US2422087A - Carrier wave detector circuit - Google Patents

Description

Jurne 10, 1947. F. c. EVERETT v CARRIER WAVE DETECTOR CIRCUIT Filed Jan. 27, 1944 vr. aL

'YG AF. AMPLIFIER.

il F.

AMPLIFIEQ \NPUT FREQUENCN EN TOR.

INV FREDERICK C. EVERETT HTTORIYEY cuitztypee- The timerconstantfof Rie-Gam iszso..` offthelilvlfcarrierwistransmittedibiently:Witli chosen tirata the.gridgliinitingzaction is fastacte substantially ynoilaii'iplitudavariationdues to? het ing; Eorexampl';resistortllcan be 10050.00fohms.. selectivity characteristic Lof each'selector cireu arrd.2condenser 2'll'aboutz100 microemicro-iarads;v This-isbest accomplished-byfliaving'tliercascadd The network .M5-25! :is-.eilTectively-.s 5f; selector: circuits-'- constructedftohave :bandLpass cuitedrduring,l AMoreceptioni, itxbein'gjnrl active.: characteristics. at least 50- kdwid and-f. Hat--l operatiomonly.duringFMrreceptionW Switcharmo topped; Y Thetunedtransformerfeediiigfgridfngf Zai-provided; with spaced contacts-,Zxand'i and li corifiprisinge` tuned Lcircuit'sl.K gaand-4 22154-2535 AMcontact Sile forzthis. purposeg. The.; contact l'. should` have: such a; .flat.;-`top1fred'lresonaricei-curve2 30: issconnected. tof.tlfreaungroundechendiof."resis;.` mi characteristic atleastl .150;-keywide'a Fmr-AM feat tor.' `24",? while .ContactTr 29iisfgfreeia Theplxedv end: ception th'eAMchar-inel selector circuits'willhavec impedance..to.-currents, of-'LitheAMi-:freduency,'Y to clroserr-"tonpassa band '10'kc. wide .withuniforr'rrthellgroundedzend': of: resistor: 2.4.1 Tl'ief fixed endarripliti'ide.` Those skilledfinir-theV artk WilF readily`voff.svvitelznarrm28)A ist additionallyV connected tocaimgknowthatisuchl a-..chara'cteristici is providedi'b'ya pointfzoff.:suitableanegativef potential.; Theifnega-r. selector circuits -h'aving'ilat-topped?bandpassfre-l# tive'ibias: to blerapplied'to thefgridgllZsistfeifectivee Sponsercurves. Y t solely. Whleniswitclr: 28;-30rfiis' closed:l 'Thea-bias.V Assuming fir-'stfthatl switch'y arms-12 Siandi'l-Sasfs wi1kbe:suiiicient; togetner'with `thec-l-f25fl`Voltswen? atssvynjh.i 5j; have been adjusted for Alg/pi onfplatefz-lfto; cause theitriode sectionutopperate 2m, reception; the triode 4'sectionof tuba lwill-fnc as a class A amplifier.A When;.switch;.28-z3ris tioninvthe-manner disclosed inLmyaforesaidc Opened E for. FM receptionvtli'e4 negativa bias.- is patent. The grid -'l 2J' vvili'V haveaa i negativevbiasf removed zfromigrid 112;; andfxnetwork: 24e-21," ist ef; supplied tof it; and tlieplate lI 31" Will'fb'e 2a't-l'a rela@ fectivelzto;providesautomaticl grid-.biasing` in `re.- tively high` positive 'voltage'.v As'explained-iinamy` SPODSG'FGO input-.Voltagesdn excess., Ofa predelli- 255. aforesaid patent,A the cathodeloadlB-'acts as the minedamagnitude'; output load of the triode section, and at-th"esame Modulation voltaga. specifically" audio. fretimeexerts minimumloadilrig k'or'.turled" circuit quencyfvoltagef isgprovdedacrossload::resistor 234-26;rv This`- providesincreased gain'eandliselce: 32?k ConleCtedt lOelNVeeILv 2110116 I4* and ground'. tivity.' It-Will' be understood-thatf for `tlie opere- Condenser; 33:'shunts resistor` 32,` Aand f condenser 30;; ating'IS F. value in Alvll'reception the Circuits22`- 33 :hasta magnitud@ such. as: to .bypass :all highv ZEP-has negligiblevimpedance: TherefregI-circuitf. frequencyomponents: Diode A l I .e-lllfrfunctions, 22l-25ffdoesinot aff-ect the AM`isig`na1sfduring:AM-

