ADF Interference Blanker Development

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  • 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 RECEIVERSA blanker unit designed for commercial ADF use and

    mounted 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 be

    expected 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 anaperiodically 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 ADF

    antenna from canopy surface charging effects. (a) Slow sweepspeed display of artificially generated pulses on the canopy.(b) Interference pulse waveform on antenna with 10-megohmprobe resistance to ground. (c) Interference pulse waveform onantenna 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 senseantenna 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 antennacircuit. 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 tofeed 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 blankersystem, since here the discriminator circuit operates topreserve the noise while rejecting the signals-it is sidechannel used to reliably trigger gating pulses whereveran 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--1 TPE Q FERRAMICM 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 afrequency 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, asseen 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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  • 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 cha...

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