US2379721A - Frequency modulation receiving system - Google Patents

Description

Jul 43,1945. y w. R. KQCH 2,319,721

FREQUENCY MODULATION RECEIVING SYSTEM A TTOBVE K 2 Sheets-Sheet l INVENTOR. Winfield E Kod/b ATTORNEY.

July 3, 1945. w. R. KOCH FREQUENCY MODULATION RECEIVING SYSTEM Filed June 4, 1943 .hw m um AAAAAAA YVVVVV AAAIAAAA Patented July 3, 1945 UNITED STATES 4mtTsNfT QFFICE i i FREQUENYMgimsnecnmno.y l

1 Winfield. a. noch, Haaaoniiela, N. .1., mignon to Radio` Corporation of i of. Delaware American, a. corporation.

Application June 4, 1943, Serialil\io.`489,ii.?.2k f q `14,V claims. (c1. 25o-20v` .My present invention relates generally to systems for receiving angle modulated carrier wave energy, and more specically to receivingsystems for frequency modulated carrier waves.

In communication by angle modulation of'higlrV frequency carrier waves, it is: necessary at the receiver to provide wave transmission networks capable of eiiiciently transmitting theV entire band off modulated `wave energy. By the expression angle modulation there is generically compre-` hended frequency modulation ("FM hereinafter for brevity), phase modulation or hybrids of the i i two. By way of specic illustration the remainder of this specication is directed' to frequency modi ulated carrier waves. In the reception of the latter with a superheterodyne"receiver, Va mode of reception which is practically universal, itis neceSSary to design the various selector circuits up tothe demodulator so that the entire range of' frequency deviation of a selected station passes eiciently;` For example, in the FM broadcast y band of 42 to 50 megacycles (mc.) there is `em-` ployed a frequency deviation up' to'75- kljl'ocycles' on either side of the midband, or center, frequency of the transmitter'. Hence, theselector circuits at the receiver have been designed to passa band 150? kc. wide. i q i q In the intermediate frequency (I. F.) amplifier circuits such a relatively wide pass band reduces the gain per stage.` I-Ience,` there must he supplied suicient stages of I. F. amplication to compensate ior the loss of gain due to the Wide pass band. Accordingly, it is one of the. main objects of my invention to provide a novel and highly proved method of' reducing the range of frequency deviation, or swing, of the FM wave energy at the converter network `so that the I. F. selector circuits feeding the F. M. detector may have a rela tively narrowed band width whereby greater gain per stage can be secured in the I. F. amplier.

In the past various methods have been proposed to provide reduction of .the ran-ge of frequency deviation of FM waves at the receiver. Some systems have accomplished the reduction by employ'-v ing a reactance tube `as a. frequency modulation device at the receiver local oscillator, the local oscillations being frequency modulated by audio frequency so phased that the oscillations deviate,

in the same direction. as the incoming signal, but` Such `systems are complex' with less deviation. and require careful adjustment of phase. relations throughout the system. Again, in 'certainsystems there has been employed prior to the demodulator a locked-in` oscillator operating ata sub-harmonic of the I. F. waveenergy. While such systems are local oscillationsgand then usingthe sum of the mixed energies as the heterodyne energy for received FM wave energy.

Another important obfect of myinvention is to provide a method ofsuperheterodyne reception of waves` wherein .therefis employed in the I. F.

amplierffnetwork a locked-in oscillator from which, the harmonic of the I`. F.` wave `energy is obtained; the I. F: networkfeedine' the` oscillator having a relatively restricted pass band width compared to the frequency swing of the receivedA FM waves whereby the lockedinosciHator does `not have to lock in over as wide a percent` devia.-

tion as would be the case if theapplied signal had a frequency deviation range as wide as that of the received waves.

