US2401214A - Receiver circuits - Google Patents

Description

May 28,. 1943; J- A. wo cEsTER, JR 1 RECEIVER CIRCUITS Filed Jan. :50, 1945 Inventor-v:

I Joseph A. WorcesteT-Jfi,- I

- His At-tO iTTe circuit of Fig. 1.

Patented May 28, 1946 UNITED STATES RECEIVER cmcorrs Joseph A. Worcester, Jr., Fairfield, Conn., assignor to General Electric Company,.a; corporation of New York Application January 30, 1943, Serial No. 474,086

8 Claims.

My invention relates to radio receiver circuits and more particularly to the elimination of noise encountered in tuning from one station to another.

It is an object of my invention to provide an improved receiver circuit arrangement which eliminates noise when tuning between stations.

More specifically, it is an object of my invention to provide an improved receiver circuit in which a, pair of electron discharge devices operating in cascade are employed, in the absence of incoming signals in the receiver circuits, to function as a multivibrator and, in alternation, block translation of noise to the receiver output.

In frequency modulation receivers it is customary, in order to obtain the full benefits of the noise reducing properties'of frequency modulation, to employ an electron discharge device asa limiter to remove amplitude modulation components from the modulated high frequency carrier waves, the amplitude at which limiting occurs being determined by the amplitude of the weakest signal which it is desired to reproduce.

Particularly effective limiting has been obtained by the use of a plurality of electron discharge devices in cascade, means being provided to pre ventregeneration between the devices of the differentstages of the limiter. In the absence of a carrier wave, however, noise impulses having a minimum R. M. S. amplitude of the aforesaid weakest signal appear in the receiver output unless special means is provided to eliminate noise pulses during such a period.

It is an object of my invention to provide an improved limiter circuit in which regeneration between stages is introduced to eliminate noise pulses in the absence of a high frequency wave.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however,

together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. l represents'a portion of a receiver circuit embodying my invention and Fig. 2 is a modification of the As shown in Fig. 1, high frequency carrier waves are supplied to the primary winding of an input transformer ll. These wavesmay be supplied directly from a receiving antenna or a radio frequency amplifier. However, in the usual form of superheterodyne receiving apparatus, they will be intermediatefrequency carrier waves andin a frequency modulation receiver are supplied from the final intermediate frequency amplifier. j

The high frequency carrier waves are subjected to amplitude limitation through limiter circuits l2 and l3. v They are then supplied to the input winding of a transformer [4 which may be part of a discriminator or detector circuit, not shown. Subsequent to demodulation in the discriminator circuit, the low audio frequency signals may be amplified'through further stages of amplificationand ultimately supplied to a sound reproducen.

'Connected across the primary winding I!) is a capacitor 15 fortuning the primary transformer circuit to the "intermediate frequency. The secondarywinding [6 of transformer II is similarly tuned by means of the capacitor I7 and the high frequency voltages in this tuned circuit are supplied to the control electrode l8 of electron dis- 1 charge device Hi. The device I9 is shown to be of the pentodej type haVing in addition to the anode20 and the cathode 2|, screen electrode 22 connected by means of conductor 23 to any suitable source of operating potential, anda third control electrode 24 which may be directly connected to the cathode 2| to act as the usual suppressor grid. The anode 20 of device 19 is connected through a load resistor .25 to a suitable source of potential. The cathode 2| is directly connected to groundand the input circuit of limiter I9 is provided with a grid resistor 26 having a by-pass capacitor 21 connected thereacross. I

The anode circuit of device I 9 is coupled by means of capacitor 28 to control electrode 29 of electron discharge device 30 ,in the second stage limiter circuit l3. The device 30 is likewise of the pentode type having the anode 3!, the oathode 32, and, in addition to the control electrode 29,-;screen electrode 33 and a suppressor grid 34,

"the lattrbeing directlyconnected to the cathode 32 andelectrode 33 being connected by means of the conductor 35 to a suitable source of potential, not shown. The cathode 32 is directly connected toground and grid resistor 36 is connected between the control electrode 29 and ground; The output circuit for anode currents of device 30 comprises, in addition to the primary winding [4 of a discriminator transformer, the capacitor 31 fortuning the output circuit to the intermediate frequency andresisto'r 38 is connected between the tuned output circuit and any suitable source of anode potential to act as a decoupling resistor, as wellasfor certain other purposes which arepOinlEd out in detail later,

The capacitor 39, connected across resistor 38, acts as the anode by-pass capacitor and is of a value such that the resistor 38 and capacitor 39 function as a low pass filter for removing currents having a frequency of the order of the intermediate frequency from the potential source.

