US2400648A - Timing modulation - Google Patents

Description

May 21, 1946. N. l. KoRMAN TIMING MODULATION 2 Sheets-Sheet l Filed June 30, 1943 ATTORNEY May 2l, 1946. N. n. KoRMAN TIMING MODULATION Filed June 30, 1945 2 Sheets-Sheet 2 INVENTOR MNM/wa 1PA/,4m

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ATTORNEY up wave is multiplied in frequency,

. rier controlled 'oscillations nor the beating oscilla-l land all of the other 'ating voltages Patented May 21,l 1946 TIMING MoDULA'rIoN.

Nathaniel I. Korman, Camden, N. J., assigner to y Radio Corporation ofAmerlca, a corporation oi Delaware Application June 30, 1943, Serial No. 492,841'

1s claims. (01. 17o- 1715) A requirement in a. frequency or phase modulator is that the mean carrier stay Within its assigned place in the frequency spectrum. The Federal Communication Commission requires that the carrier frequency must be maintained within prescribed limits, the present limits requiring a stabilityof +2000 C. P. S. in the 42-50 mc. band or better.

Two frequency modulation systems have attracted considerable attention in the radio art. In one of'these systems a crystal controlled oscillator supplies a carrier which is modulated in amplitude in accordance with modified signals,

the side bands are selected and recombined with,

the carrier which has been shifted 90 in Phase. This resulting built up wave has a Lzharacteristic of a wave which has been frequency modulated. The carrier frequency stability is that of the crystal oscillator but in order to have low harmonic distortion a small phase variation only may be obtained in the built up wave. In order to obtain the wider deviation desirable this built six thousand times and the multiplication steps are broken up by heterodyning down steps.

This sort of system is obviously complicated and uses a large amount of apparatus.v Moreover, an'ects the frequency lstability of the nal carbecause' it is no longer that of the crystal tions. It is a modulation product of the several oscillations. The stability of the crystal confrequency thereby necessitating less multiplication and. resulting in simplication' of the entire arrangement. Bycareful attention to the frequency discriminator circuit used to derive the control potentials the carrier frequency can readily be maintained within the assigned limits. However, in any system where a large frequency deviation is used, the modulated wave consists of a large number of side bands of considerable energy content and a carrier'of relatively small energy content. This renders more difficult selection and linear conversion of the saine by the discriminator (sloping circuit) to obtain control v potentials truly characteristic of carrier drift. If the frequency deviation or excursions are reduced to a large degree, say for example, to a degree less than the lowest signal frequency, the modulation then consists of a strong carrier or mean frequency component and a few upper and say five orl the use of the beating down oscillations trolled oscillations and the beating oscillations factors including temperature variations,

in the problem, thereby rendering it more difllcult to obtain the stability required.

In the second method or system which was devised M.4 G. Crosby (Crosby Patent #2,279,659, dated April 14, 1942) direct timing modulation is used.. A' generator of the selfgeneration Y type, has its frequency modulated directly by a reactance tube and its mean frequency is stabilized by selecting some of its output, subjecting the same to discrimination and rectication and using the output obtained by rectication to 'regulate the blason a control electrode of the reactance tube in la sense to oifset or nullify tendencies of the oscillator frequency to drift from its assigned frequency.

This second method and means has been widely accepted and has numerous advantages. The initially modulated oscillations may be of higher vacuum tube dissimilarity, operj and other circuit elements enter lower side bands of relatively negligible ampli.

tude. It has been suggested that this reduction of deviation be accomplished by frequency division. This, however, entails a large number of frequency divider stages and again the system becomes unwieldy due to the large number of divider stages and apparatus used with the same. vIn my present system direct frequency modulation, as illustrated in said Crosby patent, is used, and in my new improved means and method, to reduce the deviation to derive substantially mainly carrier energy with reduced modulation components for control purposes, I make use of the principle of degeneration.

An object of my invention then is to stabilize the frequency of generated oscillations.

A particular object of my invention is to stabilize the mean frequency of generated oscillations the timing of which is modulated extensively in accordance with signals.

