US2428265A - Frequency modulation receiving system - Google Patents

Description

Sept., 30, ,1947. M. G. cRosBY FREQUENCY MODULATION RECEIVING SYSTEM Filed June 4, 1945 2 Sheets-Sheet 1 INVENTOR v '/Mwffm/ @man ATTORNEY Nw. QN

Sept 30, 1947. M. G. CROSBY 2,428,265

FREQUENCY MODULATION RECEIVING SYSTEM Filed Jun 4, 1943 2 sheets-sheet 2 /DETECTUR .7N/Jur C/Rcu/T l l l l 1 l 1 M [am] `INVENTO'R ATTORNEY Patented Sept. 30, 1947 ,FREQUENCY MODULATIGNRECEIVING SYSTEM `:Murray G.."Crosby,'Riverhead,N. Y.,.iassgnor to Radio Corporation of America, avcorporation ,ofi Delaware Application June 4, 1943,"Serial-No. '489,627

6 Claims. i1

vMy present .invention-relates to receivers of .frequency modulated carrier waves, and lmore vparticularlyStola :novel method of, land means for, :detecting :frequency modulation Waves.

FAn important object of this vinvention is to provide a method of detecting-frequencymodu- :latedcarrier-Wave energy in an entirely unconvent-ionalrmanner, vand. without employingv any of the usual forms of `vdiscriminator circuits; ac-

cording-to myin-ventionthe usual discriminator networkhaving anS-shaped detection Characteristic :is .dispensed with.

.Another obj ect` of 1 this invention is to provide tion vcomponents areathoserof:the/originally collected ffrequency :modulated carrier r`Wave, -such amplitude v.modulatedLenergybeingtheni detected in ,the conventional emanner.

Thencvel features *.which I believeto be chargacteristic of :my iinvention fare :set 4Sforth with particularityfin i the appended claims i the invention :itself, i'however, :as to both .its organization -and method; offoperation Willabesty be understood Vby vreference to the 'following description, :taken .in connection `Withthe .dra-Wing, .in vWhichofhave indicated diagrammaticallyacircuit organization whereby.` my.v invention fmay v'be carriedinto effect.

In the f drawing:

Fig. 1 shows ava receiving: systemrconstructed in 4accordance with .my invention,

Fig. ,-2,1.graphical1y illustrates the characteristic `of thedetector input circuit 18,

,Fig -3 depicts -the wave aformfofzthe loutput energy of detector a9,

VFig -4 Irepresents the lWave zform of z the l.ampli- =tude modulated f .energy at the combiner output circuit.

It-isf to be lclearlyV understood :that the system shownbyumepin Fig. ;1 .is Ypurely illustrative, .and is in no Wayf'restrictingl on: the scope ofmy generic concept. Asystated;heretofore,.the systemlshovvn in Fig. llis equally capableof receiving frequency modulated or phase modulated carrier wave.- en- -ergy or evenghybridsthereof. The genericterm angle modulated ,whenevenused :hereinafter is .Y to be understoodas :includinganyof thesefforms `nation AFM :will belhereinafter employed AWhen referring to frequency modulatedcarrier Waves. `Wherever used,.the designation' AM designates amplitude modulated carrier .Wave energy. For ithelpurposes of thefpresent specific illustration it `is yassumed that'the. signal collectorfl has-applied toit-FM wave; energy. ".Tnccenter, or'midband, frequency ofthe collectedv energy may be in any `of therassigned FM bands, as, for example, the presently assigned 42-50 `megacycle (mc.) FM .band

The numeral 2 denotes .the usual .radio fre- .quencyamplier which may comprise one or Vmorey stagesof tunable radio frequency amplification. 4Theconverter tube i3 is :of the Well known fpentagridconverter type, and 'has apipliedtofits input lgridafl the selected, amplified rFM waveenergy. yThetunable local oscillation :circuit .Ehas the oscillation grid 6 connected to theihighpotential sidethereof. The cathode of ethefconverter tube is connected to a, tap on the .oscillation :coil 'I thereby to provide a Hartley :formcf oscillator circuit. Of course,.the tank circuit 5 Willbe tuned to :a frequency such that itbeats :Withzthe midband frequency ofthe kse- --lected lilVLvs/aves-to produce in the resonant output circuit f8 ofthe converter tube intermediate frequencyiL-F.) energy Whosek center frequency isof the predetermined operating LF. value.

