86 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS June

ADF Interference Blanker Development*M. M. NEWMANt, J. R. STAHMANNt, AND J. D. ROBBt

Summary-Interference blankers have been developed to giveimprovement ratios of the order of 1000 in the presence of severeprecipitation static of the order of 150,000 pulses per second. Re-cently the blanking technique has been applied to ADF receivers withthe objectives of simplification, reduced size and weight, improvedsensitivity in the presence of interference, reduced intermodulationdistortion, maintenance of the relative phases of the sense and loopsignals, and general compatibility with ADF receiver operation.

INTRODUCTIONMPULSIVE radio interference, such as that due toprecipitation static on aircraft antennas, coupledcorona, and some lightning atmospherics, can be

rejected by introducing controlled blanking circuitrybetween the antenna and receiver input. This is mostlogical wherever removal at the source is impractical orimpossible, as it would be for atmospherics. Laboratoryinvestigations, utilizing artificial lightning and precipi-tation static generating facilities, as well as flight testshave indicated improvement ratios (signal required forintelligibility-blanker out/signal required for intelligi-bility blanker in) of the order of 1000, using the LTRIdeveloped approach of controlled blanking circuitryahead of the receiver tuned circuits. Nonlinear cir-cuitry, such as limiters, introduced after the receivertuned circuits have been shock-excited cannot removeinterference having the high repetition rates of precipi-tation static because each pulse is lengthened, as shownin Fig. 1, resulting in a large proportionate loss of signalcarrier. However, by removing the interference beforeit reaches the receiver, a very much greater number ofdisturbances can be tolerated. With LTRI techniques,corona interference of the types illustrated in Figs. 2and 3(a) having average repetition rates of the order of150,000 with burst rates exceeding 1,000,000 can be suc-cessfully rejected.

In the blanking method, both the interference andsignal are prevented from reaching the receiver for theduration of the interference. The receiver is effectivelydisconnected from the antenna for short periods of a fewmicroseconds, and the signal is received in the periodsbetween the interference pulses. The flywheel effect ofthe receiver tuned circuits makes loss of signal for suchshort periods unimportant except at very high inter-ference repetition rates.The blankers developed by LTRI under ONR spon-

sored contracts with Navy BuAer, from 1946 to 1949,were general purpose units covering a frequency range

* Manuscript received by the PGANE, January 21, 1958; revisedmanuscript received, MIarch 18, 1958. This paper was originally pre-sented at the East Coast Conf. on Aeronautical and NavigationalElectronics, Baltimore, Md., October 29, 1957.

t Lightning and Transients Research Inst., Minneapolis, Minn.

Fig. I-Interference pulse lengthening in a typical receiver.

from 100 kc to 20 mc. Improvement ratios up to 1000were obtained at broadcast frequencies with theseblankers, under severe precipitation static conditions.Shorted transmission line elements were included in thesignal channel, which effectively shortened the inter-ference pulses and helped make it possible to rejectcertain lightning atmospheric pulses of the types shownin Fig. 3(b). These units did not appreciably increasethe fluctuation noise figure of the receivers, but theywere subject to intermodulation distortion from largeadjacent undesired carriers. Recently the blanking tech-nique has been applied to ADF receivers with empha-sis on reduclng intermodulation distortion, reducing sizeand weight, simplification, reducing the minimum in-telligible signal voltage available at the receiver inputin the presence of precipitation static and some types ofatmospheric interference, maintenance of loop and sensesignal relative phases, and general compatibility withthe ADF receiver requirements.

BLANKING TECHNIQUE FOR ADF RECEIVERS

A blanker unit designed for commercial ADF use andmounted on an ARN-6 radio compass is seen in Fig. 4.This unit has a volume of 48 cubic inches (2X4X6

Newman, Stahmann, and Robb: ADF Interference Blanker Development

(a)

(b)

Fig. 3-(a) Laboratory oscillograms of interference transients to beexpected on an antenna, other than lightning atmospherics, underthunderstorm conditions. (A) and (B) corona on bare wire an-

tenna; (C) and (D) from adjacent insulating surface streamer-ing; (E) and (F) from charged raindrops hitting the antenna.(b) Typical oscillograms of induced potential waveforms, in an

aperiodically terminated, single-turn 4-meter loop, 3-5 miles fromlightning area. Only the high short disturbances of (C), (D), and(E) can presently be removed by blanking techniques.