The above objects are attained in accordance with my invention by diverting wave energy from the generator, subjecting the diverted energy to degeneration to `an extent such that frequency or phase deviations thereof are reduced to a low 's value such that they may be subjected to phasedetection and the demodulation components used to control the mean frequency ofthe generator in a sense to Wipe out undesired drifts or variations in its meanlfrequency.

Details of the manner in which the above objects arc attained, as well as other objects and the manner o f attaining the same, will appear from the detailed description which follows. In this detailed description reference willlbe made to the attached drawings wherein Figs. 1 and 2 2 each illustrates schematically and mainly Aby block diagram, embodiments of timing modulation systems arranged in accordance with my invention, while Fig. 3 illustrates details of the phase detector used in the Amodification of Fig. 2.

I is a self-,excited oscillation generator designated hereinafter as oscillator #1. 'I'his oscillator generator is modulatedl as to its timing by signals fed to the modulation input of a modu- Vlatorl I4 coupled to the oscillation 'generator I0. This modulator may be of any approved type and herein is assumed to be of the reactance tube type similar to that shown, for example, in Crosby Patent #2,279,659, dated April 14, 1942.

The timing modulated oscillations are supplied to an output and also to a frequencyV converter I6 designated hereinafter as converter #1; In

, this converter, which may be of any well known type, the timing modulated oscillations are heterodyned with oscillations of constant frequency derived from an oscillator I8 which may be of the crystal controlled type.

The timing modulated oscillations are reduced in frequency in converter I6 and supplied to a converter designated hereinafter as-converter #2. In this converter oscillations of a reduced frequency are mixed with oscillations from a selfexcited oscillation generator 28, designated hereinafter as self-excited oscillator #2.

The output of converter #2, which maybe the sum or difference frequency, is supplied to a fre-- quency modulation detector 24 wherein the tim- Y ing modulations on the wave energy are detected sothat at the output from 24 is derived potential variations characteristic of the timing modula- Ation and of the slow changes or drifts in the" mean frequency of the intermediate frequency out of the converter in 20. This frequency modulation detector may be of any approved type, such as, ./for example, Vas shown in Crosby Patent #2,279,659, dated April 14', 1942, or in Seeley Patent #2,121,103, dated June 21, 193s. Y The output selected from 24 is fed through a lter 25, wherein the direct current is removed, and acts through a reactance tube modulator such as, for example, used at I4 to control the oscillator in 28. Units 2l, 24, 2G and 22 comprise a degenerative loop wherein, as will be set forth morein detail hereinafter, the modu-- lations on the wave energy are reduced considerably such as, for example, to a phase amplitude of less than one-radian. This degenerative loop utilizes the principle involved in Hansel! Patent #2,205,762, dated June 25,1940, and Crosby Patemiy #2,191,518, dated April 1e, 1940. 5

Intheoutputofconvertei-#2duetothisdegeneration the frequency of the carrier is substantially unchanged while the swing is' reduced to asmallphase amplitude andtheoutputisfed toaphasedetectorwhichalsoissupplied'with oscillations from stable oscillator il, the output frequency of which is the` same as the average frequency of the output of converter #2. A network 5l is provided between the output of the phase detector andthe modulator to filter lout all audio frequency currents.

Thephasedctectormaybeofanyapproved type and. as illustrated, utilm the principle involved 1n crosbyratenr #2,229,610, dated Jan-f nary 28, 1941.-

the phase detector it will be notedthattheoutputofconverterwisimpressed byatransformerlldiiferentiallyonapairof diodes I2 and 44, while the oscillations of con-YL etant frequency from Il ade-impressed in parallel Y controls the reactance tube in 26 to modulate the output from 20 so that the resultants on the A anodes change differentially in vamplitude as do the potential drops in resistance and condenser units 46 and 48. This producesacross 46 and' 48 a potential which represents the changes in frequency of the carrier in Ill. The demodulated components are filtered by resistance condenser units 46 and 48 and, if necessary, by additional' filtering circuits in 50 and supplied tothe react- 1 ance tube modulator I4 in a sense totend to oppose undesired changes in the mean frequency yof the oscillations generated in I0.

y The operation ofthe timing modulation system including self-excited oscillator #l and modulator I4 is well known in the art, and is described in the CrosbyPatent #2,279,659, referred to above.