Inother Words, the received FM energy has Rhadits -center frequency (Fc) `reduced to the operating I.:F. value without any change in the -frequencydeviations of the signal energy. These xfrequency deviations may be of a relatively small ratio, orthey mayfbe of a large ratio. The circuit ..8 isgiven a substantially single-peak resonance curve, the center frequency (Fc) being located at fthefpeak of-theresonance curve. Attention is .directedgtovl'ligZ which shows an idealized vchar- `iacteristicof the resonant circuit-8. The circuit ...8 functions as the input circuit of a detector tube 'shown as a diode rectifier 9. The anode Iii of rectierrg scoupled tothe high potentialside of input circuit 8 through coupling condenser Il. 'Thecathode ofrectierll is grounded. The re- .sistor.|2 connects fthe anode and cathode ofthe .diode"9. In other .Words the resistor IZ acts as 4the frectifier `.load resistor, and circuit is a selective inputcircuit for the detector Q.

According to my invention the oscillatory energyeexistinginlthe `localoscillation circuit 5 is frequency modulated in accordance with the oscillatory output of a superaudible frequency oscililator designated by -numeral I3. Essentially, this Vof modulation. For the sakeof:brevityfthedesigv'55 superaudble frequency oscillator Will provide oscillations of substantially constant amplitude and constant frequency. For example, the superaudible frequency may be of the order of 20 kilocycles (kc), and any desired form of superaudible frequency oscillator may be used. I have shown the specific form of superaudible frequency oscillator which has been disclosed, and claimed, by me in my U. S. Patent No. 2,269,417, granted January 6, 1942. The superaudible frequency oscillator shown in the present case may be analagous, for example, to that shown in Fig. 1 of my said patent.

It can be stated that, in general, the oscillator comprises a pair of triode electron sections which are enclosed in the same tube envelope. A common cathode resistance I4 is utilized for the cathodes of the two triode sections. Hence, the voltage which is applied to grid I5 is fed to the cathode circuit of the opposite triode section. Utilizing the cathode circuit of section T1 as an output circuit results in 100% inverse feedback, and the voltage applied to the grid I5 of section T1 appears unampliled on its cathode. Due to this high degree of inverse feedback the grid I5 of section T1 may be swungover a wide range of voltage without drawing grid current. Therefore, the impedance presented to the tuned circuit I6 by the grid is that of a negatively biased, Vor class A, grid which is very high. In order to produce sustained oscillations the feedback voltage at.

tuned circuit IB is fed to the grid blocking condenser I'I'.

This feedback voltage must be fed back to the grid I8 of the second section T2 with a 180 degree phase reversal in order to produce oscillations. This is readily accomplished due to the fact that the common cathode resistance I4 effects such reversal. A positive voltage on the grid I5 causes more current to ow through the cathode resistor I4 and thereby makes both cathodes more positive. The cathode of section T2 becoming more positive is equivalent to grid I B becoming more negative. Thus, a positive voltage change on the grid I5 is equivalent to a negative voltage change on grid I8, and the required 180 degree phase reversal is obtained. The tuned circuit I6 is resonated, in this particular case, to the required frequency of oscillation, which has been chosen as 20 kc. The oscillations are taken from the slidable tap I9 of potentiometer 20, and the amplitude of the oscillatory output is thus regulated. These oscillations are employed to frequency modulate the local oscillations produced at tank circuit 5. The frequency modulated local oscillations heterodyne with the incoming signal energy in converter 3 to produce frequency modulated I. F. energy in tuned circuit 8.

Considering, rst, the manner in which the superaudible frequency oscillations are utilized to frequency modulate the oscillations developed in tank circuit 5 of the converter tube 3, a reactance tube 2I is used for this purpose. The latter is shown specifically as of the pent-ode type, any other type of tube may be used. The plate 22 thereof is coupled to the high potential side of oscillation circuit 5 through a direct current blocking condenser 23. The positive voltage is supplied to the plate 22 through a radio frequency choke coil 24. The cathode of tube 2l is connected to ground through a grid biasing resistor 25. Alternating voltage at plate 22 is applied to the input grid 2S of tube ZI in phase quadrature relation. To secure this phase quadrature relation there is employed a phase shifter network which consists yof a. condenser 2l ar- I5 through ranged in series with resistor 28. The upper end of condenser 2'1 is connected to the plate 22, While the grid 26 is connected to the junction of condenser 21 and resistor 28. The lower end of resistor 28 is bypassed to ground for radio frequency current by condenser 28. The lower end of resistor 28 is connected by lead 29 to the adjustable tap I9 of potentiometer 2B.

As is well known to those skilled in the art, if the reactance of condenser 21 is relatively large with respect to the resistance of resistor 28, there will be provided across tank circuit 5 an apparent condensive effect whose magnitude will be a function of the gain of tube 2l. In other Words, the plate to cathode impedance of tube 2l simulates a condenser across oscillation circuit 5 by virtue of the phase quadrature voltage applied to grid 2.6. The gain of tube 2I depends on the voltage of grid 26. The voltage of grid 26 in turn depends upon the varying magnitude of the periodic voltage derived from the superaudible frequency oscillator. The grid 26 has a normal bias determined by the voltage across cathode resistor 25, since the grid 26 returns to ground through potentiometer 2li. It Will now be seen that the local oscillation frequency is actually deviated back and forth with respect to a selected local oscillation frequency value in dependence upon the 20 kilocycle oscillations.