therefore, as fa rectifier.oi-.modulated carrier Wave reception'. It has-been lprevious1yf=exp1aned:thatT current'appiiedito` it; Theainputelement for network, 24..- 212 is.Y effectively shmi-,aeircutm diode IIT-|41? is thefinductancefelements, IS.- It f, Th"e triode section-has :considerable/powerinane,

l5 wil'lzbe notedthatdnductance ltarranged insedli'ng capability,y fur-thier enhanced bylthefdegen ries.;with.;the.=space currentpath'oidiode .I I-I4- eration producedldue-to thefcatliode inductancei andgwithfxtheload-resistor 332;. The carriercur- His- Hence,A regulationrofl thestriodewsectionfiisi rentsacrossinductor lvwillhaveamplitude Var- Veryvgoodf thereby preventingrdi'stortionasethe@ iations regardless ofwhether VFM lor AAlvireception 4m, sig-nal Voltage'applied to diOdelI-I-MLISWmgs farf is;had.. During` EfMfzreception the action of, in.- positive with large percentages ofincdulaticnzVT` ductor; lEf is discriminatory" thereby PlOVdIlg Theerectied AMI'voltage'developed-acrossvreeL translationroffFM"Signals. into Corresponding AM sistorytfis utilized to" provideL the-auditif. free@ signals: During Alwreeption indutor I6 'iunc-v quency voltage-t0 a following f audio frequency tionsa-assa'.V cathode load-',element for` the triode 45 ampli-ner stageyas indicatedin-thefdrawingi One@ sectionofitube I0; ascmore clearlyexplained in Or-more audio-frequencyamplifiersmay'be-usedfmy: aforesaid ipatent.` and any desired-type of- "reproducen will" 'fdllovv`A Diode]v Il-lufunctionsasa rectifier during" the last audio amplifier. The gain througrm-,hee AMareCepiOmfOrtlepurpose'of providing AVC. triade section or tube ltif!isairfiity;` Haase;v no voitagei ForthisipuIDGsefload'resistor 34'iS1a1` 50 f ampli'cationfis-:obtained throughthatk'portiom lmgediimseries"between Aanode'fl and4 Cathode ofl the-tube;Y However; the-'actual overallgainfelementi-16:. DuTngAM'iSignal reCePlOIIvaIlY'ingoes-up, because offthefrelected loadinto the crease.; in 4carriery amplitudeabovef a predeterplatdof; the preceding` I. F. ampli'r Whi'c'irfis mined.1 level-r causes an:v increased rectiied voltage high/912; Thigfol-l'ws fromthgfact that.themodeA across. resistor 34.2 Thisfincrease'in Voltage is -55Useoti0m0 thetube mafuntionsias au in-finite applied inpa gain-reducing sense-to the variousr` impedancecircuit.A 'Ilnismeansfthat'tlielselec-` control grids of thc controlled tubes in networks. tiVity is, also, considerably increased; Those.- 3,?.6-and 1 overl theusual AVGY circuit. Resistor skilled in the art are Iullyacquainted with the Sin-and-lcondenser l3-provide-a'lter network'iol purpose and functions of the AVC connection...

prevent-111g: any pulsating; currentv components 60,31. Briey, the AVC bias varies directly in magfrombeingfed over1',1uelVC line 31. The-ltei.- nitude with the carrier intensity. The'gain of'f- 35?-35vis. moreover givena time Constant VValue the prior controlled ampliliers is varied in=a"sens'e-= sucnthat thef-AVC circuitis SlOW-actrlgyandsresuch as to obtain substantially uniform carrie'r SDOIldSonlyftO slow changes-in carrier amplitude. amplitude at input circuit 23-26"*regardless"of- Resistors viiifaref-included in the' connections from 65 wide carrier'amplitude.variationsiat thereceiver theffAVC line =to eacheofthe-controlled tubes to sourceZ.r`

provide additional 'lterings, The AVC circuit is 'lhepresencelof the two diode'anodes-.llilandilr. eiective only during AM reception, anddoes not inthe. doublediode-triode vtype of vtube makes it'` unGOn. dllrlgY-FMfIBCePOHbecause Of thecureasy-,toutilize one `diode to. deliver the audio freev rent-limiting characteristics of thetriode seciton 70 l quency.voltage and: theother to suppl-ysthe-AVC,t oftubenlfduring suchreception. bias;- Thispeases the lter requirementgfaids;