The portion of the receiver circuit thus far described nomprisesrthe conventional type of cascade limiter circuitemployed in a frequency modulation receiver, the two limiter devices l9 and as utilizing resistance-capacity coupling between the stages l2 and I3.

Cathode 21 is directly connected to ground and signals supplied to grid l8; cause grid; current-to flow and charge capacitor. 2l,;sthus'.providing. a

negative bias for device I9. During negative half cycles of the incoming signals, thelchargeson capacitor 21 leaks off, the rate of leakage being determined by the value of resistor 26. During the period of .positive half cycles that grid currentflows, capacitor 21 charges, rapidly,ijthe charging rate being determinedby the relatively lowjgrid resistance. of device). .Actually, be-' grid current flows .When the signal at grid 18 increases, grid I8 is driven more positive and the plate current of tube l9.tends to increase. However,,the pulse of plate current also becomes-slimmerpwith respect to time, .since asmaller perdescribed, it is well known that noise impulses pass through the limiter circuits and appear in the receiver output unless special means is provided to eliminate noise impulses during such periods. In accordance with my invention, the passing of noise impulses through the limiter circuit in the absence of a received high frequency wave is prevented by causing the: limiter circuits l2. and .131. to regenerate afterthe fashion of a multivibrator and to set up oscillations having a gfrequency outside the limits of the range of audio frequencies and having an amplitude large enough to cause a maximum or saturation current -to=fiow in-thedevices I9 and 3t and eliminate centage of the total positivegrid. cycle produces plate current. This tends to reduce the average plate l current.

'Since it is .desired'to-removesharp impulses from the output of the receiventhe-time constant of resistor 26. and v.capacitorllis made as small as possible. Whilev this permits device l9 toamplify signals a slight amount signals supplied by the output of tube. l9 to the input of tube as are restricted to a limited range of amplitudes.

Since noise impulses are reduced v substantially b the first limiter tube, the selection of the time constant of the network comprising capacitor 28 and the resistor.36 in. the circuit of-v the control electrode 29 of device .30 isylesscriticaland is' chosen with the view of further limiting the-restricted input signal amplitudes so that the output currents of the second limiter have a substantially constant value. of this type therefore, the first-limitingdevice-is effective-partially to limit the amplitude of-signals supplied to the second device and to remove a considerable amount of impulse noise from such signals. The'second limiting device is effective to provide a substantiallyvconstant output over the very restricted range of signal :inputamplitude supplied'by the first limiter. Since in this portion of the receiver circuit it is usually not necessary to provide 1 appreciable additional gain for thesignals being translated, it is customary to use resistance coupling between the limiter stages .l2 .and 13 both for simp1icityef-coupling and to prevent .unwantedregeneration.

In the. absence, of, a carrier wavexin frequency modulated receiver circuits of the type thus far Ina cascade limiter rnoise pulsesfrom the output circuit of the device .In. order. to provide a circuit which oscillates in the absence of high frequency carrier waves,

v:the. output circuit of the device 30 is coupled to the tuned input circuit of the device [9 by means of'jthe capacitori'fie and the resistor' ll so. that these devices operate as amultivibrator. Oscillations in the multivibratorcircuitthus established are started by a positive noise voltage in the input circuit or ayvoltage irregularity at the grid of one of, the tubes, grid "18' for example. This voltage isamplified by the two devices and then reapplied togrid lB in'the correct'phase and as amore positive potential to be again amplified. "The more positive potential 'of electrode fit is effective. to produce a large anode current flow ,in 1 device; l9 and through load resistor 25, great1yreducing'the potential ofv anodef2B and carrying the potential of control electrode 29 beyond the cutoff.point'of"olevice3l'l; since the charge on capacitoriia cannot change instantaneously. "Cessation 'of .currentflow in device 30 causes a sudden increase in the potential. of anodej3l and;gthroughtheaction of capacitorcfl, of ,controlelectrode '18, the increased potential of the" latter being .effectiveto increase'the current'fiow in device l9 to its maximum; or saturation, value for the screen grid and anode potentials employed. "The charge on capacitor 28 leaks I off through resistors. 25 and. 36at an exponential rate, returning thepotential of grid"29 toward its normal operating value. .At the, same time capacitor fill is charged atan. exponential rate through the series circuit comprising resistors A,i,;25;' the potentialsource and resistor38 and reduces .the positive, potential of grid 18. "When the potential of grid29jire'aches thepointthat amplification is again possible, .there is. repetition of the amplification action this time .in the opposite direction, which results. in. a rapid. rise inrpotential of -grid .29 and. a. rapid; decrease in potential ofgrid l8. (InthisJmanHBr, the ,operation of .thelmultiyib'rator is reversedtobias grid .18 beyond cutoffand reduce'the current flow in 1 device .I 9 to zero.

