US2296962A

US2296962A - Frequency modulation

Info

Publication number: US2296962A
Application number: US310495A
Authority: US
Inventors: Tunick Harry
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-12-22
Filing date: 1939-12-22
Publication date: 1942-09-29
Anticipated expiration: 1959-09-29

Description

H. TUNICK 2,296,962 FREQUENCY MODULATION Filed Dec. 22, 1959 s sheets-Sheet 1 /S i Q* TS Saw@ |El w` Jf www wm l INVENOR. H RRY TUN/CK ATTORNEY.

Sept. 29, 1942.

\NNN WNNJ up" QWN @MN Filed Dec. 22, 1959 6 Sheets-Sheet 2 H. TUNlcK '2,296,962

FREQUENCY MODULATION Filed Dec. 22, 1939 6' Sheets-Sheet 5 Sept. 29, 1942.

Sept. 29, 1942. H. TUNICK 2,296,952

FREQUENCY MODULAT ION Filed Deo. 22, 1939 6 Sheets-Sheet 4 750 7587 760, @762l I AM- DET AMPL 7 pfcr. l 754 z AMpL. Reer. DET' l 752 El TUBES B/AsED To c1/7'0F/=\ MAY BE 2y-PASS PARALLEL L//vso c/RcL//r Z/NED PPOS/ TELY 70 OPPOSITE EX TREMES 0F BAND By-PAss 850 coNoL-'N- u 9.5/25

INV EN TOR.

i RRY TUN/CK BY v/WW A TTORN E Y.

Filed Deo. 22, 1939 6 Sheets-Sheet 6 INVENTOR. HARRY TUN/CK www ATTORNEY.

Patented Sept. 29, 1942 FREQUENCY MoDULA'rIoN Harry Tunick, Rye, N. Y., assignor to Radio Corporation of America,

Waffe a corporation of Deia? Application December 22, 1939, serial No. 319,495

' 2o claims. (ci 25o-s) The inventions described herein relate to frequency modulation systems and apparatus.

A f ew of the objects of my inventions are toprovide improved automatic frequency controlling circuits for frequency modulation transmitters; to provide' 'over-modulation indicating and controlling means for frequency modulation transmitters; and to provide improved apparatus for eiecting frequency modulation.

In the drawings:

Figure 1 is a wiring diagramof a frequency modulation transmitter having a circuit for automatically bringing the transmitter back to its mean assigned frequency. Means are provided whereby this automatic correction is operative only upon cessation of the modulating signal. Other features also are present.

Figure 1A is a wiring diagram of an over-modulation indicating and correcting system which may be applied to the transmitter of Figure 1 Figure 1B is explanatory of Figure 1A.

. Figure 2 is another form of frequency mod-ulation transmitter.

Figures 3, 4 and 5 are wiring diagrams of other frequency modulation radio transmitters.

Figures 6 and 7 are characteristic curves ofampliers'which may be used in conjunction with transmitters described herein and receivers for receiving the transmissions from the transmitters.

Figure 8 is a block diagram of a frequency and phase modulation signaling system.

In Figure 1, the signal pick-up device 2 feeds signal amplifier 4. The pick-up device 2 may be a voice microphone or a television or facsimile pick-'up unit. If light signals are picked up, amplier 4 would be a video amplifier. vThe amplified signal from amplifier 4 is fed through transformer 6 and lead 8 to the screen grids I0, I2 of reactance tubes I4, I6. In case video signals are transmitted, radio frequency oy-pass condenser I8 should be made smaller so as not to by-pass the highest video frequencies from amplifier 4 or, if desired, I8 may be omitted.

The reactance tubes I4, IB act to change the tuning of the grid circuit 28, comprising coil 22 and condenser 24, an amount proportional to the amplitude ofthe signal fed through transmodulated high frequency oscillations in the tank or plate circuit 34 comprising inductance coil 3B and condenser 38. oscillations are fed through condensers 48, 42 to a power amplifier and/or limiter 44 and then to additional power amplifiers and/or limiters and/or frequency multipliers 48 and radiated sired, it may be coupled to the grid coil 22.` The' former 6 and at a rate or frequency identical to I the frequency of the signal fed through transformer 6. As a consequence, the push-pull connected

oscillator tubes

26, 28 regeneratively cross coupled by condensers 30, 32, produce frequency by antenna 48.