A more specic objectv ofmypresent invention is, the provision of an; receiver of the superheterodyne type wherein a locked-in oscillator is employed in the I. F; amplifier network, .narrowing of the frequency `deviation range of` theFlVI wave venergy being secured fat the inputfterminals of the I. F; amplifiers; orin. a` modification some narrowing canV be secured at the inputterminals of the I'. F. amplifier and some at the oscillator output terminals whereby the over-all stabilityof the I. F1 amplifier is' improved.

stm other objects` or my invention are` to improve generally the efficiency and operation of receivers; and more especially to `provide a superlreterodyne receiver of FM wave energy. of` relatively high deviation ratio wherein the I. F. selector circuits are of a narrow pass band, andi the' receiver being economically" manufactured i and' assembled. l

novel. features` which I believe to be characteristic ofmy'invention are set forth with particularityin the appended claims; the invention. itself, howeverh as to both its organization` and method ci.` operation will best be understood. by referenda` to the following. description, taken inv connectionwith the drawingarin which Ihavein.-

wherein like reference characters inthe different` figures denote similar circuit elements, Ilet it bef` assumed that the receiver in Fig 1 is of the be assumed that the operating I. F. value is of the order of 8.3 mc. Actually, I; prefer an I.y F.

value of 8.33 mc., since this value seems to give.` 4

ner so as to follow I. F. changes over a range of frequencies on either side of 8.3 mc. The local oscillations produced by oscillator 5 are transmit; ted to a mixer network 8 where the local oscillations are mixed with the second harmonic energy derived from the locked-in oscillator. The resultant mixed energyfwill be `cfra frequency within a range of 33.8 to 41.6 mc., depending upon the adjustment of the local oscillator tuning mechanism 6. The output energy from mixer Bis passed through a band pass filter 9 which i should be designed to efciently transfer any fre- Y quencies in the band between 33.8 `and 41.6 mc. The filter `9 should be'capable of passing 33.8 mc.

the least trouble from spurious responses in the*v present band.4 It `will be understood, of course, that thefinvention is'notlimited tov any spcilcfreqency values. 'The ys'ytemlnayloe used to vreceive rFM"waves 'in any` of the ultra-high amplier."'The` FM wave 4energy collector` 3 isV shown ,as a aippleieediiifgihje tunable ampliiier. The received modulated carrier'wave"4 energy will have a wave' formV wherein the mean y frequency1 isdeviated in accordance with the amplitude of the modulationsignals applied ytclzyfthe carrier at tile" transmitter. Therese of frequency deviationis' a' function"`of" the modulation frequencies perse.v It islassumedthat the maximum'frequency vdeviation fon either side of the mean frequency'of the transrrlittedr wavev Qen'ergyl is 'ap'-v p'roximately'75 Hence. the over-all maximumirequenc'y swing of vthe"recfeiwdled r'waveA Venergy is lso kc.

In suchl case theV ftunablefselector circuits of the radiofreduncy yarriplier `I yvill each have a passband widtii'cf atleast' `15oy ke. Aboveilie amplifierl there is shown ac'urve which repre# sents the pass bandwidth of each `ofthe selector circuits f the air'lplifier network. The midband `frequency`is"represented'as being capable Tof var l-A ation frolnflZ'l to"49.`9 'megacycl'cs In other words; Hthe representati fabov'e n the rectangle conveys thefinforlnation'ithat each selector cir` cuit is; capable Orbe'ing tuned'through a fre# qlllyi'rig 0f lmC., and al) each selected'mean frequency' the selectorband width` will ytelco kc. j v i i Thselectd FM"wave 'energy is transmitted to a Imixer network 4.' Concurrently with the' se`- lected and ampli'edFM'wave energy there is fed to the mixervfd" heterodyne oscillations.` The numeral i-'designates' the usual lojal oscillatory or a' 'snperh'etero'dyne receiver. The symbol w6 de?v notes the usual't'unin'g means' of the cs cliator.` The 'tuning means' Gwill be capable of adjusting the local "oscillations througha frequency range of 17.2 to mc. The dotted line 'l designate'snany wellknown forni of unicbntrol mechanism vvf orconcurrently' 'adjusting the tuning `jmeel;aiiisiiis g and 6, so `a`s` to secure'the' required' trackingf The lockdgiiij'oscillafiril isiesigned io operate at the mean I.' F. valu,"e. g.",l8.3` mc., andproduces minus -50 kc.; and should also be capable of passing 41.6 mcgplus 50 kc. The curve adjacent the g iilter represents this design of the band pass The output energy of lter 9 is fed to the mixer 4 was `the actual heterodyning energy. kAt any settingl ofthe tuning'mechanisms \ 2 and 6, the medulated waveenergy output of mixer 4 will have a'mean frequency value of 8.3 mc. The I. F. amplifier Ill may comprise one or more stages'pof intermediatefrequency amplicationand the resonantselector lcircuits 'thereof will each be tuned toaffrequency of"8.3 me. `In addition,l`each se-` lector circuit "will have a pass bandwidth of onlyf50 kc. 4 In other words', at the output terminals lofv mixer Il therehas been provided a reduc'- tion'of the frequency' deviation rangel of `lthe re-v ceived FM wave'energy. .There hasyalso, been kprovided a frequency reduction of the mean fref quency` of the received FM carrierwaves.