The selector'vcircuitsfeedingY thefsignal thetst-ability/cf-fthe receiver; and avoidsiloa-dingii entre?.V tofgrd IZt-SSu-mngf tha-System is .adoffthe-diodef aud-iceircuit wh-ich; might camedia--v justediory-.FM reception; will have in-overall setortion.- However;- it y is-` torfbe :understood: whats) lectivityf characteristic sucht'that itheentire:;swingj 75lbothw audio andi AVCy voltages: can. be. obtained;

Within'thetube envelope.' -When necessary the cathode coil I6 should be shielded.

. v,Since cathode bias cannot be used withoutlbia'sing the Vdiode-anodes, a separate sourceof nega-` tive bias Vis applied. This Vmay Ibe present in the receiyerrwithoutany additions. -Ofcourse a special biasing cell may be used, or a small dry cell. The lter resistor'usually presentiingthe' negative power supply leadof a receiver will provide the necessary fewvoltsv of negative bias..l 1f desired,v a part of the AVC bias may be used for the same purpose, although kthis is ,not an advan-` tageouszexpedient. Of course, the-diodes maybe provided by a separate GHS type of tubegwithout change in the circuit shown ink Fig. l. I K

'Ihe triode section of the tube I0 has sufficient negative bias on the grid to prevent it from going positive. Hence, the tuned circuit y23--26 is not loaded. However, the grid voltage is varied at aradio frequency rate, and the plate VCurrent` variations correspond to,Y the grid voltage excursions. The plate current flows through the YYcath-- ode'irnpedance I6 acrosswhich each of the diodes is shunted, and which provides excitation for each diode. y Y

With very slight modification, consisting only ofchanging the direct current voltages applied tothe grid I2 and plate I3, the tube II) is made to Ytranslate FM signal energy to corresponding AM energy, to remove undesired AM eects (noise), and to rectify the translated desired AM energy. In other wordsit is only necessary to shift switch arms 28 and I9 to the FM contacts, the switches 4 and 5 being as shown. The negative bias is now removedfrom grid I2, and netav crease in the alternating current component of plate current. Fig. 3 shows the resulting input v. output characteristic of the triode section of tubeV I for FM reception.

eliminated by virtue of the characteristic of Fig. 3.

The FM signals applied to grid |12 are derived from input circuit 22--2 5. The AM circuit 23--26` is ineffective, since the condenser 26 acts Yas aV short-circuit at the I. F. value'of 4.3 rnc.4 Whereas for AM reception the frequency of the Vapplied Undesired ,currentv amplitudevariations across cathode load I6 are signal energy Awas a constant and only the sig- Y.

nal intensity varied at grid I2, in FM reception f=frequency Y Lfe=inductanceof cathode coil I6.

Since the plate current is a constantY due to op-l eration along the saturation portion of the characteristic of Fig. 3, it follows from this formula that the voltage across Ythe inductor I6 will defy pend upon the frequency Variations of the ap-v plied input voltage. Also due to the limiting action Vof the circuits, undesired amplitude varia` tions which appear on the FM wave at input cir-` cuit'22-25 are substantially prevented from appearing across the inductivefload I6. Y Noise impulses will be prevented from passing to therec-l repeated across the cathode load I6. This meansf that the'various tubes in networks 3, 6 and 'I'will l Y s tier HLM by the lsame token.'

cathode inductorl; Sin'cediode il I-I4 is connectedfacross the element I6, the resulting volt-` agevariations will` loe-rectified. Since in the case of applied FM'signals the Yvoltage variations across load I6 are .the YAM variations correspondingito the instantaneous deviationsofV theFM carrier.. the inductor I6 actually functonsas a discriminator device to translate FM signals into -V corresponding AM signals.Y

The variation in inductive reactance of inductor .I6 with changes in frequency is graph'- ically presentedin Fig. 2. The dotted Vline curveV is Vthe ideal characteristic. In practice the linear relation is'secured only by r'employing aY cathode loadwhich may become somewhat complex. By

suitable selection of values the load I6 can be a simple', inductance, having' the characteristic shown by the full line of Fig. 2.

f It will be seen from Fig. 2' that the actual characteristic has a resonant peak at some frequency.

