US2186247A - Noise suppression circuits - Google Patents

Description

T. P. KINN NOISEv SUPPRESSIGN CIRCUITS Jan. 9, 1940.

Filed Sept.- 16, 193'? l mmi Nm): A \m.

V' Q N kk 9%* MAN INVENTORv WITNEssEsf m M P. e r m ATTORNE Patented Jan. 9i, .1940

. UNITED l STATES NOISE SUPPRESSION. `v'cnwm'rsf Theodore P. Kinn,l Springfield, Mass., assigner. to

Westinghouse Electric & Manufacturing Com.- pany, East Pittsburgh,

Pennsylvania lApplication september 1e,1e3"1,-seria1Noname@ ff,

izclaims.v (dise-20) g ThisI invention relates to radio receiving sys-4 tems, and more particularly to receivers of the type wherein. the` sensitivity' of the amplifier stages isautomatically varied in inverse propor- 5 tion with the strength of the received carrier wave. D f 1 An objectionable Vfeature of radio receivers equipped with automatic volume control resides in the noise appearing in the loud speakerwhen v the set is tuned from one station to another. This noise is especially strong between kstations where no carrier `wave is present, which 'would a'ctuate the automatic volumey control and thereby reduce the sensitivity of the receiver. The interfering noise is attributed to the fact 'that in receivers of the above type the amplifying tubes at no signal inputV are operated to the full extent of their amplifying power, and in this state of maximum sensitivity will amplifymi- Y2() nute electrical disturbances as well as'even slight variations of current appearing within the tube. The latter condition is usually referred tov as lthe shot-effect, and results in a steady highfrequencyhiss in the tube output due to the fact that the electron `emission from the cathode is not perfectly regular but 'spasmodic in charac` ter. A

From the foregoing, it 'is readily seen that upon the absence of the carrier wave to which the'receiver is tuned the automatically readjusted sensitivity of the set results in objectionable'inter-channel or back-ground noises. This v'may beexperiencedvnot only in the tuning of thereceiverbut also when atmospheric conditions cause fading of the signal. l

Jn order to eliminate the above disadvantage, various circuits have been designed in which auxiliary devices performed the silencing function. Some were in the form of manually operated' 40 switches, so-called muteing buttons which were pressed down at the time the tuning dial was moved and thereby short-circuited certain portions of the signal amplifying channel. Others employed auxiliary vacuum tubes per- 45 `forming a trigger function in the audio amplifier circuit. The auxiliary tu'be was subjected to the automatic volume control potential in such manner that upon the absenceofa control potential,

, ,the heavy vcurrent drawn by the tube disbalanced a iixed state of voltage distribution necessary for the operation of the tubes in the audio amplifying system. The latter'type, known as the quelch or QAVC system, while automatic in operation requires' the use of an extra tube in the ,55 receiverwhich contributes nothing to the primary function` of `the v acuum tubes in the-set. Furthermore, it complicates 'thecircuit inasmuch as several components to the receiver proper must be added' in `order yto obtain an operative arrange- The `primaryobject-of this invention is to 'eliminate inter-channel .-or back-,ground noises .in a.

radio receiver, by providing/a circuit arrangement which requires no'additional tubes 1 1o ra1 1 x iliary' devices to perform the stated objective. 1

It is a particular feature of ythis invention. that, the means providedgfor suppressing.inter-channel noise,`is simple in operationand-easily applicable to al1-types of radio receivers having automatic sensitivityy controls. Y.

A further';;advantage `resulting from the use of the proposed meansin accordance with this invention is that existing receivers mayA easily kbe modified to perform in the mannenstated without requiring any majorchanges or adjustments in the receiver circuit.

Additional objects and advantages will be apparent from the'following description of the invention, dened in particular-ity byy the f appended claims land takenin connection with the accompanying drawing, in which the invention is illustrated diagrammatically in the-intermediate frequencyamplifier-portion of a radio receiver.

Referring to the drawing, the single figure showsa portion of a radio, receiver, namely, `the signal-translating jchannel rof the intermediate frequencyamplier in a superheterodyne rtype` receiverthe `,detector stage and ythe rsucceeding` audio Afrequency stage. Therotherparts' of the circuit preceding the intermediate frequency amplier have beenj omittedv for thesake of simplicityf,inasmuchv as the invention herein described may advantageouslybe employed to one high-frequency amplifying stage. It is assumed, ofcourse, vand anybody skilled in the art will ,40 readily recognize, that an` oscillator and first detector stage will precede the arrangement shown in the dravvingqto-complete a superheterodyne receiver, `andan additional stage of audio frequencyamplication may also `be needed ifa 4 5 loudspeaker Mis l.to bel operated. lurtlfierrnore, Ait will be 'clear as thedescription yexplains .the inventionin itstechnical details, that it need vnot be restricted toa-superheterodyne type of receiver` but will operate in any radio frequency 50 st age as long as. there is an (automatic control responding to the strength of the carrierlwave. The circuit shown essentially consists of two high-frequency amplifying stages including4 the vacuuml-,ubes'V `l 4r'and @..adetector stage'.includinie,r

vgrid 6, a screen grid I and suppressor grid 8.

