Патент USA US2404338код для вставки
_July 16, ¿1946y J. A. WORCESTER, JR NOISE SUPPRESSION CIRCUIT Filed Aug. 20, 1941 cN 2,404,338 v I 2,404,33‘8 Patented July 16, 1946 PATENT OFFICE , UNITED STATES >‘2,404,3:as v f l Noise sUPPREssIoN olaoUrrs Joseph A. *Worcesten Jr., Bridgeport,z Conn., as signor to General Electric Company, a corpora tion of New York Application August 20, 1941, Serial No. 407,543 23 Claims. (C1. Z50-20) ’ 1 ’ My invention relates to noise suppression cir cuits and it is particularly applicable to noise sup pression circuits for automatically rendering the output circuits of a frequency modulation receiver some form of noise suppression circuits for Velim inating _these background" noises in a frequency modulation receiver in ‘order that the full ad vantages of “static-less” operation may be real inoperative when no signals are being received or 5 ized.` Accordingly, it is an important object of ' my invention to provide an improved background when the received signals are too weak to pro noise suppression circuitwhich is particularly vide satisfactory operation. adapted to the requirements of a frequency mod It is well known that objectionable noises may ulation receiver. ' . be emitted by the signal reproducer of a radio receiver during those intervals when no signals 1Q ’ In a preferred embodiment of my invention, ' the noise voltages appearing- as amplitude modu are being received, or while the receiver is be ing tuned between carrier channels. rI‘hese noises lationv in the limiter output under no-signal or weak-signal conditions, as explained above, are may arise from a great many causes including cross-modulation effects, natural atmospheric detected and utilized to provide bias potentials for - cetera. Therefore, various squelch circuits have the limiter'is completely blocked, and the sound reproducer effectively muted.Y However, as soon static, locally produced high-frequency electrical 15 reducing the transfer eiliciency of a portion of the disturbances, “shot" effects and thermal'agitation ` transmissionchannel following the limiter. Pref erably, one of the'amplifying "stages following in the tubes and circuits of the receiver itselfget heretofore been proposed for muting the sound re- - as a signal‘of' sufficient strengthfor satisfactory producer except when the receiver is tuned to a signal of sufficient strength to over-ride the operation is received (and such signal need be only a relatively ‘weak signal just suflicient to saturate the limiter), the "squelch -voltage au tomatically collapses and the receiver operates noise level and provide satisfactory reproduction. The operation of such circuits has variously been designated by such terms as “background noise suppression,” “inter-channel noise suppression” and “carrier-olf noise suppression.” A properly designed frequency modulationv re ceiver inherently provides a high signal to noise 2.5 at maximum sensitivity. ' " It is accordingly another'object of my inven tion'to provide an improved'squelch or muting circuit, particularly adapted to the requirements of a frequency modulation receiver, whose >opera ratio so long as the received signals exceed a pre determined level. This is due in large measure to 30, tion is automatically determined by the character the action of the amplitude nmiung circuits ` ’ and magnitude of the voltages impressed there which transmit the desired frequency modulation It is also a specific object of my invention to but discriminate against amplitude modulation. In order to realize the full possibilities of fre quency modulation reception, it is common design provide an improved carrier-off noise suppression circuit for frequency modulation receivers which practice to provide such a high degree of amplifi is very sensitive and has a trigger action. " Still another object of my invention is to pro cation in the high-frequency stages of the receiver vide an improved automatic background noise that such effects as thermal agitation in the first suppression circuit which is economical and read tuned circuit and the “shot” effect in the first tube cause voltages to be impressed on the am 40 ily adapted to existing types‘of frequency modu plitude limiter approaching values required to ' ’ saturate it. So long as any signal strong enough to operate the limiter is received, these voltages, as well as other undesired static and noise volt ages and undesired amplitude modulation present in the signal, are reduced to a very low level at the limiter output and do not cause objectionable disturbances inthe sound rep-reducer. ' However, in the absence ofsuch signals it has been found that