(d) I" = -

Fig. 2-Oscillograms of laboratory reproduced interference on ADFantenna from canopy surface charging effects. (a) Slow sweep

speed display of artificially generated pulses on the canopy.

(b) Interference pulse waveform on antenna with 10-megohmprobe resistance to ground. (c) Interference pulse waveform on

antenna with 30,000 ohm by pass resistance to ground. (d) Re-sultant interference transient at ADF receiver antenna terminal.

inches), weighs about 3 pounds, and contains eight dualtriode stages. In addition, an amplifier coupler, shown inFig. 5, and delay line are required between the sense

antenna and the blanker. A functional block diagram isgiven in Fig. 6, and an over-all schematic in Fig. 7. Therelays switch the blanker in or out of the sense antenna

circuit. The amplifier coupler is located at the sense an-

tenna. This location obviates the necessity of loadingthe antenna with the capacity of long connecting cables,and thus increases the signal available from a givenantenna since the capacity of the input amplifier issmall. The cascode amplifier improves the noise figureof the ADF sense channel slightly, and, with an unby-passed cathode resistor to provide negative feedback,also introduces little intermodulation distortion. Cath-ode followers in the amplifier-coupler unit are used to

feed the interference pulse discriminator and a 2000-ohm delay line which delays the signal and interferenceabout 1 ,usec to allow time for an interference gatingpulse to form and close the gate for the duration of theinterference. The function of the shorting gate also in-cluded in the amplifier coupler is discussed later.The discriminator produces trigger pulses for all sig-

Fig. 4-ADF blanker mounted on ARN-6 radio compass;dimensions approximately 2X4X6 inches.

Fig. 5-Antenna amplifier-coupler used at sense antenna.

nificant interference pulse amplitudes, at the same timeattenuating all signal carriers so that no spurious triggerpulses will be generated as a result of carriers. Its pur-

pose is the converse of what is desired for the blanker

system, since here the discriminator circuit operates to

preserve the noise while rejecting the signals-it is side

channel used to reliably trigger gating pulses wherever

an interference pulse occurs. For example, in a typicalspecification the discriminator must produce trigger

(a)

(b)

(c)

871958

88 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS June

Fig. 6 Block diagram of interference rejection system.

appear across the plate load, Zo/2, for each interferencepulse for a period corresponding to twice the delay ofthe transmission line length or until a reflection from theshorted end of the line appears at the input to the line.As shown in Fig. 8, the transmission lines can be

tuned to produce very little amplification at specificcontinuous wave frequencies f1, f2, f3, f4, etc. In fact, thesignal carrier at the plates of V5 may be reduced to al-most zero by compensating for the inherent attenuationof the lines by using a resistor connected between theends of the line and tapped at a point where the signalvoltage at the plate of the tube is just cancelled by thevoltage of opposite phase appearing at the other end ofthe line. The composite relative-amplification vs fre-quency curves of three transmission-line stages having

LOOP AMP REPLACEMENT

TO VIOISOCKET

AFRN-6

Fig. 7 Over-all schematic of principal components of ADF blanker system.

pulses when interference pulses as low as 1-mv peakoccur at the antenna, even though signals of 50 mv, zeroto peak, may also be present on the antenna. Thus, forthis channel, the pulse amplification should exceed sig-nal voltage amplification by a factor of at least 100(40 db).To accomplish such discrimination, shorted transmis-

sion lines may be used as amplifier plate loads as in Fig.7, tubes V4 and V5. Here, we take advantage of theshort duration impulse nature of the interference andthe continuous wave characteristic of the signals. Theinterference will be amplified, since a pulse voltage will

lengths of X/4, A/2, and X/4 at frequencyf1 are indicatedby the broken line in Fig. 8.