The latter in some tubes is tied to the cathode withiny 4the tube .and .2in others, the connection may beieffected outside of the tubeproper. The input circuit vto each pentode tube comprises a transformer Ivof the double-tuned type, generally employed in intermediate frequency, or commonly referred to as IF stages. In series with each cathode is a resistor II which is so proportioned as to effect thenecessary voltage drop vby the anode current of the-tube for the minimum operating bias required in accordance with the type of tube used. Each resistor II is by-passed by a condenser I2."`Following the input circuit of each'stage, the return of the secondary coil of each transformer I 0 connects through-resistors I3 and l'ltothe load resistance I6 of the detector tube' 3 vand thence to the catho'de "-35 thereof which is .connected to ground. An additional Aresistor I4`for filtering purposes* is included in this circuit and by-pass condensers 23, 25 `and `26 provide low'impedance paths to ground forhigh-frequency currents.

v"The circuit above referredto comprises the automatic'volume control portion of the receiver together withthe detector tube 3 which is the source of automatic volume control, or using its accepted abbreviated form,`A. V. C. voltage. The 'detector tube 3 operates as a diode and recties the 1'high-frequency current in the secondary Winding of the transformer I connected between its anode 36=and cathode 35 in series with the Iload resistance I6 previously referred to. Due to the recticationf of the tube 3, a voltage drop is' effected bythe current/flowing in the resistor ist Yits V'corresponding transformer IE. '22 by-pass the output circuit to ground. The

l'w'vhich canI` be resolved into two components: one the modulated component of the high-frequency current which istransmitted through the coupling condenser' 21 to the grid resistor I1 of the audio amplifier 4'and comprises the audio frequency; and a unidirectional component which at the point where the resistors I5, I6 and the secondary Winding vof the transformer I0 join is negative with respect to the'cathode 3,5 and proportional in magnitude to the high-frequency `voltagesulpplied` by the amplifying tubes to the detector tube `3. "Iherefore, when there isno currentflowin the resistor I6, thegrids 3 are effectively at ground potential and biased only by the minimum bias derivedfrom the resistors Il. "This is the case when there is no signal voltage tothe detector tube. I-Iowever, when the 'detector'tube' is supplied with signal voltage and rectifies the latter, the grids' 6 willbe biased increasingly negative bythe rectifiedcomponent of the signal voltage appearingy across the resistor I6. This is essentially the A, V. C. action and willbe further exemplified in describing the Aoperation of the circuit as a whole.l v

"The output circuit of eachamplier tube includes the anodes 9 and thev primary Winding of Condensers firstY IF amp-liertube plate return is connected 4directly to the power 4supply schematically shown bylter ' condensers 32, 33,-iilter reactor 34 and the voltage divider 3l. The second IF tube plate Yreturn includes a series resistance 20 which may be: short-circuited'by contacts42 and 43 of the switch 4G. The screen grid 'I of the tube I bypassed'by condenserfZI lreturns directly'to the ance shown by an audio transformer 33. 4arrows'of `the secondary winding indicate that tap 45 of the voltage divider 3l, Whereas the screen grid 'I of the tube 2, by-passed by condenser 2I includes a series resistance I9 in the return lead to the tap 45, and may be shortcircuited by contacts 4I and 44 of the switch 40. The two resistors I9 and 2U form an important partin the operation of the-system to silence the receiver upon absence of signal voltage, and more will be said thereof later.

-The rst audio amplifier tube 4 is' connected in a conventional manner, having a bias resistor I8 connecting the cathode 38 to ground and the plate circuit includes a suitable output imped- The the output circuitmay terminate in a` succeeding amplifier or a suitable translating device not shown here. Similarly the arrows afxed to the A. V.,C. potential supply lead, the screen and plate supply conductors, indicate that preceding `portions of` the receiver may be supplied from the same source for all operating potentials. The filaments forheating the cathodes of the various tubes and the source of heater current have been omitted in order to simplify the drawing. It is well known in the art that the heaters may be supplied from any suitable alternating current source, such asI a transformer, for instance.