To prevent drift in the mean frequency of the push-pull oscillator 26,'28 a coil 5I) is coupled to either the plate tank coil 36 as shown or, if deoscillator` energy picked up at coil 58 is 'fed to the

suppressor grids

52, 54 of

tubes

58, 58 through leads 8 8, coupling coil'64 and tuned circuit 62. Between the grids 64, 68 of

tubes

56, 58 there is connected a piezo-electric crystal 88 oscillating at some suitable frequency higher or lower than the mean frequency of oscillation of push-

pull oscillation generator

26, 28. Tuned circuit 18 connected. between the cathodes ofA

tubes

56, 58 is tuned to the crystalfrequency or to some harmonic thereof. The .screen grids 12, 'I4 are grounded for radio frequency currents by the action of by-passing condenser i8 and serve as the anodes for the oscillating system'formed by the crystal 88, tuned circuit 'I0 and the cathodes 'of tubes, 56, 58. Electrodes 18, 'i8 serve to screen the crystal controlled oscillating system from the suppressor grids and plates of

tubes

58, 58. All

of the electrodes of

tubes

58, 58 are suitably polarized or biased as shown. The beat frequency oscillations resulting from inter-action of crystal controlled oscillations and those from the transmitter generator fed through coupling coil 64 are resonated in the tunable plate circuit 80. This beat frequency is preferably of a. lowerfrequency than either the crystal frequency or the mean frequency of oscillator-tubes 26, 428, although if desired it may be higher than either the crystal frequency or the frequency of oscilcoils s4, es are symmetncauy coup1ed to either c0il82 or to the coil of circuit 80 or to *The frequency modulated both.

Coils

94,96 are arranged so that the electromotive forces induced therein are additiveand 'these electromotive forces are fed in push-push to the midpoint 90 of resonant line 84. If deare symmetrically variably tapped, as shown, to

the resonant line 84 and excite the grids |,I4, ||6 of the detectors II8, |20. The grids of'detectors II8, |20 are biased through lead |22 and tubes could be taken in a similar fashion from the plate circuit' 34 as was done from the grid circuit 20 and, of course, the cathodes or filapotentiometer |24, suitable chokes |26, |28 being provided to prevent high frequency currents from passing into lead |22. A resistance |30 is connected between the plates of detector tubes |I8, and shunted across each portion of re,

sistor are time constant 'condensers |32, |34.

By virtue of the foregoingarrangement, should the frequency of

oscillation generator tubes

26, 28 drift from'its mean assigned value for any reason, the rectified current output of tubes II8 or |20 will be larger depending upon the relative drift .in frequency of

oscillation generator tubes

26, 28. 'This' change in frequency produces a voltage drop across leads |36, |38 which is added to or subtracted fromthe Voltage from potentiometer |40 fed through resistance I 42 to the suppressor grids |44, |46 of reactance tubes I4, I6. Reactance tubes I4 and I6 are drawing lagging or leading currents depending upon the adjustments of condensers |46, |50 and resistors |52, |54. Inj-'other words, by virtue of condenser |48 and resistance |52 the voltage Aimpressed upon grid |56 of tube I4 is such as to make that tube draw either a leading or lagging current with re.

.ments of these additional tubes would be grounded. This m'odicatiomin which another pair of reactance tubes is connected vto the plate circuit 34, is illustrated in Figure 1C.

As shown in Figure 1C, the suppressor grids RI44 and RI46 of the plate circuit reactance tubes RI4, RIG are. connected in parallel and to the suppressor grids |44, |46 of reactance tubes tion produced by the signal, part of the signal spect to the current in the grid circuit 20. So

such a voltage as to cause tube I6 to draw either a lagging or leading current.4 The voltage impressed upon the suppressor grids |44, |46 through leads |36, I 38 andresistor |42 is made to be in suchadirection' as to correct for de- A partures in frequency of oscillation generator 26,

28 from its mean assigned frequency and to bring it back to thatvmean assigned frequency for which the voltage difference across leads |36, |38 as derived from resistor |30 is zero or negligible. The plates of the reactan'ce tubes I 4, I6 may be supplied with potential from any suitable source such' as |60 feeding the plates through choke coils' |62, |64. Bypassing condensers |66, |68 may be provided if desired. Also, blocking condensers |10, I'I2 are used to connect the plates of the reactance tubes to the grid circuit 20.