It is evident that, with a sufficient number of linesand proper line length selection, the bandwidth of thefrequencies killed may be extended as desired. For ex-ample, the composite characteristic shows how a bandof frequencies around f2 are attenuated using three lines.Since in a practical system the number of lines is usuallylimited, such a system is Inot very effective as a wide-band discriminator unless provision can be made totune the lines to the specific large carriers appearing atthe input. And since only four fixed tuned lines were

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1958 Newman, Stahmann, and Robb: ADF Interference Blanker Development

f,

Fig. 8-Relative discriminator amplification showing stagegains (solid lines) and composite gains (broken line). SiGNAL (N

(a)

1--1TPE Q FERRAMIC

M CUP CORE

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(b)Fig. 9 Typical twin diode shorting gate.

used in the system (Fig. 7), additional discriminationhad to be obtained by other means to provide adequatediscrimination over the ADF band from 100 kc to1750 kc. An additional method used was to tune thetransformer at the output of the first section of V6 to a

frequency rejected by the earlier lines. Thus, the circuitwould tend to reject frequencies to which it was nottuned but would be shock excited by the interferenceand pass it. In addition, by proper choice of the timeconstants of CAR1 and C2R2, the detected low-frequencyamplitude modulation of the signals can be discrimi-nated against in favor of the relatively steep rates ofrise of the interference. In order to reduce the effects ofvery large specific carriers, a frequency selective AGCnetwork may also be used as shown at the output ofV6 in Fig. 7.Trigger pulses produced by the discriminator actuate

the gate generator V7, which is usually a modified multi-vibrator or Schmitt circuit. In some systems the noiseitself is amplified, lengthened, and rectified to provideunipolarity gating pulses. The gating pulses are usuallyfrom 2 to 10 Asec in length, or they may vary accordingto the interference length, particularly in systems de-signed for rejection of atmospheric interference.A shorting gate V3 is used at the antenna to dissipate

charges deposited on the antenna by shorting the an-

tenna to ground momentarily when an interferencepulse occurs. The typical shorting gate is illustrated inFig. 9. The diodes are switched to conduction just afterthe start of an interference pulse. The shorting gate hastwo advantages relative to a series gate: 1) the shortingprinciple permits dissipation of the energy of the inter-ference, and 2) intermodulation distortion is not intro-duced when the shorting gate is not actuated, since onlyits shunt capacity is effective and acts as a linear passiveelement. However, since with available components theshunt gates cannot be effectively switched to zero im-

Fig. 10-(a) Series switching stage schematic. (b) Doubleshielded output transformer.

pedance, several shunt gates are required to produce thegating attenuation of a single series gate in which lowintermodulation distortion can be obtained by negativefeedback. Thus, a series gate is more effective as a finalgate, especially where the number of stages must beheld to a minimum.The final series gate is similar to that shown schemati-

cally in Fig. 10(a). The grid-cathode capacitances of thesections are balanced by the variable capacitors CA andCB cross connected between grids and cathodes of op-posite sections of the bridge circuit. To reduce inter-modulation distortion, the tube sections are connectedas cathode followers with negative feedback from thecathode-load resistors R3 and R4 and the output trans-former. The output transformer is double shielded, as

seen in Fig. 10(b).To reduce to a minimum any residue due to gating,

the tube sections A and B of V8 in Fig. 7 should havenearly identical transfer characteristics. Negative feed-back helps to attain this objective, but tube selection isusually necessary at least for the simple circuit shown.Capacitors CA and CB of Fig. 10 are adjusted for maxi-mum gate attenuation during blanking to prevent inter-ference feedthrough. Signal amplification prior to gatingreduces the relative magnitude of the gating residue,and the over-all gain is reduced to unity by means of theoutput transformer turns ratio. However, such signalamplification is limited by intermodulation distortionoccurring in the preceding amplifier and cathode-fol-lower stages which produce spurious sum and differencefrequencies. With current circuitry, employing specialcathode-coupled, grounded-grid amplifier pairs provid-ing a signal channel gain of about 20 db, the sum anddifference frequency intermodulation distortion can bekept at least 60 db below mixed input carriers of 30 mv.