Before'describing the operation of the receivingy system embodying the improvements in accordance with this invention, a'clearer understanding thereof will be had by dening the term normal order of potential which is to signify here the proportion of the voltages supplied to avacuum tube in order that it may function in the manner intended, for example as an amplier, In other words, by normal order is meant the respective magnitudes of operating voltages' required for a particular tube for its principal electrodes. In a screen grid tube, for example, to operate as an amplifier the normal order of potentials is a higher positive potential lon the anode than on the screen, both potentials being taken with respect to the cathode. Now, let` the vacuum tubev 2 be considered first in connection with its associated components but apart from the rest of the system. The circuit elements are so proportioned that with the minimum" grid bias potential derived from resistor II bythe plate current flow, the operating potentials on the screen 1 and anode 9 will be subciently lowered due to the voltage drop in the series resistors I9 and 20 and in such proportion as to upset the normal order of potential, the screen becoming more positive with respect to the cathode than' the anode. This causes blocking of the tube for signal transmission. Stated in another way, with maximum sensitivity setting of the tube, the plate current causes a voltage drop of such magnitude across the resistor turn, decreases the currents in both resistors I9 and 20. Thus, a negative increase in grid bias of the tube 2 causes the average plate voltage to rise producing thereby an increased mutual conductance with consequent increase of amplification. Considering the rst I. F. amplifier tube, which is conventionally connected and includes no means vaffecting its operating potentials, the

mutual :conductance increases with decrease ef biasv potential and 'at no signal -input to the 'systemi,f as fior instance, when the setv isI tuned between adiacent broadcast channels, the amplification is at maximum. Whereas, in the pres# ence of the lsignal with 'increased grid bias, `its amplification decreases until equilibrium "isreached between signal transfer and control potential. Bea-ring in mind thehdifferential lfunction of the rst and second amplier tubes, it will be seen 'that in the `presenceof a carrier Waveland due to the A. V. C. action, 'both tubes become operative and normal reception Yis obtained. On the other hand, in the absenceof a carrier Wave while the rst amplifier tube isat maximum gain the second amplifier tube is inoperativefand no signal in the form of interfering noises" Aand hisses is transferred pastv the stage preceding the detector.` The circuit constants and the resistance values `for resistors l and Zare so proportioned that' While there is no transfer to the detector tube of the ampliiied electrical disturbances through the tube 2, the signal voltage of a broadcastingqsta# tion considered local to the receiver, having a carrier intensity greatly inexcess of the amplied value of the above disturbances, Awill pass through by capacity effect'and in suflicient'ma'gnitude to be acted upon by the detector. The resultant A. V. C. bias offsets the inoperative state of the tube `2 and normal reception is obtained. f

Y- The simple switch 40' is included Afor the purpose that, if full sensitivity of operationisde-y sired at alltimes as for instance when extremely weak signals are to be tuned in, the resistors I9 and Zbe short circuited. 7

It has also been observed in practice that in some receivers it is advantageous to include the noise suppression feature in more than one Vstage.

From the above description, it* will be 'apparent that the onlychange necessary to a standard receiver embodying automatic volume control-in order to obtain the advantages of the invention, constitutes the mere addition of a resistor in the screen grid and plate lead of the amplifier to the voltage supply source, so proportioned that under conditions whichwould normally produce maximum amplification in this stage, the plate and screen voltages drop to a relatively low value, and with the rplate voltage at a lower value than the screen voltage, the tube becomes inoperative as an amplifier.

Through the addition of these two elements tov an otherwise standard circuit, the objects of this invention have been attained andsuch results obtained which in accordance with present practice necessitates the addition of another electron discharge device to the circuit, together with such circuit elements which are normallyv required to enable an electron discharge device t0 function in its intended manner.

While the invention has been described in i charge device having a cathode and a plurality of principal electrodes, means for applying a normal order of potential to said principal electrodes `principal electrodes causing inoperativeness said device.

withY respect to lsaid'` cathode 'duringr periods of signalfamplication, means responsive to va'ria tiens Ain the intensity of theincoming signal Wave" for'maintaining the volume level of said amplifier substantially constant during amplification of signals, and means for reversing the normal Aorder of ootentialsto certain of said principal electrodes'to rrender said amplifier substantially non-operative during no signal periods.