very substantial noise voltages appear in _the : limiter output and that, due tothe extremely high sensitivity of the receiver and the frequencychar acteristics of the receiver circuits, the audible background noise may be very severe. ’ A ’ . lation receivers. ' The 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 further objects and advan tages thereof, may best be understood .by refer ence vto the following description taken in con n'ection with the accompanying drawing, in which Fig. 1 isla schematic representation, partly in the form of a conventionalized one-line diagram, of a frequency modulation receiver embodying my invention; and Fig. 2 schematically represents modified forms of circuitsv embodying my inven tion which may be substituted for the correspond It is therefore highly desirable to incorporate `5&5 >ing portions of Fig. 1 within thedashed rectangle, 3 4 Corresponding reference numerals have 'been e. g., audio signals. They are demodulated in a’ slope filter.l or discriminator I1. Various forms of apparatus suitable for this purpose are well known to the art. One type of frequency modu lation detector circuit which I have found'to be placed upon corresponding elements in the two figures to facilitate their comparison and simplify the accompanying description. Before proceeding further with the description of the illustrated embodiments of my invention, I wish to explain the meaning of certain terms as satisfactory is described in Patent No. 2,121,103 Seeley,` granted June 21,1938,- to WhichV reference used hereinafter' in the specification'- and »ap pended claims. ` maybe `made for further information. » L vThe demodulated audio signals are supplied to ` yaì-ñrst audio frequency amplifier I8, further am pliñed in an audio power amplifier I9, and then I intend such terms as “noise” or “noise Volt ages,” unless otherwise'qualified, to include al1v undesired voltages which may accompanyfthe desired signals or be modulated upon them. `For*l example, these voltages maybe caused by‘cross-" *I suppliedtoV a suitable signal translating device, _ h. Ísuoli as the loudspeaker 2U. Y .The 'limiter |16 comprises a pentode amplifier modulation effects, adjacent channel interference 15 30, havingan input circuit including a coupling or non-linearities in the transmission channel; capacitor SI and grid resistor 32 connected be they may be caused by thermal agitation effects, tween its control grid 33 and cathode 34, and an “sho ” effects, and related phenomena which output circuit including a tuned circuit 35 con occur in the receiver itself and give rise to dis nected to its anode 36. Anode operating poten turbances of the soecalled' “continuous” or “hash” 20 tials are supplied in conventional manner from a ‘suitable source of power, which is not shown but ’ typegíor they'may beA caused by natural atmos merely indicated by the connection designated as pher'icF static, Nignition' interference, andY 'the ` like,` which' occur outside" the receiver and give rise'to “To-PB” on the drawing,> through »the usual de disturbances of the solcalled “impulse” type. i coupling resistor 33. Í‘Á‘By‘t'he term “amplitude modulation”- I intend' to 'i'r'iclucle> all amplitude variations in the envelope of highl'frequency voltages appearing at speciñed points 'in the"transmission channel, irrespective The screen grid 39~ is' simi larly supplied with operating potentials from the common power supply source through vadecou pling resistor 4U. of the precise character of these voltages.' Thus, for ~tl'ie'purposes of my'invention these voltages " ~ v ' ' tailed' explanation. ” The limiter is` self-biased' by means of the capacitor 3I and the grid resistor 32. It is so adjusted ’as to pass anode current- may constitute periodic signal voltages, either unm'odulated or modulated in frequency or phase Y by desired signals, 'or irregular voltages such as are -characteristic of any of the types of noise.v ' By the'term “frequency modulation” I mean to v The operation of the limiter I6 will be readily understood by those skilled in the art without de only between the >limits at which positive grid' _ include' any form "of modulation produced by current flows and at which anode current cutoff takesplace. ' In the'form of' 'v my «invention - ' v ï illustrated l l in Fig. Varying the frequency or phase of a carrier wave 1, the anode 3'6‘ of- the limited Sil-is' also‘coupled as 'distinguishedV from' modulation produced _by VThe generic term “angle modulated” has been to an amplitude detection circuit over a conductor 50.' The- circuit comprises a diode detector 5I’ ' shunt‘ed by a 'load`Y resistor 52'.