Accordingly, the Vselector circuitssubsequent to the mixerrnay be narrowed to one-third the pass.. band (width of theselector'circuits ahead of the mixer.. This pass Vband narrowing is due to the iam; thi-aille llieieiodyii energy ifea it mixerv 4 has an effective frequency deviation of 100 kc., while theoriginal FM wave energy fedl to l 4 has a frequency` deviation range of 150 kc. The processjof mixing these energies results in a pro'- duction of'` intermediate frequency energy. Whose frequency deviation is the diie'l'ence between the frequency deviation range'of the original signals and that of the heterodyne energy. Thev frequncy'swing" at the output of mixer 4 will be redcedfbecausefior'example, at the'instant the.

tivlyfnarrow frequency deviation range of 50 kc." atthfinput` terminals of oscillator I|,`the latter'4 oscillations whether'ioiiiojrjr. F. wave enigyfisO supplied to it. It is constructed'in a known mansignal has deviated kc.` in the higherfrequency direction, thefhetero'dyne"frequency will have deviated 504 kc'. in the same direction. This will i make'thebeat note equal to theI. F. plus 25 kc;k

' The numeral Il denotes the previously 'menev tioned locked-in oscillator of any well known construction.'e` Those skilled inthe art of FM lreception are well acquainted with such an oscillator. Such an oscillator` tends to oscillate at aboutth'e midband frequency of 8.3 mc., but is capable of following the frequency deviations of the energy at its input terminals over a restricted rangeV of frequency. .'Thus, 4the oscillator Il will lock infwitl'l the I. 1*".V energy fed. to its input Iterminals, and it willfllow the frequency deviations;A "In otherffwords, the output of oscillator llwill be areplica of the I. F. energy fed toits in'putterininals. vHowever, by providing a relados Alfrc'ltliaveto Vlock in"over as wide a Vpercent of deviation as would be the case if the signal energy fied lto the'oscillatorwere at lthe original Wide? deviation rangefof kc. Moreoverj'a greater gain'per stage of I, F. amplication: is'

mixer secured by employing the reiativelynarrow pass loandini each amplifier stage.` Infaddition, the allgmnentof the selector `'circuits in. fthe I.' F. amplifier HI isueasicr than in nthe .case ofthe relatively,widerpassband circuits. f

`The output of the -lockedfin-oscilflaton `whose i oscillations have a mean Vfrequency equal to 8.3

rnc. butwhose frequencydeviations are a replica 'of thefrequency deviations of the I. F. energy fed torthe oscillator, is then applied toan detec-` tonneilufciirkd!r 'Ii-he lattermay be ofany `Well knowntype; Its typical. Sfshaped characteristic,

with a. center frequency `of 81.3 mc. isrepresented` above therectangle l2'. Byiway of .specificex ample, there` may loe` employed for the` `.detectori IZ thetype of" circuit shown by S. W, Seeley, inhis the FM detector. t2.- For example, themeanwfre; quency of the FM waveenereyatthe oscillator output circuit`A may be `one-half of 8?-.3 ma., or 4:15 incLf The overall. frequency ydeviation: will, there fore, be reduced to 25 kc.; Tlfii's means thats the coupling network' betweeny oscillators-l t' :anni` the detector |2 needfhave a pass band'fonlytkc; wide. A However, where 'reproduction `of modulate.

