US2354508A - Noise balancing - Google Patents

Description

July 25, 1944. R. B. DOME 2,354,508

NOI SE BALANCING Filed Oct. 31, 1942 lnvefitori Robert, B. Dpme,-

Hi5 Attlorneg.

Patented July 25, 1944 UNITED STATES PATENT, OFFICE Robert B. Dome, Bridgeport,..C,onn., assignor to General Electric Company; a, corporation of New York Application-ctober 31, 19.4.2, Serial No. 464,029 7 Claims, (01. 250- -2o) My invention relates to signal translatingcirwits and in particular to'a system for eliminating transient noise peaks'from such circuits.

It is an object of 'my invention to-providean improved noise limiting circuit'whichefie'ctively discriminates between desired signals" and; un"- desired noise impulses impressed on the inputof aradio receiving apparatus and which prevents such impulses from appearing in the output circuit of the apparatus.

In the usual type of receiving apparatus for amplitude modulated radio frequency waves it is customary to'employ a 'diode'rectifierin the detecting circuits for reproducing the audio'signalsfrom the modulated waves. It is a further object of my inventiontoprovideanimproved noise limiting system for 'such' circuits-- which is relatively simplein operation, is'inexpensive to construct, and may be easily applied 'to existing receivers.

'Ihe'features of my-invention-which I believe to be novel are set-forth with-particularity in the'appended claims. My invention itself, how'- ever, together with furtherobiects and advantages thereof, may best be understood by reference-to the following description taken in cornnectionwith the 'accompanying draw-ing; in which-thesingle'figure of the-drawing represents a portion-of a receiver circuit embodying my invention. 4 i

The receiver circuits shown in the drawing comprise an input circuit-l0 supplying modulated-high frequency wa-ves to the detector"! v The waves are demodulated andthe modulating signals reproduced in the-load circuit-of the detector H, the audiovoltages developed therein being supplied overthe leads 12- to succeedingstagesof the receiver. The waves impressedacross-the input circuit l0 maybe suppliedfroma'receiving antenna or from an amplifier ofa-radio-receiver. In-the usual -form-cf super-heterodyne receiver, they are intermediate frequency waves supplied from the final intermediate frequency amplifier and are impressed on the-primary winding -l3 ofinput transformer I-4 havirigasecondary winding l5 tuned, by meansof capacitor IG,to-the intermediate frequency. 7 t i The-balanced detector I I may comprise a twin diode type o felectron discharge device havinga plurality of:anodes l1 and I8 and a plurality of cathodes l9 .and 20. It will be realizedof course thatinsteadof havingthetwin diodes in a .single envelope, two separate, diodes or. any other suitable type of rectifying device may beused.

Themathodes l9 and 20-.are'connected together and are supplied with-high frequency. oscillations from .the tuned; secondary circuit of the transformer [.4 by means of'conductor 2|. .The diode comprising the anode ,Il'and the cathode l9 acts as azsignaludetector whosecircuit extends from the-anode ll through a. loadvresistor '22 and the high-frequency by-pass capacitor 23 in'parallel and the-tuned secondary-circuit of the transformer [4 to the cathode l9.

The circuit for the detector comprising the anode l8 and the cathode 20 consists of-two resistors 24 and .25 by-passed respectively by means of capacitors 26 and 21, capacitor 26 being an audio frequency by-passcapacitor and capacitor 21 being a radio frequency-by-pass capacitor. Resistor 24 is relatively large compared to resistor 25; for example, the former maybe 10 megohms and the latter 100,000 ohms. Thus, resistor 24 and capacitor 26 and'resistor -25and capacitor 21 form two impedance networks in the load circuit of the. diode 18, 20, the former having a relatively long time constant and the latter a relatively short. time constant. The

common point of these two networks is directly connected to ground potential. Preferably, the resistor 22 is of substantially the same magnitude as resistor 25 and capacitor ,23' substantially equal to capacitor 21. Since capacitor 21 is directly connected to ground, capacitor 23 may be made slightly greater than-capacitor 21 to compensate for the added capacitance across capac itor 2'! due to the capacity of transformer I4" to ground, this latter capacity actingin shunt with capacitor 21.

