PROCEED1NGS OF THE I.R.E.

Ultra-High-Frequency Triode OscillatorUsing a Series-Tuned Circuit*

J. M. PETTITt, SENIOR MEMBER, IRE

Summary-As the conventional triode oscillator is applied tohigher and higher frequencies, one or both of two phenomena finallyprevent oscillation. These phenomena are transit-time effects andself-resonance of the tube elements. The analysis described im thispaper refers these effects to the resistance and reactance require-ments of the tube upon the external circuit. In those instances wherea given triode is limited primarily by the reactance effects, due tolarge lead inductance, it is shown that the self-resonant frequency ofthe tube is not the upper limit of oscillation if one departs from theconventional parallel-tuned circuit and uses a series-tuned circuitinstead.

HEHERE IS ALWAYS an interest in extending thefrequency range of any given triode tube to higherfrequencies. This is especially true in receiver ap-

plications, where, in addition, it is often desired to havewide tuning range and single control. The customaryoscillator used in this application is the standard Hart-ley or Colpitts circuit, but at higher frequencies themodified Colpitts circuit shown in Fig. 1 is employed.In this latter circuit the feedback is accomplishedthrough the internal tube capacitances. It has been

H c'W<'l~~~~~~~~~~~~~~~~~~~~~~~~Crute

SCHEMATIC DIAGRAM EQUIVALNT CIRCUIT

Fig. 1-Ultra-high-frequency oscillator-modified Colpitts circuit.

shown by previous authors' that this circuit is one of a

class that is most favorable in reducing transit-time ef-fects. With conventional small triodes, it provides a

useful oscillator for receiver and signal generator ap-

plications up to frequencies of several hundred mega-

cycles, often using the "butterfly" circuit for the tunedelement, as indicated in Fig. 1.

In order to analyze the requirements of the externaltuned circuit for a given triode tube, it is expedient toinvestigate the nature of the two-terminal impedancelooking into the grid-plate terminals, then to computethe series resistive and reactive components as a func-tion of frequency.2 When this is done, results such as

those shown in Fig. 2 are obtained (6F4 triode, connec-

* Decimal classification: R355.912. Original manuscript receivedby the Institute, September 12, 1949; revised manuscript received,January 4, 1950.

t Stanford University, Stanford, Calif.I J. Bell, M. R. Gavin, E. G. James, and G. W. Warren, "Triodes

for very short waves-oscillators," Jour. IEE (London), part III A,vol. 93, pp. 833-846; 1946.

2Further details on this approach will be found in a companionpaper, F. J. Kamphoefner, 'Feedback in vhf and uhf oscillators," inthis issue, PROC. I.R.E., pp. 630-632.

tions to only one pair of grid and plate terminals). Cal-culated data are presented, together with measurementsmade upon a scaled model operating at one tenth of thefrequencies shown. Simple linear theory is utilized, em-ploying a real and constant transconductance. Thisprovides, of course, no information as to the magnitudeof the oscillation nor the available power output, butdoes define a set of minimum conditions required foroscillation. The lower curve shows the negative resist-ance provided by the tube, and it follows that the ex-ternal circuit must provide a positive resistance equal toor less than the appropriate negative value at the fre-quency of interest. Likewise, the reactance of the ex-ternal circuit must be equal in magnitude to and oppo-site in sign from the reactance provided by the tube.

This paper is restricted to those applications wherethe reactance of the external circuit is the limiting fac-tor, especially operation in the region near and above theself-resonant frequency of the tube, which occurs atabout 730 Mc for the example in Fig. 2. It is thusassumed that transit-time effects have not impaired themagnitude of the negative resistance presented by thetube to such an extent that usable external circuits aretoo lossy to operate. This situation might be expected

uden9iea d iunductanceA gO 400 00 - @ looAoDO

Z , < Withoutt leacd inductance,

614

. _ ~~~~~~~~~~~~~Lg

C .OZ. fd. G.M - 6pOOOmhosCgr: I.9pefd. -16.5

4~~~~~~~~C z .Pf

ExXEUENC Y- U&4010 600 60 1. I-Loo 1.40 j--

C)-4Clp oscillator. W(t houtIaesd induteondc.scaled mod)Witht lead inductanceZ $or with los kss leas.

-14'- -

Fig. 2-Dynamic impedance presented to tuned circuit by 6F4Colpitts oscillator. (Note: Solid curves are computed. The dottedcurves and encircled points are measured, using the frequency-scaled model.)

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PROCEEDINGS OF THE I.R.E.

to prevail in close-spaced triodes mounted in miniatureenvelopes, for instance the triode-connected 6AK5,where lead inductance is large and it, instead of transittime, will tend to be the limiting factor.The manner in which the external circuit operates in

conjunction with the tube to provide the proper react-ance and thus to determine the oscillating frequency isshown in the calculated curves of Fig. 3. Here the cus-

\i570 MC| C *5 Vyf

300_L S, 3H

o0 c FREOUENCY - MC_200 t < | 200Bo

w 400 600Oi0

1oo_ 667 MC; IOOC _

l-REACTANCE-200- REACTANCE OF REQUIRED BY 6F4

TUNE0 CIRCUIT,

-300_ 11Fig. 3-Reactance of parallel-tuned circuit.

