue so t ha t it becomes negligible compared t o Rs (seeFig 9 ) When is thus made small the input termination is practically the same for reflected pulses as for the in i t ia l input pulse For example i n a specificcase i n which Rs = RL = 75 ohms and RSA = RA = 50 ohms Z~mpcanrange i n magnitude f r o m a minimum of 75rsquo

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

C

a d

b c

t

n3 id

14

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

ohms to a max imum of 175 ohms depqnding upon the value of Q A value o f ZG equal t o 5 ohms would r e su l t i n aninput tennination of approximately 80 ohms fo r all reflected signals and consequently the input tennination can be considered t o be75 ohms f o r most purposes

An example of a driver c i r cu i t having an output impedance t i e d down to less than 5 ohms i s shown i n Fig 12 This c i rcu i t may be used to deliver rf pulses

32

I IA + 2 5 0 V

la DC+PULSEt l 50 V

TI-WESTERN ELECTRIC RF OUTPUT TRANSFORMER NO 527A T2-FERRAMIC Q TOROID GENERALCERAMICSPART NO 262-

318 OD X 316 ID PRI-20 TURNS CTSEC-3 TURNS

Fig 12 - A push-pull g8tad amplifier used to drive the insertonloas mearmringcircpit

In addition to i ts low output impedance a gated amplifier circuit of the b e shown i n Fig 12 has several other advantages The average power dissipated in the plate c i r cu i t i s low because pulse durations are approximately l p s e c and repetition retes range from 60 to 1OOO pps In single-ended gated amplifier stages a pronounced transient caused by switching the plate current on and off is U s u a l l y present but this switching transient i s cancelled out by the push-pulldesign An important limitation of this type of dr iver c i rcui t is the fact that a Small amount of thecontinuous input rf signal i s fed through to the outputThis feed-through may be a source of interference i n the measurements of extremely IOW level unwanteds For the c i r cu i t shown it was possible to obtain a t the out-Put terminals a difference between the output level when the driver is turned on a d theleakage rf leve l when it is off of theorder of 80 db This difference is great enough fo r most measurements involving short delay lines

I

15

I

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)

CONCLUSION

The preceding discussion has been based largely on experimentalshort delay lines using ceramic transducers The same techniquea of may be applied t o d e w l i n e s using quartz crystal transducers

There are important differences between lines using quartz and ceramic transducersFor example i n lines wing quartz transducers thecapacitive con-ponent of the input admittance of a transducer may be satisfactorily representedby a fixed capacitor ampreas for lines nith ceramic transducers the capacitive component may vary by 20 per cent o r more Another important poiamp of difference i s that of the effect of termination impedances on curves for both the loss i n the main signal and the level of unwanted signals

Delay l ines using quartz crgstale usually have impedances at midband of e

order of thousands of ohma In both measurement and use they are frequentlydeliberately terminated i n impedances of the order of hundreds of ohms i n order to widen their pass band As a resalt of the large mismatch small variations of the order of 10 to 100 ohma I n the termination impedances make l i t t l e difference in either of these two Qpes of quantities On the other hand lines with ceramic

1 transducers usually have low impedances of the order of 20 to 100 ohms at midbandIn this case variations of 10 t o 100 ohms in t e m i m t i o n impedances can make appreciable changes i n measurements of both l o s s and unwanted signals and the change i n urmanted signals may be especially apparent i f a l ine i s short The differences observed i n the two types of delay lines am basically caused by the considerably higher electromechanical coupling factor of ceramic transducers

BIBLIOGRAPHY

1 Mason W P NElectsomechanicalTransducers and Wave F i l t e r s New York D Van Nostrand Company Inc pp 230-237 and 399-bOh 1948

2 Fagen M D nPerformance of ultrasonicvitreoussilicadelay linest1 ProceedingsNationalElectronicsConference 7380-389 (1951)

3 May J E nCharacterirstics of delay lines using quar tz and barium t i tanate ceramic transducers J Acoust Soc E 26347-355 (May 1954)

I--

4

5 Meitzler A H A procedure f o r determining the equivalentcircuit ele-ments representing ceramic transducer8 used i n delaylines Proceedings-1957 Electronic Components Symposium pp 210-219

6 Schulz and Andereon nExperimenks i n Electronics and Conammication Engineer- ing New York Harper Brothers pp 118-119 1943

7 Cady W G A Generalized Theory of the C r y s t a l Receiver for Plane Waves ONR Contract N6 ONR-262 Task Order 1 (Nov 1950)