Returning, now, to the detector 9 there is de- Veloped across resistor I2 a voltage Whose idealized wave form has been represented in Fig. 3. The anode II] of rectifier 9 is connected to ground through a parallel resonant circuit 3B which is tuned to the superaudible frequency (specifically 20 kc). The high potential side of circuit 30 is connected by i'llter resistor 3| to the anode I0. By virtue of this network 30 there is developed thereacross the wave form shown in Fig. 3, and' it will' be noticed that at each half cycle of the Wave form there occurs a polarity reversal. Actually, the modulated energy appearing across circuit 30 is carrier-eliminated modulation.

The processV of producing this carrier-eliminated modulation is as follows: The intermediate frequency energy appearing in tuned circuit 8 is modulated by two frequency modulations. The normal desired modulation of the incoming signal is present, and, in addition, the superimposed sub-carrier which modulates at a superaudible rate. In the unmodulated condition of the incoming signal the intermediate frequency energy in tuned circuit 8 assumes the frequency Fc shown in Fig. 2. In this condition the superaudible frequency sub-carrier varies the frequency over the round top of the selectivity curve in a manner which does not produce amplitude modulation having a fundamental component. Second harmonic amplitude modulation will be produced, but will be rejected by tuned circuit 30 in the output of detector 9. Consider, now, in Fig, 2 that the incoming signal is deviated in accordance with the modulation so that theintermediate frequency energy in circuit 8 is shifted to the frequency F1. The superaudible frequency modulation now varies the intermediate frequency energy over the range between zaaezsgzes wave in .the same mann-er i but in Aopposite esensa .sincefthe signalis tuned to `the .opposite Aside of the resonance y curve.

The result of "this 'dual modulation-in-tuned circuit-8 fis an .amplitude modulated Waveiwhi'ch, when detected,=produces thewwave formas-shown inrFig, 3. have termed a Wave formofthetype shown in'Fig. '3 carriereliminated modulation. The amplitudes modulation of this Wave is detected'bydetector ilpand Wave :ofzo kc. Whose .phase andamplitude are constant. "Hence, the phase and amplitudevariationsatlgrid 32 .will add or subtract from the fixed phase and amplitude at grid 35 to provide a resultant z-kcwavewhich is amplitude modulated inaccordance with the frequency swings of the FM signals at collectori. Theresulting output of tube 33 is an amplitude modulated Wave composed ofV carrier and side bands which produces a Wave form asshown in Fig. y4. `This `Wave form is detected in detector [i to produce the desired modulation which was present on the original incoming FM signal.

The energy at the output of circuit 30 is trans mitted to the intermediate control grid 32 of combiner tube 33. Grid 32 is coupled to the high potential side of tuned circuit 3@ through a coupling condenser 34. The grid 32 is returned to ground by resistor 35. The combiner tube is shown as being of the pentagrid type, but it is to be understood that any other type of tube may be utilized. The function of the combiner tube is to combine the carrier-eliminated modulation with the original superaudible frequency oscillations. This is done by connecting the first grid 36 to the resistor 3'! of a phase adjuster network. The resistor 3l has one end thereof grounded, While its upper end is connected by adjustable condenser 38 to the condenser Il of the superaudible frequency oscillator network. The adjustable slider 39 selects from the phase adjuster resistor 3l the desired magnitude and phase of superaudible frequency oscillatory voltage. The phase of combination is selected by adjusting condenser 38 of the phase adjuster Sii-3l.

The resultant amplitude modulated signal voltage is developed in the resonant output circuit @il connected to the plate lll of the combiner tube 33. The circuit @il is tuned to the superaudible oscillation frequency of 20 kc. The detector is represented by diode rectifier t5 vvhose tuned y input circuit 42 is resonated to the superaudible carrier frequency. The rectifier input circuit l2 is reactively coupled to combiner tube output circuit All. Across the output load resistor 53, which is by-passed by condenser 44 for superaudible frequency currents, there are developed the modulation signals which may be amplified in any desired manner.

'organizationishown and described, `butv that; many v'modifications:may lloe made Without ":departing fromithe scope-ofimy invention, as fsetlforthin ther app ended claims.