IThe frequencypf the oscillations, or reversals of current fiow,-in the-.multivibrator circuitis determinedchiefly by thetime constants of the resistors and capacitors used.in..the grid circuits of Ithetwomlevices I9.and. 3U, that is, the time constant of resistor 25 and capacitorl lll and of resistor 36 and capacitor 28. Preferably, these valuesare ,adjustedso that, the .multivibrator. oscil- Iatesata frequency intermediate the. frequencies ,oftheaudio signals andthecarrier wave, for example about 30 kilocycles.

. In the. absenceof vhigh'frequency carrier .waves 7 in the..primary circuit-of. ,the';transformer l I, in

7 accordancewith my invention, therefore, the.mul-

tivibrator circuit operates to provide voltages having a frequency above audibility and below the frequency of the tuned input and output circuits of the devices I 9 and 30. This multivibrator voltage has sufficient strength to produce, in alternation, saturation current flow in one of the devices i9 and 30 at the anode and screen electrode voltages at which they are operating and to block current flow in the other of the devices. Since one or the other of devices 19 and 30 is thus nonconducting at all times, any noise currents in the preceding circuits of the receiver are prevented from reaching the output circuits of the receiver.

When a carrier wave from the preceding portion of the receiver is again supplied to the tuned input circuit of the device I9; it is effective to produce a negative bias across resistor 26 and capacitor 21., decreasing the transconductance and reducing the gain of device l9 to the point that oscillation in the multivibrator circuit ceases. For alternating currents a portion of the output circuit of device 30 comprises two parallel paths to ground; onecomprising the capacitor 39, and the other comprising capacitor 40, resistor 4 I, and resistor 26 in parallel with capacitor 21. The elements 26 and 21 are also present in the input circuit of the device 19 and voltages developed thereacross by the alternating currents in the output circuit of device 30 are of such polarity and phase that they comprise a source of positive, or regenerative, feedback to said input circuit. Operation of the multivibrator ceases when the negative bias voltages developed across resistor 26 by signal currents in the limiter l2 are of a magnitude sufiicient to overcome the positive peaks of the feedback voltage thereacross.

I Since resistance 4i does not affect the normal operation of the cascade limiter circuit, the intensity of the signal modulated carrier wave which is required to stop operation of the multivibrator may be determined by selection of the value of this resistance. Capacitor 3s likewise does not affeet the normal action of the limiter circuit so the cutoff point of the multivibrator may be determined also by the value chosen for this element. Adjustment of either resistance 4! or capacitor 30 controls the division of alternating currents in the above-described parallel paths of the output circuit of device 30. Increase of resistance 4| is effective to decrease the amount of output current of device 30 which flows through the path common to the input circuit of device l9. Hence, when either the resistance 4| or capacitance 39 is increased, multivibrator action is stopped by a weaker signal supplied to limiter 12.

In Fig. 2 is shown a modification of the limiter circuit shown in Fig. 1. In the two figures corresponding reference numerals have been placed .on corresponding elements to facilitate a comparison.

In the circuit of Fig. 2 the grid resistor 26 for device I!) is shownv as directly connected between the control electrode I8 and ground. Likewise, the control circuits for screen electrodes 22 and 33 are shown in detail. These circuits include capacitors 50 short-circuiting these electrodes to the cathodes of the devices for currents of the frequencies to be amplified and the voltage divider comprising resistors and 52 connected across the source of operating potential shown as the battery 53, the common point of resistances 5| and 52 being connected to the screen electrode;

vider circuit between the control electrode l8 and the cathode 2|. The capacitor 56, connected across the resistor'55, forms therewith a path of sufilciently long time constant that any hiss or transient currents introduced into the limiter circuit by the currents of the multivibrator are bypassed to ground preventing their reproduction. in the output circuits of the receiver.