Frequency modulation of the oscillations produced by

tubes

26, 28 is accomplished by impressing signal voltages through transformer 6 Vand lead 8 upon grids I0, I2, thereby effectively varying the amount of leading or lagging current 'drawn by the reactance tubes in accordance with

system

26, 28.

If desired, another pair of reactapce tubes may be connected similarly to the plate circuit 34 of the push-pull oscillation generator, in which case the suppressor and screen grids would be connected in parallel to the suppressor'and screen grids of tubes'I4 and I6. Phase displaced voltalso for reactance tube I6, the condenser |50 and resistor l|54 cause grid |58 to be subjected to' voltage may be fed through switch |80, signal amplifier |82 and rectifier |84, Whose rectified voltageis impressed across the resistance condenser .time constant circuit |86. Arrangements should be made as regardsthe amplifying power of the amplifier I 82 and voltage developed by rectifier |84 so that in the presence ofthe lowest value of signal desired to be transmittedithe voltage impressed by rectifier |84 in series with the voltage from potentiometer |24 is such as to bias detector tubes |20, |I8 to cut off. In this way, when the device 2 picks up signals to be transmitted, the frequency controlling circuits are rendered inoperative. Obviously, the system of Figure l may be operated without this auxiliary control by opening switch |80, in which eventautomatic frequency control of the system will take place at all times. With switch |80 closed, frequency correction takes place only in amplitude changes in the oscillation generator is ages on the grids ofthese additional reactance fed from time-constant circuit |94 and leads |96 to, resistor- .|98 in series with the grid biasing lead 200 connected through coil 22 to the grids 202, 204 of the push pull

connected tubes

26, 28. Leads |96 should be so connected that-the voltage inserted in series\ with thegrid biasing lead 200 from the fast a. v. c circuit |94 is in such a direction as to wipe out amplitude changes in the oscillation generating circuits to which coil |92 may be coupled. If desired, of course, coil |92 may be coupled to an output circuit of either limiter 44 orl an output circuit or input circuit of circuits =46 should circuits 4 4 or 46 imperfectly eliminate undesired amplitude variations.

Leads y2|0, 2I2 -of Figure 1 are identical to leads 2|0, 2I2 of Figure 1A which illustrates an over-modulation system for a frequency modulation transmitter. Assuming, as shown in Figure A 1B, that the frequency channel allocated to a frequency modulation transmitter is represented by-the frequency range between points A and:B, then tuned circuits 2|4, 2|6 connected to. the coupling antennas 2|8, 220 are tuned so as to have resonant points R, R respectively at or slightly beyond the frequencies A-and B. AAs a consequence, should the transmitter frequency be shifted an amount greater than the deviations A and B away from the mean assigned frequency M ofFigure 1B, current will be induced in the amplifiers 222, 224l in turn coupled to

rectifiers

226, 228. The rectifiers are connected so as to feed current through resistor 230 in series with the polarizing battery or source 232. In the presence of over-modulation, leads 210, 212 are so connected as to inject a voltage in resistor 234 (Fig.

l) in' series with-the signal lead 8 in such a di` rection as to cut down the amount of frequency swing of the oscillation generating system. This auxiliary control may be eliminated, -if desired, by opening switch 236. In the alternative, or in addition to this auxiliary control, a bell 238 and connected .through lead 244 to resistance 230 of Figure 1A.

In the transmitter of Figure 2, microphone 300 feeds an audio frequency amplier 302 whose characteristic is as shown in Figure 2A. That is, amplifier 302 should have a fiat 'characteristic along the range Ato B of about to 500 or 10 to 1000 cycles. From points B to C the characteristic should rise preferably linearly. 13 to C represents theA remainder of the audio frequency range which may be as desired from 500 or 1000 to-5000, '1500 or- 10,000 or more cycles. When such an amplifier is used at the transmitter, the frequency modulation audio frequency amplifier at the receiver should have a complecomes active to produce an indication in the bell or lamp indicator 354 and also inject a' gain reducing voltage in the automatic volume controlling lead 356 leading to the same or another a light 240 may be actuated through relay 242 amplifier stage in audio frequency amplifier 302. If desired, the system may be operated without the automatic volume controlling feature, in which case switch 360 and/or switch 362 may be opened.