Some of the timing modulated output energy is fed to I6 and heterodynedwith oscillations from crystal oscillator I8 and the difference .frequency is selected inv converter #l a'nd fed to converter #2. The difference frequency is taken here in preference to the sum frequency in order that the self-excited oscillator #2 may operate at a low frequency and a relatively'low frequency output may be derived from converter #2. The second intermediate frequency output of converter #2 may beat the sum or difference frequency and this output is fed to the phasefdetector 38 and tothe frequency detector 24. The

detected modulation out of the detector in 24,

self-excited oscillator #2 in a degenerative sense, so that when Athe output of 28 is mixed with the output of IB in converter #2 the timing modulation is reduced to an extent such that it isv rep# resented by not more than one radian phase l amplitude. vWhen the sum intermediate frequency is selected at the output of converter 2U the self-excited oscillator #2 is to be modulated in 'such a direction thaty the modulationenvelope thereof is'opposed in phase with respectA to the modulation envelope on the energy atthe output of converter #l in I6. By the expression modulation envelope used here I use the term somewhat in the same manner in which it is used in connection with intensity modulation.`

When the difference frequency is selected at the output of converter 20, the self-excited oscillator #2 in 28 is timing modulated in such a sense that the modulation envelope 0f the output thereof is in phase with the modulation envelope of theenergy in the output of the converter #1', i. e., the frequency deviations are in the same direction instead of opposed, 'as -is the case when the sum frequency is selected from converter #2.

polarity of the control DOtentials The proper supplied to the reactance tube modulator in 26 to obtain the desired sense f control of the self-y excited oscillator #2 in 28 to make the loop degenerave, may be obtained-in various manners. For example, reversal of the detector 24 output leads reverses the polarity ofthe control potentials. The use of an additional modulation ain-f pliiler coupling stage between detector and 26 will also reverse the polarity of thecontrol potentials; In this respect, it is noted thatA if the ^control potentialsl are of insuillcient amplitude to v accomplish the desired degeneration to obtain not more vthan one phase swing in the output anodes y of converter #2, an amplifier may be included in the regenerative loop and this amplifier may be at the output of detector 24 and serve the double purpose of correcting the control potential polarity and increasing its amplitude the required amount. The output of converter #2 of reduced phase swing is then fed to phase detector 88,*as is output from source I8 and the detected. components are filtered and used to control the moduvl quency for the #l converter in I8, and the delator .I4 to tune theoscillator V#1.

In the modification of Fig. 2, the phase detector 38" is coupled to converter #l in .I 8, to carrier source I8 and also to oscillator #2 in 28. The output of the phase detector- 88' is coupled through the audio filter 50 to the reactance tube I' modulator in I4 and through the direct current filter to the modulator in 26.

The phase detector 38' in the embodiment of Fig. 2, performs the functions of the converter #2 in I 8, the detector 24.and the phase detector 88 of Fig.. 1. As shown in Fig. 3, a phase detector somewhat as used in Fig. 1, 'modified to meet the requirements of Fig. 2, is satisfactory. The in5- put supplied .at the terminals marked Il in Figs.

2 and 3, from oscillator #2, isfapplied dinerentially to the diodes 42 and 44 by transformer 40'.

This. input is mixed in 38' withthe input Ifrom converter #1 applied in parallel to the diodes by transformer 45. Moreover, this combination or mixing is in a degenerative sense and the polarity of control of modulator 26 is made such by steps described above to insure degeneration in thev loop. 'I'he sum or difference frequency, sayvthe sum 1.01 mc., plus the radian or less phase modu- 'lation resulting from this mixing in the degenerative sense, is also detected in 88'.

'I'he output from I8 is also supplied at the leads ings appropriate frequencies for the various waves A used in the various parts of the circuit.