2;4 Incombination, means for amplifying'highfrequency'energy comprising an electron dise'. charge device having a cathode and a pluralityof principal electrodes, means for applying a f normal vorder of potentials to said principal elec-i' trodes Awith respect to said cathode duringl periods ofV signal amplification, means responsive to 'variations of the incoming carrier wave for varying the amplification of said amplifier vin accordance with said variations in carrier intensity, and means for reversing the normal o`r-y diate electrode `during no signal periods.

4. In combination, anfampliiier including jan' electron discharge device IhavingA a cathode,l an anode andan intermediate electrode,means for applying positive potentials tol said anode and intermediate electrode, the potential on said ancde being of aliigher order than that Qn said intermediate electrode during amplification `periods, means for maintaining the Vvolumey levell amplification of signals,`and means for reducing the voltage oi said anode to a value below that applied to said intermediate electrode during no signal periods.

5.y In combination, an electron discharge device'having a cathode, anr anode and an inter. mediate electrode, means for applying positive potentials to said anode and intermediate electrode, the potential of said anode being of al higher order than that of said intermediate electrode during amplification of signals, a voltage dropping device in circuit with said anode and having a value of impedance sufhcient to drop said anode potential below that of said intermediate electrode potential when said amplier is otherwiser adjusted for maximum amplification.

6. In'combiriation, an electron discharge device having a cathode and principal electrodes, means for applying a normal order of potential to 'said principal electrodes with respect to said cathode essentialr to the operation of said device during periods of said signal amplification, means for applying a'biasing potential tor said amplifier to from a predetermined maximum value and means of saidy amplifier substantially constant during I operative upon loss of said bias for reversing said normal order of application of potentials to said 7. Incombination, an electron discharge device to said anode andy intermediate electrode, the

potential on said anode being of a lower` order than that on said intermediate electrode during the absence of signal energy in the input circuit .5, of said amplifier, and means responsive to the application of signal energy to the input circuit of said amplifier for causing said anode potential to rise in value above that of said intermediate electrode potential.

8. In a signalling system, a signal-translating channel including amplifiers producing an amplied signal output, means responsive to said amplified signal output for producing a unidirectional voltage, means for controlling the ampliiication in certain of said signal ampliers in inverse proportion with said unidirectional voltage, and means for applying said unidirectional voltage to a control element of one of said ampliers to control the responsiveness thereof in direct proportion therewith, whereby said signal amplifier is rendered inoperative 'when the signal input applied to the other of said ampliiiers decreases below a certain value.

9. In a signalling system, a signal transmission channel including an output coupling element, a pentode amplier coupling said output to a rectier producing a rectiiied signal voltage, said pentode ampliiier having in its plate lead a high resistance which reduces its average plate voltage to a point of inoperativeness, means for rendering said pentode amplifier operative upon signal transmission of a predetermined intensity by making its control grid more negative in accordance with said rectiiied voltage and thereby increasing its plate voltage. n

10. In a signalling system, a pentode amplier,

a rectifier for producing a unidirectional voltage dependent on the amplification in said pentode amplifier, a connection from a point in said recti- 40 fier circuit to a control element of said pentode amplifier for automatically adjusting the control grid bias, a high resistance in the plate lead of said pentode amplifier proportioned to cause said amplification to increase in predetermined relation to increase of control grid bias, whereby in response to a Variation of input to said signal ampl-ierl said unidirectional voltage is caused to vary with respect to said control grid bias substantially in accordance with said predetermined relation. i

11. In a signalling system, a signal 'channel producing a signal output, a pentode amplifier coupling said output to a circuit including a rectiier for producing a unidirectional voltage dependent on said output and on the amplication in said pentode amplifier, a connection from a point in said rectifier circuit to a control element of said amplier for controlling said output in response to said unidirectionalpvoltage, a high resistance in the plate lead of said pentode amplifier proportioned to cause said amplii'lcation to increase in predetermined relation to increase of control grid bias, and means for preventing saidl amplifier from responding until the output from said rectifier exceeds a predetermined value, whereby in response to a variation of input to said signal amplifier said unidirectional voltage is caused to vary with respect to said control grid bias substantially in accordance with said predetermined relation.

l2. In a signalling system, a signal amplifier producing an amplified signal output, a pentode amplifier coupling said output to a rectifier producing a rectied signal Voltage, said pentode amplifier having in its plate lead a high resistance which reduces its average plate voltage to a point of inoperativeness, means for reducing the ampliiication in said signal ampliiier in accordance with said rectified voltage and simultaneously rendering said pentode amplifier operative, by making its control grid more negative and thereby increasing its plate voltage, in accordance with said rectied voltage.

THEODORE P. KINN.

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