This peak frequency should be chosen, of course, so that it is'not too far'from the center frequency Fc of the applied FM energy. ForV example, employing for coil I6 a magnitude (of, about 200 microhenries the coil will have a naturaly peak frequency of about 4.2 mc. `It is desirable to have the operating I. F. value fall on a linear point on either slope of. the solid line characterisek tic shown in Fig. 2. Two possible points of operationa. and b are shown in Fig'. 2, and inthe case of the operating point a on therlowerfrequency slope the Ycorresponding operating point a on the ideal characteristic is shown by Way of comparison. In actual practice it is found that the high frequency slope of the characteristic is more linear than the low frequency'slope. Point b on' the solid line curve would be the operating point (4.3 me). Where the frequency deviation ratio of the' cathode load. Where the applied kFM signals, however, are of a relatively small deviation ratio, as where the frequency swing is of the order of v1,5 kc., it is preferred toemploy an oper-` ating I.F. value such that operation willY take Vplace on either slope'of the characteristic close to the peak where the slope is a maximum; The reason for this difference in operating point vis that the greater the slope at Vthe operating point on the'characteristic shown in Fig. 2, the greater Y ilvill be theV audio voltage delivered by rectifier VYShould it be desired to approximate the ideal characteristic; instead of aY simple coil I6it may e Y be necessary to employ in place Ythereof Va specially designed network wliich rhasra more linearim- Y patience-frequency characteristic-3 lln general,

the designof'the cathode impedance I6 dependsV Y upon the frequency swingl of the applied signals@ and 'the'permissible distortion. Y Combinations of series'and parallel resonant circuits could be used f in place'of coil YI6 to improve linearity ofV the Vimpedance-fr'equency characteristic. 1 Y

The modulationvoltage developedacross -resistor 312 is utilized in the following modulation Y amplifier network. The nAVO circuit 3'Iiis-y au#YYY tomatically inoperative for FM reception, VVVbecause input signal amplitudevvariations are not However, any.v variations in fre'quency'of input energy will causeVH corresponding changes in the voltage across the engages? 9 i.-=0perafteV atlmaximumeg 'n @sincejlNC-i bias has ff'been'f 'removed l'irom-'t fv'a i-ousereceiv-er itu-bes. important-advaiitarge?I '-in therceiver I on/becausethefvarious tubese-areieperated ffatealiixed 'biasitherebyf'augmeting the'lirn-iting action in the triode section of tube I9. f

'fWhilelIfhaveindieatedland described a system for carrying my linvention into effect, it Will be apparentitorrone"'skilled"in"the` art that my A-,sifnvention :isi by*L noumeans limitedrto' the .particular organization shown andxdescri'bedfbut that many modicationsimayn-be .Inadenyithout departing fromrthe"scopeol'my invention, as set forthrin the appended claims. Y

` liNhatQIi-:olaimis: fr

1. In combination with a tube having at least a cathode, input electrode and output anode, a frequency modulated carrier wave inputl circuit connected between the input electrode and ground, an inductive impedance adapted to be traversed by space current of the tube connected from cathode to ground, said impedance consisting of an unbypassed inductance whose natural frequency is of a magnitude such that it acts as a frequency discriminator, at least one diode connected to include said impedance as a signal input element. and means in circuit with the inductance and diode for developing modulation voltage from amplitude modulated carrier current rectified by the diode.

2. In a demodulator system for modulated carrier current, a tube having at least a cathode, a control grid and plate, a carrier input circuit between the grid and ground, a source of amplitude modulated carrier waves coupled to said input circuit, an unbypassed inductance connected from said cathode to ground, a rst diode in shunt with said inductance, a resistor in circuit with the diode to develop rectiiied amplitude modulated carrier voltage, a second diode in shunt with the inductance, a second resistor in circuit with the second diode to develop a second rectified carrier voltage for gain control, a second input circuit in circuit with said control grid, a source of angle modulated waves coupled to the second circuit, said second source providing waves of a frequency widely spaced from the frequency of the first source, means for changing the operating direct current voltages of said grid and plate, when demodulating angle modulated waves, to an extent such that the space current through the inductance is limited at a low wave amplitude, said inductance having a natural resonant frequency such that it functions as a discriminator to translate angle modulated current into corresponding amplitude modulated current, and said nrst diode rectifying said translated current to provide modulation voltage across said first resistor.