`tion; frequencies up to" 115,000 Lcyclesisdesirect; f

the couplingfnetworls must, of course.. havea rpases band 30` kc. wide. Thetfourth harmonicofwthe U. apparent No. 2,121,103,.gr,anted Junezi, 193s.

lit isindicated by arrow =indications. that the response characteristic shown `above rectangle` I 0 is employed forthe various selector circuits ahead of the` locked-in oscillator I l `andffor theihput circuitof the detector"l2.` i i l"A portion of the oscillatoryoutput `offlocked-"in oscillatorjll istransmitted to` a harmonic pro-` ducer I3. Any well known forincf devicefor producing harmonics of oscillations" may be eniployed. general, there may be used any nonlinear device capable of providing nlc'fhharmcnics. i

Specifically, there is usedthe second" harmonic of the energy delivered by oscillator H. This means i that the output ofthe harmonic network i3 will have a`mean frequency of 16.6 mc. The `frequency deviation range will be multiplied by two so as to.` provide an over-all frequency swing `up 1301.100; kc. lI-Ience, the band passi filter M is designed `to pass energy having a centerfrequency' of.16.6 imc., and'r a. .nfiaximumu frequency 'deviation range of"100 kc. The output of filter Mis applied to the mixer. Accordingly, itllwi-lll@ seenthat theoutput of the mixer Vnecessarily will @over ag` range off3`3.8 to 41161 Inc., depending upon the "adi-i justrnent of the oscillator tuning 'device'l i i ingeneral, then; Fig. l shows a method "ofre ducing the `frequency deviation range of,` received FM'sghl'rie'reylbr @perenne @lacked-1e esel.- latorat the operating. F. value, and theseconci harmonic" of the locked-ieri` oscillations beingeru-` ployed to mix withthe locally-produced osci1lations to provide fa range `of",s uiin heterodyne oscillations. i 'AeaCh selected sum frequency, the frequency 'deviation range is two-thirds that of the original frequeneydeviaton rangei Further mixing with thewrig'inal FM Wave energy Vpropassing a portion of the fundamental oscillations to the" harmonic producer. t3; as; the Vcase.u of F frequencyideviation range reduction to one-sixth inthe system. of Fig; 2. This has. the advantage i of reducing` the noise aheadlloffthe loclredeirrosr-v cillatonobecause of `time narrowedwband. andthe frequency` reductionbetween` the I. amplifier and discriminator.A The over-al1 stability :of the amplie-r is improved, becausethe oscillator output, which is relatively large, is ata different frequency from the I. F.1amp1i1ier which responds to` relatively small signal voltages. i

i Theremay be employed` for theloclrecl-infy oscil-A lator H a system. of the typedisclosedi and claimed in` application SerialNo. 430588l of G.L.f

Beers, .filed February 12, 1942, granted AgugustZZ,

y194A, aSzU. S. Patent NO. 2,356,201. that 10Cked.