It is noted that the crystal oscillator I8 supplies to the converter #1 in unit IB a 4.04 mega- 6 cycle wave and to the phase detectors I8 and 88! a 1.01 megacycle wave. This unit I8 accordingly includes in addition to a crystal oscillator the-l necessary frequency multipliers or dividers to sup. ply appropriate outputs'to obtain the desired fresired frequency for 'the phase detectors 88;and 88.

I claim 1. 'I'he method of generating oscillations the timing of which is varied in accordance with signais, and of compensating variationsin the mean frequency of the timing'modulated oscillations to obtain timing modulatedoscillations of substantially constant mean frequency which includes I these steps, heterodyning oscillatory energy characteristic of said timing modulated generated oscillations with otheroscillatory energy character- `istie of said timing modulated generated oscillations in a degenerative sense to derive'resultant v energy the timing of which varies within small limits in a manner corresponding to Avariations .in the timing of said generated oscillations, .fle-'l tecting said resultant energy and varying the mean frequency of said generated oscillations in a sense to opposeand nullify said first variations .in

a0 the mean frequency thereof. n

2. The methodv of generating oscillations the -freluency of which is varied in accordance with signals., and of compensating variations in the i mean frequency of the frequency modulated 04s--l 35. cillations to obtain frequency modulated oscilla'- tions of substantially constant mean frequency` which includes these Stepaheterodyning oscillatory energy characteristic of said frequency modulated generated oscillations with other os'- 40 cillatory energy characteristic of .said frequency modulated generated oscillations in a degenerative sense to' derive resultant energy the fre,-

lquency of which varies withinabout 1 radian in marked 3 in Figs. 2 and 3, and fed in parallel'by transformer to the ` diodes 42 and 44. lThe phase detectorv 88' therefore, in addition to mix- ./.ing the outputs of converter #l and oscillator #2y in a degenerative sense and detecting the resulting current'of reduced phase swing, also detect the phase relation of the demodulation compoy 4s quency of said generated` oscillations. detecting nents so that the direct current components in modulator I4, as well as the audio components used in modulator 26, appear in the output leads,

marked 4 in Figs. 2 and 3. The diodes 42 and 44, which are non-linear circuit elements, form all possible beats lbetween the various signals impressed on them: All the beats with frequencies greater than audio frequencies are bypassed by the capacitors of resistance andcondenser units 46 and 48.. 'I'he net result is that output 4 .con-

tains only the audio corresponding to the dierin I0 relative to those generated in I8.. The oono densers 41 and 48 are to tune to resonance .the coils across which they are connected. This is not essential but is helpful in obtaining good eiliciency.

Referring again to Fig. l, it is obvious that the output of the phase detector 88 contains audio as well as direct current. This audio may be used to feed the circuits in 25, thereby dispensing with the need of the frequency detector in l24. In addition, as explained above, the #2 converter in unit 120 and the phase detector 88 may be comblned as shown-in Figs. 2 and 3.

Although I do not propose to limit my system to waves of particular frequencies, I have, merely i v for purposes of example, indicated on the drawA 1s .varied oscillations to obtain oscillations'of sub. -l

a manner corresponding to variationsin the fresaid phase variationsand varying .the mean fre- .y quency of said generated `oscillations in a sense. I

to opposev and nullify said rst variations in the meanfrequencythereof. 3. The method of kgenerating oscillationsthe phase or frequency of whichL is Varied extensively'l in accordance with signals and of compensating variations in the mean frequency of the phase or frequency varied oscillations to obtain oscillations of substantially constant mean frequency which includes these steps, heterodyning oscillatoryenergy characteristic of said phase or frequency varied generated oscillations with other 'oscillatory energy characteristic of said phase or frequency varied generated oscillations in a degenerative sense to derive resultant energy the phase of'which `varies within the limits of about A one radian' in a manner corresponding to variations in the phase or frequency of said generated oscillations, detecting said phase variationsv of said. resultant energy and varying the mean frequency of said 'generated oscillations in accordv y ance with said detected phase variations and in a sense to oppose and nullify said first mentioned 7o variations in the mean frequency thereof. v 4. The method. of generatingoscillations the.