Incidentally, in connection with Figure 1, the

condenser of tuned circuit 80 may be omitted.

and, if desired, may be replaced by a resistance to broaden the pass band of energy fed into coil 82.

In Figure 3 I ,have illustrated another irnproved frequency modulation transmitter. The crystal 100 controls the frequency of'oscillation of the crystal controlled oscillator tube V102 which is-provided with an inductive output circuit or mentary characteristic, namely, fiat along the frequency vrange from A to B and then :drop o from the frequency range B to C, as shown in Figure 2B.

The output of audio frequency amplifier 302 is fed through transformer 304 and additional transformers 306, .308 to thegrids of reactance tubes 310, 3 I2. Audio frequency by-passing con-y densers 314, 3I6 are provided as are also radio frequency chokes 3I8, 320. Depending upon the adjustments ofjreactance tubes 310, 3I2 the crystal 322 is effectively shunted by variable capacity or inductance represented by tube 310 andthe plate circuit ofthe crystal controlled oscillation generator tube 324 is also shunted by a variable inductor or capacity represented by reactance tube 3I2. Both Vtubes 310, 3I2 should be adjusted so as to addA similar reactances,

namely, capacitive or inductive, to the grid and plate circuits of crystal controlled generator 324.

If desired, of course, thecrystal may be replaced by a tuned circuit or a quarter wave resonani; line. The audio frequency voltages applied-to the reactance tubes 3|0, 3|2 may be adjusted by virtue .of

taps

330, 332 on the potentiometer shunting the secondary of transformer 304. The output of the oscillation generator may be fed through a limiter and/or power amplifier and/or frequency -multiplier 334 whose output as radiated by the antenna.336.

As before, over moduation pickup antennae or coils 338, 340 are provided, antenna 338 feeding into a circuit 342 tuned to or slightly beyond one 4extreme frequency of the channel assigned to the transmitter of Figure 2. Similarly, tuned circuit 342 is tuned to or slightly beyond the other extreme frequency of the channel assigned to the transmitter. Circuit 340 energizes amplifier 344, in tum connectedto rectifier 346.

' When the swung exceeds the channel allotted to the transmitter, an indication is produced in 'indicator l340 and to automatically prevent this coil 104 which may be shunted; if desired, by a tuning' condenser. In the absence of signal from the signal amplifier 106,.tubes 108, are, biased to cut off by voltage source 112. Tubes 108 and 1I0 are made alternately conducting with audio frequencies transferred through transformer 114. Coil 1|6 is so wound as to increase the inductance reected into coil 104 with v'current flow through tube H0 and coil 118 is so wound and polarized as to decreaseby refiection the effective amount of inductance in coil 104. Consequently, the frequency of oscillation of oscillation generator tube 102 is varied in accordance with signals from amplifier 106. The output of the oscillator is fed toA power amplifiers and/or limiters and/or frequency multipliers and then radiated over antenna 132. The crystal may, of course, be replaced by a parallel tuned circuit by throwing switch 142 to its upper position 144.

Preferably, by adjusting the volume of signal fed to the various transformers described herein and by suitable adjustment and choice of the frequency multipliers, I prefer to radiate a wave whose frequency is equal to, for the loudest signals, any integer up `to one hundred times the highest modulating frequency plus the highest modulating frequency. Thus, if we assume the highest .modulating vfrequency to be 5000 cycles and the integer Ato be, for example, five, then the.radiated carrier frequency deviation should be five times 5000 plus 5000 or 30,000 cycles, making the total band width used by the transmitter 60,000 cycles. For the case where the integer is, for example, 100, the deviation is 500,000 plus 5,000 under the above rule,

thereby making the total band width employed .when frequency modulating with speech waves,

the transmitter may go beyond one .extreme of its assigned channel and not beyond the other,