In order to provide means for adjusting the multivibrator circuit for operation at a desired frequency, in the modification shown in Fig. 2, resistor 4l is made a variable resistor and switch 51 is provided in the feedback circuit'between the two electron discharge devices. For reason explained above in connection with the description of the operation of the circuit of Fig. 1, by varying the positive feedback between the output circuit of device 30 and the input circuit of device l9 by adjustment of the resistor 4|, the noise suppression circuit may be adjusted to operate at a desired level to permit reception of signals of a minimum intensity. Also, the switch 51 is provided to facilitate, in the absence of a carrier wave in the input circuit of the device 19, comparison of noise in the receiver during periods when the multivibrator is operating and periods when its eff ect is removed from the receiver circuits.

A thus described, my invention provides, in a translating circuit for carrier waves which are' modulated by desired signals and subject to undesired noise transients, an inter-channel noise suppression circuit which combines, with a pair of amplifying devices connected in cascade, a feedback circuit forproducing in the cascade circuit, in the absence of received carrier waves, an oscillation having a frequency intermediate the frequencies of the carrier waves and signals by which they are modulated and which, by alternately stopping current flow in one of the devices while causing saturation current to iiow in the other of the devices, prevents translation of noise to the output circuits of the receiver.

Merely for the purposes of illustrationfthe following data is given for a radio receiving-appae ratus 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 embodiments of my invention.

Resistors 38, 54 ohms 22,000 Resistors 26, 55 do 47,000 Resistor 4| do 100,000 Resistor 25 do 10,000 Resistor 36 do 180,000 Capacitor 40 mmf 470 Capacitor 56 do 220 Capacitors 21, as do 47 Capacitor 28 do 22 Tubes l9 and 33 Type 6SJ7 It is found that in the absence of signal, the two devices 19 and 30 act as a multivibrator oscilla tor having afrequency around 30 kilocycles. It is also found that when resistor 4| is adjusted to a value of 50,000 ohms, a 3 to 1 noise reduction is obtained; whereas when this value is adjusted to 10,000 ohms, a 20 to 1 noise reduction is obtained. The action is such that if a higher level of noise suppression is used, stations above the In the form of the multivibrator circuit shown in Fig, 2, a network comprising resistors 54, 5'5 and capacitor 56 is included in the multivibrator circuit, resistors 54 and 55 forming a voltage dinoise level are also suppressed which otherwise could be received at the expense of tuning through objectionable off-station noise. In other words, the level is set where the weakest desired stations are received satisfactoril and the noise between stations is determined by the setting of resistor. M. or the :value of. capacitance .35.: Ina receiver which develops .noisepeaks equivalent to a .4. .microvclt signal on :the antenna, .a signal or '7 microvolts is suflicient to completelystopthe multivibrator and provide...essentia-lly noiseless reception.

It will of course :be apparenttothose skilled in the art that many-modifications withinthescope of my invention may be made. .For example. ,.instead of varying resistance dL'the signallevel at which multivibrator operation startsmay be ad-. justed by varying capacitor ddxorany other element whosesvalue is not. determinedby other considerations, such-as the operation of the. cascade. limiter. Also, althoughtheinvention has beendescribed as employed in conjunction with a. cascade limiter circuit, it is not tobe construed as beinglimited thereto since it may be used with any two amplifiers connected in cascade, either in the radio frequency or-the intermediate frequency stageatheamplifiers being so connected. that in the absence of a signal-they operate as a multivibrator and,--since 'for sub-stantially thecom-plete time of such operationat least one of thetubes-is biased to-cutoff, noises in-the preceding portionof the receiver circuits are not translated through this multivibrator circuit to subsequent stages of the receiver. In such a case, instead of developing signal voltages across resistor 28 to stop operation of the multivibrator circuit, conventional automatic volume control voltages may be employed for this purpose.