'in oscillator, the energy fed tothe discriminator section: of the .following demodulator is ausub.

multiple harmonic of the LF. energycontrolli-ng theV oscillator. This presentlsystem i tends to pron vide automatic frequency control (AFC) action. Itis no harder to tunethe@ receiver, or tofkeep the latter tuned, than: other forms of receivers.` The tendency of` the receiver to .hang onf.l to-.a signal when: the tuning controlis. operated canne readf-` i ioily` controlled` by the band pass fil-ters employed 75 kc.) l

applied to it the sum frequency energy fed from' r` saryito apply oscillations 'to the harmonicfproduce'r to start thercceiver'." Byusing. the,loeliecl` inoscillator, heterodyneenergy is always present, and" no `build-uptime or starting oscillafionjare required. i. I. c w1 i i..

l@further deviaticnfreduction maybe secured" at the output terminals of the-'locked-iir oscillator. iger 21 `tl'ie-ILCFL amplifier 44) `s-showrr"coupled tofanoscillatortnetworkfH-I-il' 'whose output circuit and by adjustment ofthewlock-inrange ofthe locked-in oscillator. i In Fi'gf I have` shownea completewrecei-viihg system embodying the general method ofFis.` .2;

There is secured a 3:1 narrowing. prior `tothe lockedl-in oscillator. and'` an additional 2:'.1 nare rowing in the` locked-in oscillator. Thespeciiic i form `of the locked-in oscillator is that of, the` aforesaid Beers application, save `that I; derive fourth harmonic of the oscillatorenergy fromthe screen circuit of the locked-in `oscillator' tube. The i amplifier l has its tuning device 2 tune therese-n` nant input circuit over a range of mean frequenciesfrom 4.1 to 49.9 mc.V The over-.all frequency swingis` 150 kc. The mixer tube` 4, of the well` i known pentagrid tube form,` has its; input grid; 2l),y

coupled to the `plate circuit of amplifier I` by a Wind-band transformer 2|. i i

Thus, the tuned.` primary `and `secondary cir`- Y cuits of transformer 2l are designed to pass a bandfrom 42.1 mc. (1ess75 kc.)` to 49.91110. (plus The grid 22 ofthe mixer tube 4 has.

band pass lter 9; lThesum `frequency energy,

asshown in Fig. 1, has a band of kc., and` covers airange of 37.8 .to 45.6 mc. Sincethe `opere` ating I. F. used in this system is to be 41.31 mc.,

the. `local oscillator tuning means-will c-overa rangeofl 29.2 to 37 mc. The I. F. amplifier l0 has 'its input transformeren designed to passa band 50 kc. Wide at a mean frequency of 4.311110. `The youtput transformer 24 is` similarly `designed `to pass the same band of FMlenergy tothe input g-rcll25 oflocked-inoscillatortube Il:` As stated previously this oscillator circuit is constructed Accordingly,` there has been. secuimd` a high frequency currents.

in accordance with therteaching of G. L. Beers inhis aforesaid,application'.K 1 y vThe plateg26 is connected to asource -of positive potential through'a resonant circuit 21,which is tuned to `one-half the I. F.` value.

In other words, .the circuit 21` is tuned to;.2.15 mc. The

resonant circuitZ'I is magnetically coupled to` e resonant circuit-28l. -Circuits 21 andA 28 are cou-` pledrto cause thetubev to oscillateat the resonant frequency ofgcircuit 21. The high potential side of.: circuit i 23. is.' connected `to the-intermediate grid 29-ofthe pentagridftube I l'. The low potential side ofcircuito28.v is connected to ground through a resistor 38 which is bypassed for high frequencies by. a condenser 3|. The positive shielding grids 32-are located on either side of grid 29. As disclosed in the aforesaid Beers quency plusgthe. harmonic energy fed to` grid 38.-.

The frequency deviation range is equal .to-109 kc.: iIn the system-of Fig. `3 the frequencies. ofthe heterodyning.oscillationsyare below; the FMwave frequencies. It. is necessary,` .therefore,`rto feed the signal energy to mixer 4 with lthe sum frequenciesvdeveloped at the output of tube 8.

. Where the heterodyne oscillations are higher in frequency than the received signalait isfneces-v sary to use rthe resultant of the heterodyne oscill-A lator minus the fourth harmonic energy; 'Ingnf eral, it is moredesirable to use the lower oscil'v application, theplate 26 is coupled'to grid 29 to provide oscillations at. a sub-multiple harmonic of the input I. F. energy. In other words, oscillator II functions in the manner of; a-locked-in oscillator which has an output reduced in` fre- VAutomatic volume ooiot'ioi bias for .the 'control grids of the prior tubes is provided by direct current voltage developed lacross gridA resistor 33.