>> The intermediate heretofóre employed in the art in connection with frequency potentials occurring at theanode 36 waves'of- this character 'to indicate either'frequen areimpressed on'the' diode 5I through Ythe con ductor 5u anda capacitor A53. This detection varying the amplitude of such a wave. ' cy modulatedV or >phase modulated waves or waves which 'are hybrids of the two; In the specific example described in the following discussion of circuit functions in a manner shortlyV to be de' scribed ingreater detail; '» Brieiiy, it develops rpo tentials across the load-resistor 52 in accordance with amplitude modulation components appear the.y drawing, the" received signal energy is Vas sumed to befrequen'cy modulated wave energy.' " 'In' they diagrammatic representation of a fre quency modulation receiver Vshown in Fig. 1`, many l. of the elements- maybe conventional and'their details of constr'uction'are not material to my in vention'.,` Therefore, to simplify the drawing, such elements 'have'merely been indicated in block form. Thus the receiver’ illustrated is'for the ing inthe output- of the limiter I6; ' ' Potentials developed- acrossv the diode '5IH are impressedy upon a' second.- diode detector 54' through a- capacitor 55, which removes direct cur rent, and through a'low-pass filter, comprisingY a resistor> 56 and> a capacitor- 5?-, which removes in’ termediate frequencies. mostparta conventional superheterodyne re- ' ~ > ' ' . - The load for the diode detector 54 comprises a ceiver.> Signals received 'at 'an antenna, Ii] are resistory 5B. amplified' in the -usual radio frequency amplifiers I I vand combined in a'mixer. I2 with locally gener ated waves supplied from an oscillator I3. The carrier waves of fixed intermediate frequency are then amplified in intermediate frequency ampli; ners I4. ‘In frequency modulation receiversof usual design it' is often the practice to pass the intermediate frequency waves through two ampli across this resistor are lfurther filtered by' a low pass filter; comprising- a- resistor' 59j and a ca-1 tude limiters in cascade in order to secure a bet bodimentsomeofj the amplitude modulationîis removed in a ñrst limiter I5 and' practically 'all of the remaining VamplitudeV modulation isf're' moved in a second limiter I6.V ' ' ' " ` ' „ pacitor G6', and> impressed- between the control grid 1u and cathode'l-I of the first audio ampli@ii'er tube, which is illustrated asÍ a triode 12; It will be observed that a conductive connection `is provided betweengrid T0 and cathode 'H' through a resistor 6 I‘v and resistors 58 and 59'. These'v'olt 65 ter limiting action. Thus in theillustrated' em ' The intermediateV frequency carrier waves atY the'olltput ofthe secondv limiter I6 'are therefore of substantially constant amplitude'and contain only'theffrequ'ency 'modulation which is repre, Unidirectional- voltages appearing ages are of such polarityas to bias’the `grid 7_0 negatively with respect‘to‘the 'cath‘odeïfl"and`, as will shortly'be:explained;- under 'certain conditions » their'magnitude is sufiicienttc‘reduce the `ampli-` cation of the audio ampli’ñ'er tube 'I2` toa very 70 low îlevel; Preferably tube 'I2A is Vthereby >biased completely to cutoff, preventing transmission 'of signals'to the power amplifier I'Sf and therloud speaker20'." f ' ’ ` ' " ^ " Y i It has beenfoundîthat there are alwaysî noise sentative of the intelligence being transmitted, 75 voltages present -at -the second’ limiter even'when 2,404,338 .the diode 54. The time constant of the network no voltages are impressed upon the receiverin put. It has been demonstratedlthat the Ynature of these voltages is that of a spectrum contain associated with this -detector‘is made very long as compared to theperiod of the lowest; audio fre quencyL in vthe signal channell gThe yresulting unidirectional potentials, after being filtered to ing an almost infinite number of component fre quencies, some of which always coincide with the frequencies associated with the desired signals. It is generally accepted that irregularities in the motion of electrons in the tubes and circuits is largely responsible, i. e., the thermal agitation and “shot” effects and related phenomena. The 10 first radio frequency amplifier stage undoubtedly contributes the greatest proportion of these dis turbances because of the high'gain' provided in the stages between it and the limiter. Further smooth them out further, are then utilized to-pro vide the automatic squelch or muting bias for the audio frequency amplifier IO, as previously ex plained. ` The modified form >of my invention represented~ in Fig. 2 differs' from that of Fig. y1 only in cer tain particulars. Corresponding elements have y been designated by the same reference numerals and the function of these elements is essentially the same. Therefore, they need not be repeated here. In this modification the pentode 33 is it~self utilized to detect the aud-io noise voltages> more, it has been demonstrated that extraneous disturbances received at the antenna l0 ~ also comprise an infinite frequency spectrum contain present at the anode 36, permitting the first diode ing» frequencies capable of being translated in the detector to be eliminated. This is possible be' receiver channel.