Hence, while I have shown particular embodiments of my invention, it will be understood that I do not-wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Th combination, in a noise suppression device for a signal channel, ofan electron discharge device connected in said channelto translate signal transmitted through said channel, and means effective when signals in'said channel are below a predetermined amplitude to render said device alternately saturated and non-conducting at a super audible rate thereby to prevent'translation by said device of noise current ,in-said channel,

2. The combination, in a noise suppression device for a signal channel, of an electron discharge device connected in said channel to translate signals transmitted through said channel, and means effective when signals in said channel are below a predetermined amplitude torendersaid device alternately saturated and non-conducting at a super audible rate thereby to prevent translation by said device of noise current in said channel, and means responsive to desired signals in-saidchannel to interrupt operation of said last means.

3. The combination, in a noise suppression device for a signal channel, of an electron discharge device having an anode, a cathode, and a control electrode, means to supply signals. in said channel between said control electrode and cathode and to. supply signals. reproduced between said anode andcathodeto said channel, a second electron discharge device having an anode, a cathode, and a control. electrode, a multivibrator circuit including said two devices whereby oscillations of multivibrator typeare produced, and said first device is preventedhthereby from translating noise currents appearing: in said channel, and means responsive to desired signals to interrupt operation ;of saidv multivibrator whereby said first device translates'said desired signals.

4. In combination witha source of signal modulated carrier waves subject to undesired noise impulses, .amamplifier comprising a pair of electron discharge devices connected in cascade, input.-and..output..circuits connected to said amplifier,-;said: input circuit being. connected .to saidsource,.and meansfor; supplying a'positive feedback voltage from said output circuit to said input circuit, said feedback means being effective inthe absence of a wave to be. amplified in said input circuit to cause said devices to regenerate and-produce therein oscillations of a frequency intermediate audibility and the frequency of said wave whereby, in alternation, one ofsaidzdevices is. biased to cutoff and translation of noise .impulsesto said output circuit is prevented.

5. In combinationwitha sourceofsignalmodulated carrier waves subject .torundesired noise impulses, anamplifier comprising a pair of electron discharge devices connected in cascade, input'and output'circuits connected to said amplifier; said inputcircuit beingconnected to said source, means for supplying a positive feedback voltage fronrsaid output circuit-to. said input circuit, said feedback means. being efiective in the absence of a wave to be amplified in said input circuit to cause said. devices to regenerate and produce therein oscillations of a frequency intermediate audibility and :the frequency of said wave whereby, in alternation; one of said devices is biased to cutoif and vtranslation of noise impulses to said outputcircuitis prevented, and means for preventing said amplifier from producing said oscillations when a signal is being amplified thereby.

6.. Anoise suppression circuit for a signal translatingchannel comprisinga pair of electron discharge devices connected in cascade in said channel, input and. outputelectrodes-connected to said devices, positive feedback means connected to supply voltages .from said output electrode to saidinput electrode, said feedback means being efiective tocause said devices to regenerate and produce therein oscillations of a frequency intermediate audibility and the frequency of saidsignals, whereby, in alternation, one of said devices is biased tocutofhand translation of noise by said channel is.prevented,-and=means for interrupting said=oscillations whena signal-is being translated by. said channel, said-means 'comprising. a. resistance connected between said input electrode and a point 'of fixed potential and arranged to have developed thereacross signal voltages having a polarity opposite to the voltage supplied to said input electrode by said feedback means.

7..In a receiver for radio signals the combination of an amplifier having an input circuit and anoutput circuit, means supplying a feedback voltage from said output circuit to said input circuit, said feedback means being effective when signals to be amplified in said input circuit; are below a predetermined mplitude to cause said amplifier to regenerate. and produce oscillations of a frequency intermediate audibility and the'frequencyof said signals, whereby noise currents are eliminated when said signal currents are below said predetermined level, and means in said input. circuit for eliminating any audio frequency currents produced in said circuits by said oscillations.

8. The combination in a radio receiver for frequency modulated waves, of means for limiting the intensity of said waves to a predetermined value to remove noise currents therefrom, said means comprising an electron discharge amplifier having an input circuit and an output circuit, means for impressing a wave to be amplified on said input circuit, and feedback means connected between said circuits and effective when said Waves in said input circuit are below a predetermined amplitude for causing saidamplifier to regenerate at a super audible rate and produce oscillations having an intensity of at least said value to remove noise currents from said output circuit.

JOSEPH A. WORCESTER, JR.

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