51. lIn xa rreceiver .of .angle y*modulated carrier waves, a rconverterhaving input .and output circuits, .means :applying said Waves to the .'converter inputfcircuih'a local'oscillator applying high frequency wai/.esto Ythe converter to pro- -fvide-ibeat frequencyvvaves inthelconverter output circuit, said ,output .circuit v.being tuned Yto a.'predetermined beat ifrequency'and having a resonance .'curvexpeake'd .at -said predetermined frequency, meanslforvarying. the oscillator Waves La superaudible frequency, ydetection zmeans .coupledtozsaidrconverter output circuit to pro- .'cluce Wavesl of esaid:superaudibleifrequency varyu ing iniphasefandzamplitude inaccordance with the:rangle "modulation A4of the aforesaid carrier waveaxand'meanszto.combine said variable phase fand'amplitude Waves Ywith Waves of the same superaudiblefrequencyand xedphase to pro- -sduce `nesultantWaves .of said superaudible'fre- `.quency `and amplitude .y modulated in` accordance with-said zangle 'modulation .2.1Inza :receiver `of angle 'modulated carrier :wai/ea?a'converter `havinginput and output circuits, imeans lforaapplying.t received Waves to the converter.' input circuit, meansfor applying high fregquency-wavesLto'the converterito provide beat frequency Waves' in the converter output i circuit,

-said-output'circuit ybeing sharply tunedto a'. predetermined beat frequency and having a frequency response curve peaked at said predetermined frequency, means for varying the high frequency waves at a superaudible frequency', detection means coupled to said converter output circuit to produce waves of said superaudible frequency and varying in phase and amplitude in accordance with the angle modulation of the aforesaid carrier waves, means to combine Waves of variable phase and amplitude with waves of the same superaudible frequency and fixed phase to produce resultant waves of said superaudible frequency and amplitude modulated in accordance with said angle modulation, and means for rectifying the resultant amplitude modulated Waves.

3. rlhe method which includes receiving angle modulated Waves, locally producing superaudible frequency Waves, concurrently reversing the phase and varying the amplitude of superaudible frequency Wave energy derived from said locally produced Waves in accordance with the angle modulation of said received Waves, combining said superaudible Wave energy of reversed phase and variable amplitude with Waves of constant amplitude and phase of said superaudible frequency to produce resultant amplitude modulated Waves of said superaudible frequency, and detecting the resultant Waves to produce modulation signals representative of said angle moduiation.

e. in a receiver of frequency modulated carrier Waves. a converter having input and out'put circuits, means applying received Waves to the converter input circuit, a local oscillator applying high frequency Waves to the converter to provide beat frequency waves in the converter output circuit, said output circuit being tuned to a predetermined beat frequency and having a resonance curve peaked at said predetermined frequency, means for varying the oscillator Waves at a superaudible frequency, detection means coupled to said converter output circuit to produce Waves of said superaudible frequency reversible in phase and variable in amplitude in accordance with the frequency modulation of the aforesaid carrier Waves, means to combine said reversible phase and variable amplitude waves with Waves of the same superaudible frequency and constant phase and amplitude to produce resultant Waves of said superaudible frequency and -amplitude modulated in accordance with said frequency modulation, and means for detecting the resultant Waves.

5. In a receiver of angle modulated carrier waves, a converter having input and output circuits, means applying received Waves to the converter input circuit, a local oscillator applying high frequency waves to the converter to provide beat frequency Waves in the converter output circuit, said output circuit being tuned to a predetermined beat frequency and having a resonance curve peaked at said predetermined frequency, means for varying the oscillator waves at a superaudible frequency substantially less than the carrier frequency, a diode rectifier coupled to said converter output circuit to produce waves of said superaudible frequency reversible in phase and variable in amplitude in accordance with the angle modulation of the aforesaid carrier Waves, a selective filter coupled to the rectifier tuned to said superaudible frequency, means to combine said reversible phase and variable amplitude waves with Waves of the same superaudible frequency and constant phase 8 and amplitude to produce resultant waves of said superaudible frequency and amplitude modulated in accordance with said angle modulation, and a rectifier coupled to said combining means.

6. In combination, a source of high frequency waves of variable frequency, a second source of superaudible frequency waves of constant amplitude and constant frequency, means responsive to the superaudible Waves for angle modulating said high frequency waves, a detector provided with a resonant input circuit having a frequency response characteristic peaked at the center frequency of said variable frequency Waves, means applying said angle modulated high frequency Waves to said resonant circuit thereby to produce at the detector output variable amplitude waves of said superaudible frequency reversible in phase as said angle modulated variable frequency Waves pass through said center frequency value, means for combining said detector output Waves with waves from said second source to produce resultant amplitude modulated Waves of said superaudible frequency, and means utilizing the resultant Waves.

MURRAY G. CROSBY.

REFERENCES CITED UNITED STATES PATENTS Name Date Roberts Aug. 14, 1934 Number

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