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1958 89

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90 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS

The successful use of specially designed transformers atthe input and output of the gate helps to simplify thecircuitry and does not introduce intermodulation dis-tortion.Another type of noise or gating intermodulation ap-

pearing at the output of the blanker is that which iscaused by the gating of a large undesired carrier about40 db greater than the amplitude of the desired carrier.All signals must be gated by a wide-band blanker, and,as a result, the chopping of a large undesired carrier cancause noise at the desired frequency, depending on thecarrier amplitude and blanking pulse characteristics.rhe best method of reducing this type of noise appearsto be by providing shorted transmission lines or othernetworks in the signal channel to attenuate the unde-sired carriers. Flight tests have indicated very goodblanker performance in regions (such as Alaska) that arerelatively free of large carriers.A delay network is required in the loop channel to

compensate for the delay purposely introduced in thesense-signal channel of the blanker. This delay can beadded by replacing the loop-amplifier tube with anadapter unit and using a relay to bypass the circuit whenthe blanker is not in use, as shown schematically inFig. 11. A loop channel adapter is pictured at the top ofthis figure.

In applying the blanker to a radio compass, it is im-portant to equalize the phase shifts introduced in thesense channel with an equal phase shift in the loop chan-nel at any frequency in the range of the compass inorder to obtain correct bearings and compass action. Ifthe phase shifts per stage of the signal channel are keptbelow about 20 degrees they can be compensated bymerely adding delay to the loop-channel network de-scribed in the preceding paragraph.Another consideration is whether or not to provide

blanking for the interference induced in the loop an-tenna. In the system described such blanking is not pro-vided, and satisfactory compass bearings have been ob-tained with signals as low as 10 mv on the sense antennain the presence of precipitation static. However, furtherdevelopment requiring good performance with smallersignals may require loop-channel blanking.

CONCLUDING DISCUSSION

The blanking approach to the rejection of precipita-tion static interference has been successfully flight-tested with aircraft receivers. Since low-frequency ADFreceivers have been especially vulnerable to interference,an interference-free, combination blanker-ADF receiveroffers distinct advantages in range and signal availabil-ity in comparison with VHF systems.A simplified eight-tube blanker can be applied to

ADF receivers, maintaining relative loop and sense-sig-nal phase relationships and remaining compatible withthe other ADF operational requirements. A two orthree-tube amplifier coupler is provided at the sense an-tenna to obtain maximum sense-signal sensitivity andto allow for insertion of circuitry which prevents inter-

+ F-Fig. 11-Photograph and circuit diagram of

loop delay amplifier.

ference lengthening and rejects large undesired carriers.A loop-adapter unit also is required to replace the nor-mal loop amplifier tube in the ADF and provide a delaynetwork in the loop channel which compensates for thedelay and amplifier-phase shifts introduced into thesense channel when the blanker circuitry is switched in.

Satisfactory compass operation with antenna sensesignals as low as 10 mv has been obtained without theuse of loop-signal blanking. The intermodulation distor-tion of the system, producing sum and difference fre-quencies, can be kept at least 60 db below two mixed30-mv signals at the sense antenna, even where signal-channel amplifier gains of the order of 20 db are pro-vided ahead of the blanking gate. The discriminatorwill produce gating pulses for all interference pulsesranging from 1 mv to several volts at the antenna.

Strong undesired carriers in the pass band of theblanker can produce spurious intermodulation signals,disable the discriminator, or introduce interference bygating carriers that are much larger than the desiredcarrier. Therefore, some ADF blankers which are to beused in areas where there are large undesired carriersshould employ networks of special limited-shock-excita-tion characteristics to reject undesired frequencies. Spe-cial AGC circuits and improved discriminator circuitryreduce undesired carrier effects in the discriminatorchannel.The blankers can improve receiver performance under

some lightning atmospheric interference conditions, butwith precipitation static it appears quite difficult to ob-tain performance as good as can now be obtained. How-

June

Curtis and Kelly: Improvements in Radar Data Presentation

ever, ADF operation under interference conditions willbe improved sufficiently to warrant immediate applica-tion to the ADF receivers for the major problem of pre-cipitation static, especially in regions where no VHFfacilities are available. The more difficult atmosphericstatic problem is being studied in long range programsat LTRI.The dimensions and weight of the blanker units can,

of course, be further reduced by replacing most of thevarious tube stages with transistors. Vacuum tubes havebeen used in the developmental model because of theirgreater availability and temperature-change reliability.