- These disturbances further increase -the voltage impressed upon the limiter 20 cause the time> constant of the grid circuit' 3|, 32 is such that the pentode 30 functions essentially under no-signal conditions. as a grid power detectorfor amplitude modula As indicated earlier in this specification, it is tion components within the audio frequency preferable to provide such a high gain through band. Therefore, it produces, audio voltages the radio frequency and intermediate frequency across an audio frequency load in the anode cir amplifiers that the noise voltages impressed upon cuit. Such a load is provided by the decoupling the limiter grid 33 develop self-bias potentials network comprising the capacitor 31 and decou approaching values required to saturate the pling resistor 38. Consequently, the audio noise pentode 33 even- under no-signal conditions. voltages appear at the point 13 and maybe sup Nevertheless, under such conditions it has been plied directly to the filter and detector-network found that these voltages are not appreciably 5x5-'65, as illustrated. ' y ’ limited in amplitude in the limiter i6 and that In a frequency modulation receiver‘of` .com substantial noise voltages are developedat the limiter anode 36 including> modulation frequen cies-within the audio band.` Reception of exter mercially acceptable sensitivities, the audio noise voltages at ythe point i3 have been found to `be noise level, voltages within lthe intermediate fre-' quency band are continuously impressed upon the squelch voltage is automatically increased when limiters to maintain them operative. Y Noise volt tions.y nal “static’f disturbances at the antenna l0 mere 85 at least of the order-of lO'volts under all operat ing' conditions. This provides a unidirectional lyincreases their magnitude. However, as soon >squelch voltage of > at least four volts atjthe grid as' a carriersignalis received which is suiiicient of the audio amplifier 12, which is sufficient Yto to maintainrthe limiters continuously saturated, cut off a typical high-mu triode driver tube.> vEx these voltages at the anode 36 almostv completely disappear.' Inl other words,` so long as the re 40 traneous noise voltages received at the antenna I0 further increase these values so that the ceived signal is strong enough to over-ride the ages alone, on the other hand, apparently con the receiver is operated in excessively'noisy loca 45 tain no such continuous components, but are more like a highly over-modulated carrier vof practically zero intensity and are not removed by the limiters. ’ ' ~ e As soon as even a relatively weak signal is re ceived the squelch voltage immediately collapses and the receiver operates at full sensitivity. Such a signal need be only of a few microvolts intensity, i. e.`, just sufficient to render the limiters fully ef In accordance with my invention these phe 50 fective. As previously explained, the background noise at the loud speaker is now reduced to‘a vvery noména are utilized to provide background noise suppression in a manner now to be detailed. In the embodiment of my invention illustrated low level through the inherent capabilities of this type of system to discriminate between signals in' Fig. V1 the vnoisev voltages appearing at the and noise and to reject the latter. anode 36 are detected inthe diode'äl and appear 55 It will therefore be apparent that my inven tion combines a highly sensitive background noise suppression action with` a fast-acting trigger action. Furthermore, the noise suppression cir cuit is also `effective in removing side responses caused by the slope characteristics of the sides of the intermediate frequency response curve.4 lIf the receiver is mistuned or misaligned, frequency modulation signals will be detected in the inter mediate frequency circuits _through Well known -upon the load resistor 52. Preferably the time constant of this detector circuit is such that the noise voltages appearing on the resistor 52 lie within the audio range, higher frequency com ponents being filtered out. Itis