frequency of which is modulated extensively in n accordance with signals and of7 compensating var iations in the mean frequency of the frequency stantiall'y constant mean frequency which includes these steps. heterodyning oscillatory energy characteristic of said frequency modulated gena timing modulation detector, for deriving the modulation, a secondv generator modulated by the output of the detector and a second converter eratedoscillations with other oscillatory energy i characteristic of said frequency vmodulated generated oscillations in a degenerative sense to de. rive resultant energy thev phase of which varies within the limits of one radian in a manner corresponding to 'variations in the frequency of said generated oscillations, beating said derived phase varied resultant energy lagainst oscillations of a xed frequency which is the same as'themean frequency of said resultant energy to-detect saidv phase variations, and varying'the mean frequency of said generated oscillations in accordfor heterodyning said low frequency oscillations against the output of said second generator in a degenerative sense, a timing modulation detector coupled, to said loop and responsive to said selected energy and a control circuit coupling said timing modulation detector to said first generator for controlling'its mean frequency of operation in a sense to nullify tendencies of the mean frequency of operation thereof to drift.

.9. In apparatus of the class described, a frequency modulated oscillation generator the mean ance with said detected phase variations and in a sense to oppose and .i nullify'said first.. mentioned variations in the mean frequency thereof.

5'. In a timing modulation system comprising a self-generating generator and a modulator for modulating the timing of the generated oscillations invaccordance with signals, a mean fre' A .quency stabilizing circuit for said generator ins cluding a degenerative loop to which said modulated energy is fed and from which energy comprising primarily carrier energy is g selected, a timing modulation detector coupled tosaidloop trolcircuit couplin-g said detector to said generator for controlling its mean frequency of op eration in a sense to nullify tendencies of the mean frequency of operation thereof to drift.

6. In a timing modulation system comprising a 4and responsive to saidselected energy and a confrequency of operation of which may change,` a source of oscillations of fixed frequency and a first converter for derivingcorresponding modulated and changing oscillations, means for com-- bining said derivedyoscillations and said frequency modulated oscillation in degenerative sense to obtain oscillatory energy wherein the modulations have been reduced to" phase librations of the order of one radian, a phase detector coupled to ,said last named means, and a control circuit coupling said phase detector` to said first generator to change itsmean frequency of operation in a sense to Anullify said changes in its frequency of operation.

10. In apparatus of the class described, av frequency modulated oscillation generator the mean frequencyof. operation of ,which may change,

self-generating generator and a'reactance tube coupled to said selective circuit and responsive to said selected energy'and a control circuit cou, pling said detector to said reactance tube for controlling the mean frequency of operation of said connections for deriving corresponding modulated oscillations .from said generator, other means for deriving other oscillatory energy the frequency variations of which corresponds to modulation of the frequency of said generated oscillation, means for combiningsaid two derived oscillations in degenerative sense to obtain oscillatory energy wherein the modulations have been reduced to phase librations Vwithin one radiari, a phase detector coupled to said last named 4 means, and a control circuit coupling said phase the energy of reduced deviation, a phase detector generator in a sense to nullify tendencies ofthe y mean frequency of operation thereof todrift,

7. In a frequency modulation system comprising a self-generating generator and a reactance tube modulator for modulating the frequency of the generated oscillations in accordance with signals, a frequency stabilizing circuit for said generator including a .degenerative loop to which said frequency modulated energy is fed and from which energy comprising primarily carrier and signal variations .representedby phase deviations within one radian is selected.- a phasevmodulation detector coupled to said loop and responsive to said selected energy and a control circuit cou- Fling said detector 'to'said reactance tube for controlling the mean frequency of operation .of said generator in a sense'l to nullify tendencies of themean frequency of operation thereof to drift.

8. In a timing modulation system'comprising a self-generating generator and a modulator for modulating the timing -of the generated oscillations in accordance with signals. a frequency stabilizing circuit for said generator including ill degenerative loop to which said modulated energy is fed and from which energy comprising primarily carrier energy is selected, said degen-v erative loop including a converter wherein 'said oscillations areheterodyned to alow frequelncy.

detector tosaid rst mentioned generator to change its frequency of operation in a sense to nullify saidv changes in its mean frequency of operation.