ACKNOWLEDGMENT

The authors gratefully acknowledge the basic supportof the USAF, the Communications and NavigationLaboratory, Wright Air Development Center, and theNavy Department, Bureau of Aeronautics, in continu-ing researches on improved receiver circuitry for opera-tion in the presence of impulsive interference. Also,major portions of the development in connection withthe ADF application have been supported by researchgrants from Bendix Radio, Collins Radio, and WilcoxElectric, cooperating with them in a joint program.

Improvements in Radar Data Presentation*K. V. CURTISt AND T. J. KELLYt

Summary-This paper describes an application of the storagetube to marine radar, for providing an automatic plot of the equippedvessel as well as all other vessels within the radar's range. It furtherdescribes a new type of presentation which combines relative mo-tion with true motion in a manner easily understood by the marinerand which requires no understanding by him, of new concepts. Inaddition to providing six different types of presentation, the systemfor the first time furnishes a means of directly determining theaspect, true course, and true speed of other vessels without auxiliaryplotting and without calculations.

A DECADE AGO when the war-developed miracledevice, radar, was adapted to the use of the mer-chant marine, the world praised it and pro-

claimed an end to collisions at sea. The record of theintervening years has proved that the praise was over-done and the proclamation premature-radar has noteliminated collisions. Indeed, we find instances where,under certain circumstances, radar may have contrib-uted to development of the collision, if we may be for-given such an observation.Why, with modern radar on one or both of the ships

involved, must we still suffer disasters like the Benevo-lence-Mary Luckenbach, the Duke of York-Haiti Victory,and, most recently, the Andrea Doria-Stockholm? An-swers that deal with radar hypnosis, application of theRules of the Road, or failure to use the equipment mustbe pursued by others; however, as engineers we shouldlike to answer this "why" in terms of the equipment andits design.

* Manuscript received by the PGANE, January 21, 1958. Thispaper was originally presented at the International Meeting on theUse of Radar at Sea, which was held at the St. George Palace, Genoa,Italy, on May 16, 1957. It was later presented at the East CoastConf. on Aeronautical and Navigational Electronics in Baltimore,Md., October 30, 1957.

t Raytheon Manufacturing Co., Waltham, Mass.

Remember that radar is, essentially, radio directionand ranging equipment-equipment whose function isto provide a viewer with information pertinent to thedirections and distances of observed objects. That radaris performing this function is verified by careful evalua-tions of published collision reports. These reports indi-cate that radar is doing its job well enough that suchcollisions should not have happened. But they did hap-pen, and more collisions are occurring with dismayingfrequency and at a tremendous cost in ships and humanlives. What can be done to help the mariner understand,evaluate, and apply the information that is alreadyavailable in his radar so that collisions may be avoided?

Fig. 1 is a representation of what is seen on a marineradar scope, using the 20-mile range, relative bearingpresentation, in open water, with only one object inview. What can we learn from this presentation? Onlytwo things: the observed object is 12 miles away, and itbears 315 degrees. Here it is important that we realizethe fact that a single observation gives but two pieces ofinformation: bearing and range. No assumptions can bemade from a single observation, and no navigational ac-tion should be taken. It is only through the passage oftime making possible a series of observations that furtherinformation, such as direction of movement and relativespeed of the observed object, can be deduced. Since aseries of observations is required, a memory or a record-ing means is therefore implied, if the resulting deduc-tions are to be of any value. But we submit that humanmemory is highly fallible, particularly when it must copewith a series of radar observations. Yet, all too often un-sound deductions concerning the direction and speed ofobserved objects result from dependence on just thishuman memory.

1958 91