desirable to vuse the audio noise voltages for control since volt ages of these frequencies tend to be transmitted through the signal channel following the limiter and to cause audible disturbances in the loud speaker 25. Furthermore, voltages at frequen cies within the intermediate frequency pass band are of course not removed‘by the limiters but , are developed at the anode 36 even when signals ' ‘ , slope filter action, producing amplitude modula-tion signals across the limiter anode resistor and resulting in the production of a squelch bias in the same manner as noise voltages. Since noise voltages within this range cannot be f For lcompleteness of illustration only, and not in any sense by way of limitation, the following readily distinguished from signal voltages, such circuit constants are given as being typical for higher frequency voltages must be filtered out a frequency modulation receiver of the type rep resented in Fig. 2. These constants'have been found to be satisfactory yfor the circuit elements of satisfactory strength are being received. in order for the desired control effect to-be re sponsive to noise voltages alone. The audio noise -voltages are again detected in of a frequencymodulatio'n broadcast receiver 2,404,338 adapted? to receive signals in the vicinity >of 42 detector also coupled to the'output of rsaid lim iter, ;and means_for biasing said control'electrode in response to voltages developed by said detector and. in asense to reduce the ampliñcation of said t'oï 5U megacycles'and having' anr intermediate fre quency channel tuned to about. 4.3 megacycles. In this particular receiver the .tube 30 was a type 6SJ7 pentode and the audio frequency vamplifier 12, the «diode 5I andthe diode 54 were all em device. bodied »ina single duo-diode, high-mu triode, type GSQ'T. bination comprising a» signal ampliñer, an ampli tudeV limiter coupled to said amplifier,` output cir Other circuit constants follow: . Capacitor 31 __________________ __mmf__ `22 Resistor 3'2 .... _e _____________ __o'hms__ 180,000 Capacitor 3L` _________________ __mmf__ Capacitor 55 _____________ r.. .... „mid“ cuits coupled to. said limiter, said circuits in 10 cluding an electron discharge device having a control electrode', an amplitude modulation de 47 Resistor 38 _________ __; _______ __ohms__ tector connected to the output of saidlimiter, and means for biasing said control electrode _in response to modulation voltages developed by said detector- to prevent translation of saidV signals by 22,000 f ‘.01 Resistor 56à _____________ __'__I__ohm"s__ 10,000 Capacitor 57 _______ _~_ _________ __inmf" 100 Resistor. 58 _____ __' ________ __megohms__ 1 Resistor 59 ____________________ __do_~_„„ 2.2 Capacitor 6'0 ..... __. __________ _I__mf'du .01 Resistor 61 _______________ __megohms__ 6.8 . 4. In a» frequency modulation receiver, the com said electron discharge device. « , 5. In combination, a signal amplifier adapted to amplify high frequency voltages modulated in frequency in accordance with a band of signal frequencies, said amplifier having such a high gain that undesired noise voltages of. interfering my invention, it will of course, be understood that frequencies are continuously Ápresent in its out I do not wish to be limited thereto since various put,` an. amplitude limiter coupled to the output modifications may be made, and I contemplate of .said amplifier, said limiter being effective to by the appended claims to. cover any such modi« 25 remove` amplitude modulation, yat signal» frequen fications as fall Within the true spirit and scope cies, from said 'voltages only so long.r as high fre ofl my invention. ' » , Y quency voltages exceeding a predetermined limit ' What Iv claim as new and desire> to secure by inglevel are supplied from said amplifier, a signal Letters Patent of the United States is: _ . reproducer coupled to said limiter, and means 1. In combination, a signal channel for trans 30 responsive to vamplitude modulation at signal fre lating signal-bearing waves,.am-plitude limiting `„quencies in the `output ofv said limiter for pre means in said channel effective substantially to ventingoperation of. said reproducer. remove amplitude modulation. from waves im 6. In radio receiving apparatus for operation pressed thereon only so long as a predetermined on Vhiglfi-frequency Waves modulated in frequency limiting rlevel is exceeded, and means responsive in accordance with a band of signal frequencies, to amplitude modulation in waves in the output the combination comprising a thermionic ampli of said limiting means for disabling subsequent ner; for said waves having .such av high gain