3. In combination with a tube having at least a cathode, input electrode and output anode, a frequency modulated carrier current input circuit connected between the input electrode and ground, an inductance coil connected in circuit with the input electrode and from cathode to ground, said inductance having a natural frequency such that it possesses a frequency discrimination characteristic, at least one diode having its anode connected in a closed series circuit With said inductance through solely a resistor, and means for developing audio voltage across said resistor from rectified modulated carrier current owing through the diode.

4. In a demodulator system for frequency modulated carrier current, a tube having at least a cat-node, a controlI grid and plate,fa"carrierfinput circuit, an Y unbypassed inductancemonnectejdiin quency'imodulated input-*'curren't'is vtra'rrslated -ageiacrossitheinductance. y IInv a idemddul atorsy'sten `f or mddi-rlated'l vc arv*rat illea'stlafcath'ci'de,fea-"control )grid J andfl'zal'arte,J a --carrierinputrcircuitbetween thegridandfground,

a source 'of amplitude modulated carrier Waves coupled to said input circuit; the improvement which comprises anl unbypassed inductance connected from said cathode to ground, a first diode in shunt With said inductance, a resistor in circuit with the diode to develop rectied amplitude modulated carrier voltage, a second diode in shunt with the inductance, a second resistor in circuit with the second diode to develop a second rectied carrier voltage for gain control, a second input circuit in circuit with said control grid, a source of angle modulated waves coupled to thesecond circuit, said second source providing waves of a frequency widely spaced from the frequency of the first source, means for changing the operating direct current voltages of said grid and plate, when demodulating angle modulated waves, to an extent such that the space current through the inductance is limited at a low wave amplitude, and said inductance acting as a discriminator to translate angle modulated current into corresponding amplitude modulated current.

6. In combination with a grid-controlled electron discharge device, said device including at least a cathode, anode and control grid, an angle modulated carrier current input circuit connected to the grid, an unbypassed coil connected between said cathode and the low potential side of the input circuit, said coil being located in the space current path of the device, the operating direct current voltages of said device being chosen to permit it to saturate at a low input current value, said coil having a natural frequency differing from the carrier frequency by a predetermined amount so that the coil acts as a frequency discriminator of the input Waves, and means in circuit with said coil to derive modulation signal voltage therefrom.

'l'. In a demodulator system for frequency modulated carrier current, a tube having at least a cathode, a control grid and plate, a carrier input circuit connected between said grid and ground, an unbypassed inductance connected in series between ground and said cathode, said inductance being the sole reactive element between the cathode and ground and having a frequency discriminating characteristic whereby frequency modulated input current is translated into corresponding amplitude modulated carrier voltage across the inductance, and means in circuit with the inductance for deriving modulation signals from the amplitude modulated carrier voltage.

8. In combination with a tube having at least a cathode, input electrode and output anode, a frequency modulated carrier input circuit connected between the input electrode and ground, an unbypassed coil connected from cathode to ground, and said coil being naturally resonant to a frequency sufficiently different from the carrier Vfrequency*Wherelcyv the coil acts as a frequency discriminator. y

j 9. In a demodulator systemga tube having at least a cathode, a, control grid and plate, a signal input circuit between the grid and ground,

a.v source of amplitudemodulated signal waves Ycoupled to said input circuit, an unbypassed inductance connected from said cathode to ground, a rectifier in circuit with said inductance, a resistor in circuitwith the rectifier to develop modulation voltage from rectified amplitude modulated signal waves, a second signal input circuit in circuit between saidV control grid and ground, a source of angle modulated Waves coupled to the second circuit, said second source providing waves of a, frequency substantially different from the Y frequency of the rst source, and said inductance current.

being of a magnitude such that it functions asa discriminator to translate angle modulated current `into correspondingV amplitude modulated FREDERICK c. EVERETT Hunt Y oct. 21, i941.

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