11. Inapparatus cillation generatorand modulator wherein the generated oscillations are modulated through a wide range and wherein the mean frequency of operation may change, connections for deriving oscillations correspondingly modulated and changed in mean frequency from-said generator,

other means for deriving other 'oscillatory/energy the frequency modulations of which corresponds to modulations in the frequency of said generated oscillation, means for combining saidtwo derived oscillations in degenerative sense to obtain oscillatory energy wherein the modulations have been reduced to phase librations with'one radian, a vsource of oscillations of ,a iixed frequency equal to the mean frequency of said last mentioned oscillatory energy, a phase detector coupled to said .last named means and to said last named source, and a control circuit coupling said phase detector to'said rst mentioned generator to change its frequency of operation in a 4sense to nullify said changes in its mean frequency of operation. t f

12. In apparatus ofthe class described, a frequency modulated oscillation generator the mean frequency of operation of which may change, connections for deriving. corresponding oscillations from said generator, other means for'deriving other oscillatory energy the'frequency variations of the class described, an os-' of which corresponds to changes in the frequency' said other oscillations, and a control circuit coupling ,said phase detector -to said first mentioned generator to 'control its frequency of operation in accordance with the direct current component obtained by said phase detection.

13. The method of compensating variations in the mean frequency of timing modulated oscillations to obtain timing modulated oscillations of substantially constant mean frequency which includes these. steps, heterodyning oscillatory enerey characteristic of said timing modulated oscillations with other oscillatory energy characteristic of said timing modulated oscillations in a. degenerative sense to derive resultant energy the timing of which varies within small limits in a manner corresponding to variations in the timing of said oscillations. detecting said resultant energy and varying the mean frequency of said first mentioned timing modulated oscillations in a sense to oppose and nullify said first' variations in the mean frequency thereof. v 14. The method of compensating variations in the mean frequency of phase or frequency varied oscillations to obtain oscillations of substantially y within the limits of about one radian in a manner corresponding to' variations in the phase or frequency of said first mentioned oscillations,.

detecting said phase variations of said resultant energy and varying the mean frequency of said first mentioned oscillations in accordance with said detected phase variations and in a sense to oppose and nullify said first mentioned variations in the mean frequency thereof.

15. In wave frequency stabilizing apparatus to be used to stabilize' the mean frequency of wave energy the instantaneous frequency of which is modulated in accordance with signals and the mean frequency of which may change in an undesired manner, a current frequency converter,

means to feed to said converter currents of difjferent frequencies, the instantaneous frequencies of which are modulated in a manner corresponding to the modulations on the said wave energy and the .mean lfrequencies of which may changein said undesired manner, in'a degenerativesense such that the modulations on a, beat note resulting from mixing said currents in said converter are reduced to a small phase amplitude, and means for selecting said beat note and opposing said changes in the mean frequency of said first mentioned wave energy in accordance with the phase changes of the selected beat note.

16. In wave frequency stabilizing apparatus to be used to stabilize the. mean frequency of wave energy the instantaneous frequency of which is modulated in accordance with si nais and the mean frequency of which may change in an undesired manner, a` current frequency converter, means controlled by said wave energy r to feed to said converter currents of a first frequency the instantaneous frequency of which is modulated in a manner corresponding to the modulations on the said wave energy. means ccntrolled by said currents of said first frequency to 17. Apparatus as recited in claim 16, wherein said first means includes a 'current mixer excited by oscillations of ilxed frequency and by said first mentioned wave energy with a coupling between the mixer and converter, and wherein said second means includes an oscillation generator with a wave frequency demodulator excited by l the selected beat note', a modulator coupling the demodulator tothe oscillation generator, and a coupling between the generator and the converter.

18. In a signalling system comprising a source of oscillations the timing of which is modulated in accordance with signals'and the mean frequency of which may drift, a circuit for stabilizing the mean lfrequency of said oscillations including a degenerative loop to which said modulated energy is fed, and in which the timing l

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