that portions of said'channel comprising means for undesired. noise voltages of interfering frequen rectifying said amplitude modulation to produce cies,. due to received disturbances,A thermal agita a control voltage varying in accordance with the tion,~ shot. effects,y and the like, are continuously magnitude of said modulation. present in its output, an amplitude limiter cou 2. In combination, a signal translating channel pled to said. amplifier comprising a thermionic having means therein adapted to translate fre device-having an output electrode, said limiter quency-modulated high-frequency waves, said being. of` such' a character that amplitude modu V While I have shown particular embodiments of means also being effective to supply substantial `z. lation components. at signal frequencies are con- ` noise Vvoltages to its output, amplitude-limiting means coupled to said translatingmeans to limit tinuously present in potentials appearing on said electrode so long as received high-frequency Wavesy do not exceed a predetermined limiting the amplitude of high frequencywaves supplied through 'said limiting means, said amplitude-lim level but are reduced toa negligible'value so long ' ` iting means having amplitude modulation com as said waves exceed said level, signal translating circuits including a signal reproducer coupled. to said. electrode, an: impedance element coupled to said electrode> and adapted to have voltagesV of ponents' due>y to said noise voltages developedin itsV output when said high-frequency AWaves» sup plied therethrough are of intensity less than a signal. frequencies developed thereon in response being substantially removed from its output so 55 to. said components, and means for muting said long as the intensity of said waves exceeds said signal; reproducer in response to said last-named voltages. . level, means for detecting said rcomponents and deriving a unidirectional potential therefrom, and 7. Inradio receiving apparatus for translating means responsive to said unidirectional potential high-frequency Waves modulated. in frequency by ' for substantially reducing the transfer efliciency l60 desired signals, the combination comprising ther . predetermined level, said‘modulation components 3. Incombination, a signal amplifier adapted to amplify high' frequency voltages modulated in mionic means for amplifying received waves. to such. a .high level that substantial undesired noise voltages of the frequencies of said signals and frequency in accordance with .a band of signal due to received. disturbances, thermal agitation, of said channel. frequencies, Y said , amplifier having undesired 65 shot effects, and the like, are developed, ampli noiseA voltages of interfering frequencies continu ously present in its output, an amplitude’limiter coupled to said ampliñer, said limiter being ef fective to remove amplitude modulation, at signal frequencies, from said voltages only so long as high frequency voltages exceeding a predeter - mined limiting. level are supplied- from said am plifier, output circuits coupledto said limiter, said circuits: including ‘an amplifying device have ling‘a :control electrode, an amplitude. modulation tude limiting means coupled to said amplifying means, said last means being effective to reduce said undesired voltages to a very/low level in its 7.0 output only so long as high-frequency waves ex ceeding a predetermined minimum limiting level `areimpressed on its input, means coupled to said outputfor translating said high-frequency waves, andA means responsive to said undesired voltages appearing in said output for disabling said trans lating means` comprising ~means for rectifying 2,404,338 ¿9.5; 10 said voltages to .produce a'control voltagevarylng output from said receiver'in accord with'the rec in accordance .with the lmagnitudeof said unde tified'noise voltages.y sired voltages." “ ' y , f , 8.'v In a frequency modulation receiver of the type 'comprising -high frequency amplifiers for ’ y -method which comprises controlling the‘output vfrom-said receiver .in accord with noise voltages of varying amplitude and of the lfrequencies of said amplifiers provide sufliciently high amplification that undesi?ednoise voltages ~are continuously supplied to -said limiter, and wherein said limiter is effective substantially to :prevent rtransmission signals appearing 'atfthe output fof" saidlimiter s during periods of reduced intensity ofithevre of noise voltages therethrough only so long as said , ceived carrier wave. L41.5 detecting noisevoltages transmitted through said limiter, -m'eans‘for` rectifying said detected volt ages to provide unidirectional control potentials, and» means for utilizing‘said control potentials to 1 , . î. p L_v :> Y 13.