tomary parallel-tuned circuit is indicated, although thercactive component plotted is the equivalent seriesvalue. The point of intersection of the reactance re-quired by the tube and the reactance available from thecircuit determines the operating frequency, shown to be570 Mc in the example under consideration.3 It will benoted that the resonant frequency of the circuit aloneis actually 667 Mc. The tube acts like a capacitance,very closely equal to the grid-plate capacitance, andthus requires an inductive reactance to produce reso-nance. Obtaining this from a tuned circuit requires thatthe circuit be tuned to a higher frequency than the de-sired frequency of oscillation of the combined circuit.It can be plainly seen from Fig. 3 that, no matter howhigh the resonant frequency of the external circuit maybe, oscillations cannot occur at a frequency higher thanthe self-resonant frequency of the tube, namely 730Mc. At this frequency a short circuit across the grid-plate terminals would suffice equally well.

It is noted that the behavior of the reactance requiredby the triode in the vicinity of self-resonance resemblesthat of a series-tuned circuit. Again considering theexamples shown in the illustration, if a small induct-ance and a series capacitance are connected to the grid-plate terminals as in Fig. 4, the dotted curves in thisfigure show the calculated reactance presented by theseries circuit for different values of tuning capacitance.Once more the intersections of these dotted lines withthe solid lines, representing the reactance required by

' There is also an intersection at approximately 1,000 Mc, butthe lower frequency provides the more favorable resistance condi-tions.

Fig. 4-Series-tuned circuit for tuning through theself-resonant frequency of the 6F4.

Fig. 5-Capacitance required to operate a 6F4 triodeabove its self-resonant frequency.

Fig. 6-Series-tuned ultra-high-frequency oscillator.

the tube, determine the operating frequencies of theoscillator circuit. It is seen that with the capacitancevariation shown, a substantial tuning range is covered,extending from well below to considerably above theself-resonant frequency of the tube.

634 June

PROCEEDINGS OF THE I.R.E.

Fig. 7-Schematic diagram of series-tuned ultra-high-frequency oscillator.

It is indeed feasible to operate such a circuit by add-ing only a capacitor as the external circuit, using the in-ductance of the leads as the inductive portion of thecircuit. This is doubtless familiar to a few readers. Thetuning ranges calculated in this circumstance are shownin Fig. 5. Such an oscillator, using the triode for whichthe previous calculations have been made, is pictured inFig. 6, with its detailed schematic in Fig. 7. The capaci-tor used was a concentric air trimmer, modified to pro-vide a split stator and an isolated rotor. The frequencyrange is approximately 630 to 900 Mc, with a cor-responding capacitance variation of 8.1 to 1.5 micro-microfarads.

For applications in which a large tuning range is de-sired, both capacitance and inductance variation can beemployed simultaneously. The chart of Fig. 8 shows therange of possibilities for the 6F4 example.

It will be noted that, from the viewpoint of oscillationtheory, the active part of the circuit, the electronstream, continues to see a parallel reactance. The ex-ternal series circuit merely serves to adjust the magni-tude of the inductive branch paralleled by the grid-plate capacitance.

0.2 r0f r, ph

i r8Fr e rag p

qzi

LLII AssuwmptUons:Dinomac. Ci Z1ji 'rf. f

Total 9&iplead L-OOLrh

zoo 15OO puo0 4,000!FREQUENCY vc

Fig. 8-Frequency range of a 6F4 Colpitts oscillator for single series-tuned circuit with both inductance and capacitance variable.

In conclusion it should be emphasized that the so-called self-resonant frequency of the tube used in atriode oscillator need not be a limit on the frequencycoverage if proper choice of the external circuit is made.

ACKNOWLEDGMENT

The assistance of F. J. Kamphoefner in both theanalytical and the experimental work is gratefullyacknowledged.

The Theory of a Three-Terminal Capacitor*ROBERT E. CORBYt

Summary-Theoretical equations are derived for the insertionloss of a three-terminal capacitor which check the experimentalcurves within their precision of measure. The condenser is assumedto behave like some equivalent transmission line and the line con-

stants of this line determined as a function of frequency. Skin effectand proximity effect are taken into consideration, and engineeringcurves are plotted to make the prediction of the behavior of such a

condenser easy to determine.

INTRODUCTIONT HE THEORY OF a three-terminal capacitor de-

scribed by Allison and Beverly' can be developedfrom Maxwell's equations and transmission line

theory. The insertion loss2 of this filter is given as* Decimal classification: R215. Original manuscript received by

the Institute, August 22, 1949; revised manuscript received, January13, 1950.

t University of Arizona, Tucson, Ariz.1 W. M. Allison and N. E. Beverly, "New high-frequency capaci-

tor,' Trans. AIEE, vol. 63, p. 915; December, 1944.2 Alan Watton, Jr., "The duct capacitor," PROC. I.R.E., vol. 36,

pp. 550-554; April, 1948.

P = 20 log (R1o/Z,),

where R1o is the impedance level of the line (usuallychosen as 10 ohms), and Z. is the impedance of the de-vice whose loss is desired. From transmission line theory,the sending end impedance of an open-circuited line is Zocoth 'yl where the characteristic impedance Zo is to a

close approximation,

Zo = (L/C)1I2((co2 + k2)/CO2)114,

I

1950 635