As one'iway of remedying this situation, I have provided in connection with the transmitter of Figure 3 two pick-up

antennae

150, 152 coupled to the radiatingantenna 132. Antenna 150 feeds tuned circuit 154, tuned to or below the lowest frequency of the frequency channel assigned to the

transmitter

102, 130, 132 and tuned circuit 158 is tuned to or above the highest frequency of the channel assigned'to the transmitter of Figure 3. Tuned circuit 154 feeds amplifier rectifier 158 and detector 160. The detected output, when the frequency of the transmitter goes beyond the lower limit of the,assigned channel, causes indication in an indicator 162 and also impresses a voltage across resistor 164 in series with the grid 168 of a tube 1| 0. The voltage impressed in resistor 184 is in such a direction as to cut down current flow through tube 1|0 and, hence, reduce the effective inductance reflected into coil 104 by coil 1I6. 'I'hat is, assuming coil 1I6 withl increasing current therethrough causes an effective increase in inductance of coil 104, Vover-mod ulation in one direction as detected by the action of coil 154 will manifest itself through the voltage drop in resistance 164 to reduce the frequency swing in that direction in which overmodulation is being produced.

Coil 1I8 acts effectively to reduce the effective inductance of coil 104 and hence increases the frequency of oscillation of tube 102 with increasing current flow through coil 1|8. When the radiated wave from antenna 132 goes above its I upper assigned frequency current will be produced 840 is identical toqthat within the dotted rectangle 842. By the action-of switch 844 either the crystal 848, parallel tuned circuit 848 or reso-,-y nant line 850 is connected inl the grid circuitfof the high/ frequency oscillation generatdrm852. The output of the generator 852 vis fed 'through power amplifier and/or limiter and/or frequency multiplier to antenna 856.

` Obviously, both

reactance tubes

800, 802 may be adjusted so that they both draw leading currents or both' draw lagging currents. Preferably,

coil 8|6 is omitted and condensers 8|,2, 8|8 are made identical in size so that both

reactance tubes

800, 802 act alike.

-In the transmitter of Figure the oscillator tube 800 may have either crystal 802 connected lbetween the plate 804 and grid 806 or, as shown,

` the series tuned circuit 808 is Aconnected by means of switch 8|0 between the plate 804 and v grid 808. The screen grid 820 is supplied with voltage through radio frequency choke 822 and by-pass condenser 824. The plate 8044 is supin tuned circuit 152 and the current in this circuit will be amplified by amplifier 180 and rectified by rectifier 182.- This rectified current produces an indication in the indicator 184 and also produces a voltage drop across resistor 185 in series with the grid 188 of tube 108. This voltage drop should be arranged in such a way as to reduce the current flow through tube 108 and hence through the action of coil 1|8 upon coil 104 bring the frequency of oscillation of tube 102 WithinA its assigned channel.

plied with plus voltage through choke 830 by passed by by-passing condenser 832. lTransformers 834 and 838 insert alternating signal voltages in the plate and screen voltage leads. 'I'hese signal voltages are derived from potentiometer 840 connected to the secondary of transformer 842 whose primary is connected across the plates of push-pull amplifier 844 energized from audio or signaling amplifier 946. Since like voltage changes applied to the screen 820 and plate 804 produce opposite effects on the frequency of oscillation of tube 800 transformers 836 and 834 are so polarized as to oppositely swing the screen 820 and plate. 804 inl voltage in Incidentally, it is to be noted that` the unsymmetrical characteristic of the amplitude time characteristic of speech waves may cause the automatic frequency controlling system of Figure 1 to drag the oscillator over a period-.of time to one side of its mean assigned frequency especial-- ly when the system of Figure -1 is operated with switch |80 open. To correct for this, taps 880, 882 of Figure 1 may be unsymmetrically tapped about the midpoint of resistor |30 until the unsymmetrical characteristic'of the modulation in.. troduced at point 2 is compensated. To facilitate vthis adjustment, bearing -in mind the unsymmetrical characteristic of speech waves, a reversing switch 884 (Figure 1) may be provided for reversing the polarity of the signal fed into lead 8 through transformer 8.

In the transmitter of Figure 4,

reactance tubes

800, 802 are made more conductive by the action of transformer804 associated -with-the cathodes of

reactance tubes

800, 802. Transformer 804 is energized by signal amp1iflerv808 in turn connected, if desired, to a'microphone 8|0.l Condenser '812 and resistor 8|4 are adjusted so as to energize the grid 8I6 with a relatively leadingA voltage. Condenser 8I8, coil {H6-and resistance 820 are adjusted so as to impress a relatively lagging voltage with respect to the voltage at point 822 uponthe grid 824 of tube 802.l The cathodes of the reactance tubes are connected to a source of. potential 830 such that both tubes draw a mean current in the absence of signal from amplifier 808. The apparatus Within box accordance with signal voltages derived from amplifier 846. The output circuit 850 of the oscillator 800 is fed to the limiter 852 which may be a power amplifier and/or frequency multiplier and in turn 852 energizes the antenna 854.