` The combination', in a‘receiver fofrîfrequen cy modulated carrier/waves of an .amplifier for frequency modulated carrier waves, _ a .-limiter’to limit-waves amplifiedbysaid amplifier to a‘ pre determined value, saidamplifier havingi ampli fication` s'ufñcient to produce noise` voltages " of objectionable intensity and variable amplitudeat prevent transmission of noise voltages from said limiter tov said signal reproducer. ` 9. In radio receiving apparatus adapted to translate high-frequency waves modulated in fre quency by desired audio'frequency signals, the combination of means for limiting the amplitude of said frequency modulated Waves to a substan tially constant level, said means comprising a thermionic device'having an anode circuit, fre quency modulation signal detecting means cou pled to said anode circuit, an electron discharge amplifier coupled to the output vof vsaid detecting means, said amplifier having a _control element, ' ceived carrier waves are first amplified to intensi tyY in excess of a ‘predetermined value and then transmitted through `a llimiter Which-limits said waves- to a uniform. intensitylof saidî value, the amplifying waves modulated in frequency by de sired signals,-an amplitude limiter, a frequency demodulator, Va lowV frequency signal amplifier and a signal reproducer, wherein said high frequency waves exceed a predetermined limiting level, a noise suppression circuit comprising means for . 12. In-.a `»receiver for carrier waves; frequency modulated by desired signals in which/the' re -- 1.39 the output, of said limiter in the absence" of -re ceived carrier waves, said noise voltages" disap peeringv from said output in the presenceof re-` ceived carrier Waves of greater thany a predeter mined intensity, and means> to disable said? Are ceiver in response to the amplitude variations of said noise voltages at the' outputjof said`Ã limiter. 14. The combination, in a lreceiver for frequen cy modulated carrier Waves,l of 'an .amplifier for signal `modulated carrier waves,` a limiter 4to limit waves amplified by said ampliñer to a‘pr'edeter mined value, said amplifier' havingamplilicat‘ion lsufficient to` produce noise- voltages of `objection means comprisingV a diode detection circuit for developing audio frequency voltages in response 35 able intensity at the output of said limiter in the ' absence of received carrier Waves, said noise volt to audio-frequency amplitude modulation compo ages disappearing from said output in the pres nents present in Waves developed in said anode ence of received carrier waves of greater than a circuit, means comprising a second diode detec predetermined intensity, a rectifier, means to trol potential in response to said audio frequency 40 supply said noise voltages to said rectifier, said means including means to eliminate voltages of voltages, and means for biasing said control ele the frequency of the carrier wave amplified by ment negatively in accordance with said control said amplifier, and means responsive to rectified potential. ~ 10. In radio receiving apparatus adapted to 45 voltages produced by said rectifier to disable said translate high-frequency Waves modulated in fre 15. The combination, in a frequency modula quency by desired audio frequency signals, the tion receiver, of an amplifier for frequency mod combination of means for limiting the amplitude tion circuit for developing a unidirectional con receiver. , v ulated carrier waves, a limiter to limit Waves am of said waves to a substantially constant level, plified by said amplifier to a uniform intensity, a said means comprising a self-biased thermionic 50 rectifier connected to rectify noise voltages at the limiter having an anode circuit, frequency modu output of said limiter, a second rectifier, means lation signal detecting means coupled to said an 0de circuit and an audio frequency amplifier cou pled to the output of said detecting means, said to supply variations in voltage at the output of a resistance-capacitance network inrsaid anode tified voltage produced by said second rectifier to reduce the output from said receiver. said first rectifier to said second rectifier and to prevent said carrier waves from reaching said audio frequency amplifier having a grid electrode, 55 second rectifier, and means responsive to the rec circuit adapted to have audio frequency noise voltages developed therein, means comprising a diode detection circuit for developing unidirec tional control potentials in response to said noise voltages, said circuit having a time constant