'1 The fourth harmonicrenergy tto be fed l,to the mixer tube 8 is derived in a novel manner from .the screen circuity of the oscillator tube Il. The

, harmonic energy is. produced in the screerrcircuit,` since its characteristic is non-linear. Thus,` lead 35 is shown connected from `the screen-'grids 32 to the high potential side of the resonantv the mixer tube is connected to a'tap on the tank circuit; `Itwillbe recognized that thereis thus -providedla.'source` of localoscillations which is capablefof .being tuned.. through. a frequency range of 29.2 to 37 mc. f

fir Hence, at the various settings of the tuning means 6 .there will loe-.developed inthe plate circuitv of mixer tube 8 thesum frequency energy of theloc'alioscillation energy and the harmonic energy `fed .to grid 3&5.I `The-grid 36 is grounded through resistor-36 which is bypassed. The plate 4&1'Y of mixer tube 8 is connected to a suitable source of positive potential through the primaryv resonant circuit ofthe band pass filter 9, refer-` ence. being made to Fig. l. The transformer 9 has .its primary and secondary circuits ,tuneddand designed so as ,to have a pass. band covering a range of 37.8 to 45.6 mc., as previously` explained.

The cut-off frequency will be decreased by 50 kc.

at, its llower range, and increased by.50rkc; atthe upper end of thebandi: In, this Way, there is fed `.to the grid .22 o f mixcritube the sum frequency energy which at` any setting of the tuning means 6; is,.- .equal to the. selected, local, oscillation lfref,v

from the receivedfmodulated energy and whose latory frequency (37.8 to 45.6 vma), since the same percentage stability results ina lower number of cycles which Ithe oscillator can'` drift orl wander.

Generically, therefore, I` have provided a novel method of frequency deviation reduction in su'- perheterodyne reception of FM waves. My method involves the use of a locked-in oscillator operating in response to the I. F.y energy isoproe` vide harmonic energy of a proportionately increased deviatiori range..The harmonic energy is mixed with local oscillations to provide hetero,-

dyne FM wave energy. Thev latter is mixed V,with

the original FM wave energy of wide deviation range .to produce the I. F. energy whose deviation range is the difference between the` deviation ranges ofthe mixed FM energies.

Y While I have indicated andgdescribed several systems kfor carrying my invention into eifect,it will be apparent t0 one skilled inthe art-that my invention is by no means limitedto` the 4par-f` .ticularorganizations-shownand described,l `but i that many modifications may be madewithout departing from the scope of my invention .asset forth in the appended claims. v v

`W'l'iatIclaimis: 1...

1. In combination, a` mixer having `an-input circuit upon which is applied received angle modulated'carrier wave energy' of apredetermined center frequency anda relatively x'yideffre-e quency swing, a sourceof local oscillations'relaf tively lower in frequencythan said center fre"n quency, a demodulator of angle modulated wave energy, a beat frequency network coupled to 4said mixer output circuit, said beat' frequency network being constructed to pass modulated wave energy whose center frequency `is substantially reduced frequency swing is also reduced, fa locked-in o'scillator coupled to the Voutput'of the beatI freie" quency network 'and feeding its outputoscillations to said demodulator, means for deriving harmonic energy from saidlast-mentioned oscillator, means mixing the harmonic energyland 'thelocal oscillations to provide heterodyne, angleixiodulated, energy whose center frequency differsfrfin said original received center frequency bythe value of the frequency of said beat network, and means for applying the heterodyne energyQto said first mixer. f

j 2. A method of receiving frequency liriodulated waves in a superheterodyne system, comprising deriving intermediate frequency energy 4from the received waves, converting the energy into 4oscillations of .double frequency and doubleV the frequency swing f of the intermediate frequency energy, mixing the oscillations with uninodulatedoscillations ofa predetermined frequency value toV provide heterodyne frequency modulated energy whose center frequency differs fromv the received wave center frequency by theintermej-f,