If desired, the audio frequency amplifier at the transmitter, such as the amplifier 4 of Figure l or the amplifier 302 of Figure 2 or the amplifier- 106 of Figure 3, may have a Acharacteristic such as shown in Figure 6, in which case the receiving audio frequency amplifier should have an amplitude frequency characteristic such as shown in Figure 7. In the transmitter amplifier characteristic of Figure 6, the gain is flat from about 50 to 100 cycles, rises linearly from about 100 to 500 cyclesand then rises according .to the sloping characteristic shown from 500 cycles on. The receiving characteristic of Figure 'Lis-complemen-` tary to that of Figure 6 and is fiat from 50 to ,100 cycles, decreases the gain linearly from to about 500 cycles and then slopes off in gain as shown from 500 cyclesup. s u

Clearly, many of the features of my present invention are applicable to phase modulation systems. For example,v the over-modulation indicating system may be used exactly as shown for a A phase modumnon transmitter. A130, au' of the frequency modulation transmitters described herein may be caused to produce phase modulated waves by adding an additional correcting circuit in the audio stages of the transmitter which serves to increase the audio amplifier out- 1|8 are preferably tuned by condensers 1|6' sub-carrier system 3002. The output offrequency or phase modulation receiver 3006 is then used to frequency or phase modulate the radio transmitter 3008 whose output is in turn radiated by radiator 30I0.

The transmitted frequency modulated waves are received on thereceiving antenna 30l2 and translated by the frequency or phase modulated receiver 30M into the modulation frequency waves corresponding to those at the output of apparatus 3000 at the transmitter. These de-modulated waves of modulation frequency are used to frequency or phase modulate a sub-carrier wave in the apparatus represented by 3016. This frequency or fed over land line 30|8 to a distant receiving point at which a frequency or phase modulation receiver 3020 is located. Thereceiver 3020 de.

modulates the frequency or phase modulated subcarrier and feeds the de-modulated waves into a loudspeaker facsimile recorder or television reproducing apparatus 3022. The foregoing system has among motely located from the transmitting antenna 3010 and that the de-modulating receiver 3006 f may be used bothfor' monitoring as well as for modulating the radio transmitter 3008. Similarly, the receiving arrangement is advantageous which event-the urban noises generated in urban areas are kept from affecting the output of 3022.

Any of the transmitters or receivers-heretofore specifically described may, of course,be used in the system of Figure 8, as will' be apparent to those skilled in the art.

In connection with Figure 3, as shown on the drawing, the coils 116, 1I8 are preferably shielded and uncoupled from Veach other although individually coupled to the coil 104. Also, coils 1|6,

1|8' to opposite extremes of the frequency band transmitted by oscillation generator 102. From another point of view it may be said that these off tuned circuits 1|6, 116 and 1|8 1|8 serve to swing the resonant frequency of the oscillation generator plate circuit 104 through and about its mean frequency in accordance with the modulation fed through transformer 1H.

In connection with Figure. 5. the relative voltages applied to the plate 904 and screen grid 920 may be adjusted as shown by the taps on resistor 940 and/or by adjusting transformers 934. 936.

Instead of manually reversing the polarity of the signal modulation input for the transmitting system shown, for example, in Figure 1 the polarity thereof may be automatically reversed by systems such as shown by J. L. Hathaway in his U. S. Patents 2,158,820 and 2,158,821.

In the claims which follow, the term "angular velocity modulation shall be understood to include frequency modulation or phase modulation or hybrid modulations suchv as produced by predistorting or pre-emphasizing circuits at the transmitter. y

Having thus described my invention, what I claim is:

l. In combination, an oscillation generator, a source of signals, means for angular velocity phase modulated sub-carrier is then of a city and the reproducing apand its advantages the fact that the pick-up unit 3000 may be reassigned frequency, and means, responsive to signals to be transmitted. for disabling or rendering inoperative said automatic frequency correcting means.

2. Apparatus as claimed in claim 1, characterized by the fact that means are provided responsive to signaling waves for rendering said automatic frequency controlling means inoperative in the presence of signals and operative in the absence of signals.