The operation of the circuit of the diode detector'l'l, I9 is well known to those'skilled in the art and will not be detailed here. Briefly, this detector is efiective to demodulate the'carrier wave impressed upon the transformer M. De-

modulation products are developed across the Detector I8, 20 operates in a; similar manner to demodulate the carrier diode load resistor 22.

waves impressed upon the transformer. 1.41 Since resistor 24 and capacitor 261 are both large with respect to resistor 25 and capacitor 21, in normal noise-free signal reception, capacitor 26 is maintained chargedatsubstantially-the peak-of the radio frequency signal waves and follows the envelope of the wave.- Both-the audio frequency voltages andthe'unidirectional potentials developed across resistor 25 are negligible, the'unidirectional potential acrossresistor 25 being in'the orderof 1 per cent of the unidirectional potential across resistor 24, .since resistor 24 is approximately: times .aslarge. as :resistor 25.

Since the voltage across resistor 22 is approxisuch modifications as fall within the true spirit mately equal to the sum of the voltages across and scope of my invention. resistors 24 and 25 in series, the audio frequency What I claim as new and desire to secure by voltages across resistor 22 are approximately 100 Letters Patent of'the United States is: times as large as those appearing across resistor 1. The combination in a noise reducing sys- 25. The audio potentials appearing between tem for signal modulated carrier wave receivers, anode H and ground may, therefore, be supplied of a detector for the received carrier wave havto succeeding stages. of the receiver through ing a load impedance upon which the detected couplingcapacitor-Rand leads [2. This poten-' signal and noise voltage appear, a second imtial comprises the sum of the'potentials across pedance in series with said load impedance, resistor 22 and resistor 25 in series, the potential means to reproduce upon said second impedance across resistor 25 being of opposite polarity to said signal and noise voltage in opposed relation that developed across resistor 22 and approxi-lfmqi th ,VOltage n Said load impedance. Said mately one per cent thereof. The audio potential noise voltage being reproduced with substansupplied to the succeeding stages is' thereforeap -f f tially equal intensity on said two impedances proximately 99 per cent of the audiopotential al s l-voltage bemg I D Wlth output of diode l1, [9. This 99 per cent of the substantially smaller intensity on said second usual output potential is scarcely distinguishable imp an than 011 Said load Impedance, and a from the normal potential. a signal responsive means connected across said The direct current component of the demo'dutwo p nces in series.