long er than the period of the lowest desired audio fre quency, and means for biasing said grid elec trode negatively in accordance with said poten tials. ’ ` ~ yto 16. In a receiver of angle modulated carrier wave energy of the type provided with an ampli tude limiter stage, a demodulator and a modu lation signal amplifier; the method which com prises detecting from the output of said limiter noise voltage components when the received sig nal-to-noise ratio is excessively low, rectifying the 65 detected noise voltage components to provide a 11. In a receiver for carrier Waves frequency control voltage, and impairing the operation of modulated by desired signals in which the re said modulation signal amplifier in accordance ceived carrier waves are first amplified to inten with said control voltage. sity in excess of a predetermined value and then 17. A method of receiving frequency modulated transmitted through a limiter which limits said 70 waves which comprises limiting the amplitude of waves to a uniform intensity of said value, the the waves, demodulating the limited Waves, am method which comprises rectifying noise voltages plifying the demodulated Wave energy, detecting of the frequencies of said signals and which ap from the limiter output noise voltage components pear at the output of said limiter during periods in response to the decrease of received wave en of reduced signal intensity, and controlling the ergy below a usable value, deriving from the de 2,404, 338 y 12 All.. tected -noise voltagecomponents a Vcontrol >volt age, andpreventing said amplification of the de modulated wave venergy in response ¿to said con 18.?¿In a receiver »of angle modulated carrier Vvv'aveienergy `of_ the type provided with anvampli tudeflimiter stage, the method which comprises " detecting from the limiter output noise voltage Vlcon‘iponents when the :received signal-toFnoise ratio is excessively low, rectifying the detected '10 components .to provide a control voltage, and uti.; lizing theco'ntrol Vvoltage to impair receiver re production. ` ' Y ` ’ . . » 'amplitude limiter, a detector and a modulation amplifier; the improvement which comprises rec, tifying amplitude modulation noise energy ap. pearing in the limiter output when the angle lmodulated energy decreases 4to -a level such that peakAv carrier voltage is substantially equal to peak noise voltage, deriving a mutingY bias voltagefrom tlie -rectii'ied noise energy, and suppressing oper. ation of the .amplifier with the bias voltagel V22.111 _a frequency .modulation receiver’of the type having an amplitude modulation limiter, a detectorhand an audio ampliñer; »the improve ment which includes means for detecting ampli -„19. A method of receiving frequency modulated tilde modulation appearing in the limiter output _ waves which .comprises -limiting the amplitude of l15 energy in response to adjustment Lof the tuning theA Waves, detecting from the limiter ,output noise of said'receiver between stations,î means for rec-` -voltage components in response to the decrease of tifying the noise modulation produced as a result l.of said detection, and -further means lfdr muting received" wave »energy below a usable value, de riving `from the noise voltage components .a con said gaudio' ampliñer with the rectified voltage trol voltage, :and preventing reproduction of the 20 thereby y-to prevent the rush oineisercommonly wave energy `in response to said control voltage. produced during absence of carrier. ' 23. A system of receiving angle modulated .car .20.' linv a method of frequency modulation re ception wherein the amplitude of frequency mod rier energy including means limiting the ,energy ulated vcarrier Wavesvis limited in a limiter prior to reduce amplitude modulation effects, means tov detection; :the improvement which comprises demodulatzing the limited energy, vmeans fOr am deriving »a .control voltage from amplitude modu-` plifying »the demodulated energy, means deriving f 'lation appearing in the output of the limiter when ` a :control voltage from amplitude modulation the 'received :signal to noise ratio is below a pre :noise energy appearing inthe output of said lim ‘ f determined threshold level, and suppressing the iti-ng means When the received modulated-carrier reproduction of detected energy in response to l30 energy decreases below a `predetermined thresh 1 . .said control 4.voltage thereby to prevent reproduc Y old, _and means >for inhibiting said ampliiication with said control voltage. tion of noise. . ` y. :21. In. = e operation of a receiver of angle mod ulated carrier ywave energy which >comprises an JOSEPH A. WORCESTER, JR.