dial@ ,flquerlwyfsand 'mixing the heterodyne o vanvarai modulated energ'y `and ithereceived- A:Waves :to 4toro-'el `iride said intermediate frequency energy." i

s; Af vnieuwe 1 er Oreceiving frequency mod-mated waves in a superheterodyne syetemffcomprisi-ng deriving intermediate frequency" energy from the received waves, converting'the intermediate ,fre-

quency energy into oscillations o'fffdou'ble, fre-y quency" and double the frequencye` lswinger' the intermediate frequency swing, meting thefoscrzil`- lations` with unmodulatedfoec rationsofl a pre-r determined frequency valueto'pr vide heterodyne frequency modulated energy whose centerfre quency differs from the received .wavecenter frequencyby the intermediate frequency, mixing .the heterodyne modulated energy andthe received.

waves toprovide said intermediate frequency'en`` ergy,l producing oscillations yvlose center free` cuencyfis a predetermined subLmultipleharmonic `offtlie `intermediate `frequency value and whese frequency. swing is reduced toa proportional l degreeland demodulating thesub .-multiple harmbnic oscillations.

4. 11n .a superheterodynel .of

moeunteieearrief waves. .of the type employing a locked-in oscillator. prior to thedemodul'ator; the methed which. includes reducingthemean frequency .of received .waves concurrentlyllreducing the frequency swing .of thejwavesMoperatinejsaid `oscillator ineresponse tothe frequency# reduced energy. controlling the said frequency eycls `lfromfrtheireceived waves producing oscil- "lations'tofdoub'le 4"frequency anddoublelthe frey Yquency, 1 bswin'gl yof'. 1 said lintermediate frequency,

Air1iXing1-tlne voscillations withf unmodulated oscillations 'fof a f predetermined frequency value to provide heterodyne lfrequency modulated energy whosecenter frequency differs "from" the received Wave center 4frequencyby thel intermediate fre` quem'zy,4 mixing the `heterodyne modulated venergy and vthe i received waves to providefsaid. interrne' 'diaterrequencyenergy.

"8. @method "of receiving frequency modulated Waves of 42ito 5`0"mc`., in` a rsuperfluetf-:rodyrie sysf tem mpr'isin'g deriving,intermediatefrequency 1 converting the intermediajte"frequencyenergy i intaipihannd reduction and concurrent frequency swing reducoperating said oscillator in response to the frequency-reduced energy, controlling the said frequency reduction and concurrent frequency swing reduction in response to said ylocked-in oscillations, further frequency-reducing thelocked-in oscillations `with concurrent .proportional frem quency swing reduction, and demodulating the last-named frequency-reduced oscillations.

6. In combination, a mixerhaving an input circuit uponwhich is applied received angle modulated carrier wave energy of a predetermined center frequency and a relatively wide frequency swing, a source of local oscillations relatively lower in frequency than said center frequency, a demodulator of angle modulated wave energy, an intermediate frequency network coupled to said mixer output circuit, said` network being -constructed vto pass modulated wave energy whose center frequencyis substantially reduced from the received modulated energy and whose frequency swing is reduced to the same degree, an

voscillator coupled to the intermediate network and feeding its output oscillations to said demodulator, means for deriving harmonic energy from the oscillator, means mixing the harmonic energy and local oscillations to provide heterodyne, angle modulated, energy whose center frequency differs from said original received center frequency by the value of the intermediate frequency, and