lated signals appearing across resistorszz and 2 Ina translat ng circu t for a carr er wave in series may be used likewise/for. automatic gain i t e combllrlatlon wllfh a r lm fi control. The potentials betweenwanode I'Iand uponrwhlch the demodulatlon, Voltage of Sald detector circuit in which, .while noise; transients carrier wave and an undesired noise voltage appear, of means for producing in series with said .voltages demodulation and noise voltagesfof opposite polarity, said means comprising a second impedance having a longtime constantand connected in series with said firstimpedance, said opposed noisevoltage being-of an intensity substantially equal to the noise .voltage across said first impedance andsaid opposeddemodulation voltage being substantially'smaller than the demodulation voltage' across said first impedance. 3, In a carrier'wave receiver, the combination of an impedance. upon which the demodulation voltage of. said "carrier wave andv an undesired noise voltage appear, a resistance connected in series with said impedance, and means to repro- 40 duce on said resistance said demodulation and noise voltages in opposed relation to the Voltages ,on said impedance, said opposed noise voltage .being of substantially equal intensity tothe noise voltage on said impedance and said opposeddemodulation voltage being substantially smaller than the demodulation voltage on said impedance. 4. In a translating circuit for signal modulated carrierwaves subjectfto noise impulses, the combination with means for rectifying said waves, of means including a load circuit connected across :said rectifying means for reproducingsaid signals fromsaid waves, and means including a path connected in parallel. with saidfload circuit for eliminating said noise impulse from said translating circuit, said path comprising a pair of impedances having long and shorttime constants respectively, the common point of said impedances bein con.- ne ted t a'p t of fixed potential and said im- =pedance having-a short time constant being con- .nectedin series with said load circuit between said rectifying means and saidpoint. 5. In a noise eliminating system for. highfrequency receivers, the combination of a pair of paths each comprising meansto rectify the re,- ceived highfrequency oscillations and;to;produc e ground aresupplied through a suitablenaudio removing filter, consisting-.ofthe series resistor 25 29 and the shunt capacitor 30, to conductor 3| and applied to the gain control connectionsof preceding stages of the receiver, not shown- The arrangement and operation of the gain control connections will readily beunderstood by those skilledin the art without elaboration. The potentials are applied to control electrodes of one or more of the tubespreceding the detector to vary the receiver gain in accordance with the strength of the received signals, as is well understoodinthe art.' l l If a sharp noise'transient voltage is'impressed across the input terminal Hi, this noise voltage, when" it appears-in the tuned secondary circuit of transformer 14, is rectified by diode I}; I9 and a certain voltage is developed across resistor-22. The noise' voltage is'likewiserectified by diode 18,20. Capacitor 26', being large, 'actsas-a shortcircuit for such a transient. Consequently, a noise voltage is developed'across resistor25which 45 is of substantially the" same "magnitude as the voltagedeveloped across resistor 22. These voltages across resistors 22"and-25 areconnected in series and appear withopposite polarityin that series connection: 'Asafresult, they effectively cancel each other so thatsubst'antially zero noise appears across the audio output ,terminals I2.

It is thusseen that my invention provides a are effectively balanced out, substantially. normal audio potentials'are supplied for the succeeding stages of the receiver. I

Merely for the purpo s e'sof illustration,'the'fol "lowing data is given for a'particular radio'receiving apparatus embodying my invention. These values were found to give satisfactory results in a"particular case'although they are not to be regarded as necessarily applicable to all endbodiments'of my invention. 1

Resistors 22, 25 ohms 100,000 unidirectional signal voltages varying with varia- Resistor 24 megohms l0 tions in intensity ofthe received oscillations, at capacijiors 23, 27 signal circuit connec'tedacros on of mp t Capacitor 2 anda portion of the other of saidpathsin series, Tube H YD H saidzone of said .paths' and saidportioniofzsaid while I have shown a particula embodiment other path having substantially equalimpedances 'of myinvention, it will of course be understood therein, a d al impedance having? a relatively that I do not wish to be limited thereto since long time c s a t 'the remaind'efofs'aid other various modifications may be made; and I-con- -path. a

templateby. the a pendedi claims to cover "any 575*- 6. In' a translating circuit for signalvoltag'es may equal to po l ar -thevnoise transie r n e er, the cpmh nation with a 1 10 ne cted to saidsource, a'recti fier' subject to undesired noise transients, the combination of a pair of parallel paths, one of said paths and a portion of the other of said'pathsvIoeing connected jn series 101' the transletion of said signals, a d zngns in t ne remain er of said "d 0156 trgnsients t0 otherpath for can'si ian ari s 'ppriti 1 v withamegn tudesubsta i e-( inter onetpa h source. of s ig no'lsn rodolaited gcgrrierjwayes ubjcot to noise impulses. ofla-pairaof pargllel mm on- 'inuee ngor sfaia path th fl said path and po t nfl fth second of said paths being connected in series between said source andua point of fixed potential iorthe translation of signal voltages, impedances .of substantially eqnal'msgnitude and. sign con.- nected respectively in said first path and said portion of said second path. and along time conjjstgnt network connected in the remainder of said second path, said network presenting a, path of low imp'edgnce to-noise impulses ,appearing in said receiver and said noise impulses producing voltages pfsgibstentislly equal magnitudes and oppo- 'site across said equal impedances in said series circuit f ROBERT B. DOME.

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