` means for applying the heterodyne energy `to said first mixer. l

'7. A method of receiving frequency modulated inte losciiiauoris' or double, `frfeque'rmy and" identi@ the "'frequency swing ofthe `interm'ediate fre'l 'qiiencyj energy; .mixing the oscillations with Vunmodulated 'oscillations of a" predetermined l frei quency value to provide heterodynefrequency modulated jenergy Whose centerf frequency differs from the reeeived ave centerfrequency jby the intermediate frequency; mixing theffneterodyne modulated energy andthe "received waves lto provide Sait intermedia querreyerier'syieeriving oscillations e lfre'nr the 'ntermediate energy whose center frequency is *a predeterm'ned @fue 1 'termed-metalen@ value1andy dem'ozlulatingv the sub-multiple har monic oscillations. e e v i9... In a superheterodyne` receiver of frequency modulated carrierwavesof the type employing a locked-in oscillator prior` to the demodulator; the method which. includesfrequencyfreducing the mean frequency of` received waves and concurrently reducing` the frequency swing of the waves, directly applying to said locked-in oscillator said i frequency-reduced energy thereby further frequency reducing the modulated carrier waves with concurrent proportional frequency swing reduction, and demodulating the further frequencyreduced waves derived directly from said lockedin oscillator.

10. In a method as defined in claim 9, the ad,- ditional step of deriving from the locked-in os-` cillations an harmonic thereof, and employing the harmonic `energy in the said first frequency reduction.

r source of local oscillations relatively lower in l frequency than said center frequency, a demodulator of angle modulated wave energy, an intermediate frequency network coupled to said mixer outputcircuit, said intermediate network being constructed to passmodulated wave energy whose center frequency is substantially reduced from` thereceived modulated energy to about 8.33 megacycles and Whose frequency swing is reduced to about 50 ki1ocycles,`.a locked-in oscillator coupled to the intermediate network and feeding its output oscillations to said demodulator, means for deriving" second harmonic energy from the oscillator, means mixing` the second harmonic energy and local oscillations to provide heterodyne angle l modulated energy whose center frequency differs waves ofthe order of 42 to 50 megacycles in a l superheterodyne system, comprising deriving infrom said original received centerfrequency by e r the value of the intermediate frequency of said l2. A system of receiving frequency modulated waves 'comprising means deriving intermediate frequency energy from the received Waves, means converting the intermediate frequency energy intoV oscillations of double frequency and dou-blethe `frequency swing of said intermediate-frequency energy, means mixing the oscillations Vwith unmodulated oscillations of a predetermined frequency value to provide 'heterodyne frequency modulated lenergy whose center frequency'differs from the received wave center frequency by the intermediate frequency, means mixing the heterodyne'modulated energy and thereceived'waves to provide said intermediate frequencyenergy, means deriving oscillations from the intermediate energy whose center frequency is `a pre- 1 determined sub-multipleharmonic of thev inter-` mediatenfrequencyvalue and whose frequency swing is reduced to a proportional degree, and meansffor demodulating the sub-multiple harmonic oscillations.

` '13; In a superheterodyne receiver of frequency modulated carrier Waves provided with a locked-` in oscillatorprior to ademodulator; the method which includes receiving. .frequency modulated carrier waves of a relatively wide frequency deviaf tion range, reducing the received modulated carrier Waves to an intermediate frequency, applying intermediate frequency energy to said yoscillator to provide looked-inV oscillations of a substant tween said wide 'deviation range andsaid har` monic deviation range.

tially'narrowed deviation range, 'demo'dulating the locked-in oscillations, deriving fromthe locked-in oscillations harmonic energy -offaproportionately increased deviationfrange, controlling the frefquency reduction of `said received Wavesinresponse to said harmonicenergy so-as to cause said narrowed deviation rangetobe equal to the difference between said wide deviation range plying intermediatefrequency energyto said os-` cillator to provide locked-in oscillationsfofwasub-f stantiallynarrowedv deviation range, demodulating a sub-multiple harmonic of the locked-in' oscillationsV with proportionately reduced deviation range, deriving from the locked-inloscillationsharmonic energy Qf a' proportionately increased deviation range, controlling the frequency reduction `of said received, wavesin response :to said harmonic energyv so as to causev said narrowed deviation'range to be equal to the difference'bef

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