3. An angular velocity modulation over-modulation system comprising a circuit tuned to or beyond an extreme frequency of the frequency channel to which an angular velocity modulation transmitter has been assigned, saidtuned circuit being coupled to said transmitter so that usable current ows in said circuit only when'said transmitter produces waves of frequencies exceeding the channel assigned to said transmitter, means for rectifying the current produced in said tuned circuit, and means for indicating presence of said rectified current, said apparatus being characterizedby the fact that means are provided responsive to said rectified current for reducing the frequency swing of the transmitter so that it does not produce angular velocity modulated oscillations of a frequency extending beyond the frequency to which said circuit is tuned.

4. An angular velocity modulation over-modulation system comprising a circuit tuned to or beyond an extreme frequency of thefrequency channel to which an angular velocity modulation transmitter has been assigned, said tuned circuit being coupled to said transmitter so that useful current flows in said circuit only when said transmitter produces Waves of frequencies exceeding the channel assigned to said transmitter, means for rectifying the current produced in said tuned circuit, said apparatus being characterized by the fact that means are provided responsive to said rectifiedA current for reducing the lfrequency swing of the transmitter so that it does not produce angular .velocity modulated oscillations of a frequency extending beyond the frequency to whichsaid circuit is tuned.

5. In an angular velocity modulation system, an angular velocity modulated transmitter, a pair of tuned circuits tuned to or beyond extreme frequencies of an assigned frequency modulation.

channel, means for rectifying currents produced in said tuned circuits, and means for utilizing the rectified currents to reduce the frequency swing of said transmitter so as to confine the angular velocity modulated waves produced thereby within said assigned or allocated frequency channel. 6. In a frequency modulation system, an oscillation generator, means for frequency moduized by the fact that said oscillation generator is provided with automatic frequency controlling means, and being further characterized by the fact that said automatic frequency controlling means is adjusted in such a way as to compensate for the unsymmetrical nature of voice waves used to produce the frequency modulatedoscillations in such' manner that the frequency controlling system brings the oscillation generator back to its mean assigned frequency rather than 'on the average to a different` frequency which source of oscillations to be modulated, a reactance tube connected to said source, said reactance tube having a multiplicity of electrodes, means subjecting one pair of said electrodes'to phase displaced oscillations from said source, mean'l subjecting another pair of electrodes of said reactance tube to signaling current waves, and means for subjecting still another pair of electrodes of said reactance tube to automatic frequency controlling voltages derived from said source of oscillations, and characterized further by the fact that a pair of electrodes of saidreactance t'ube is provided with and subjected to over modulation control voltages. Y

8. In combination, asource of complex signal modulating waves, an amplifier to amplify said waves, an oscillation generator, a reactance tube circuit coupling said amplifier and generator whereby the Waves derived from said generator are angular velocity modulated in accordance with waves from said source, a pair'of frequency discriminator circuits, a pair of rectifiers connected to said discriminator circuits, said discriminator circuits being energized by modulated Waves derived from the generator and being adjusted so that said rectifiers produce substantial rectified voltages when said generator is overmodulated, and means connecting said rectiflers to saidreactance tube circuit in such a way as to produce a modulation to oppose the signal modulation only when over-modulation occurs.

9. Apparatus as claimed in the preceding claim, characterized by the fact that the rectifiers are connected to the signal amplifier to reduce the gain thereof when over-modulation occurs.

10. Apparatus as claimed in claim 8, characterized by the fact that the rectifiers are connected to the reactance tube circuits to. reduce the amount of angular velocity modulation when over-modulation occurs.

11|. In combination, a generator of carrier waves. a source of control waves, means to anell- 'lar velocity modulate carrier waves from said generator with waves from said source, and means responsive to the modulated waves for reducing the degree of angular velocity modulation thereof only when the degree of modulation exceeds pre-determined limits.

12, In combination, a generator of carrier Waves,a source of modulating waves, means. to angularyelocity modulate waves derived from said carrier generator with waves from said modulating Wave source, and means responsive only to over-modulated waves for reducing the amount of over-modulation.

.13. In combination, an oscillation generaton-a over-modulation of said generator over-modulation thereof.

14. In combination, a generator of carrier waves, circuits for transmitting waves derived from the generator, a source of variable control voltage, circuits arranged to frequency modulate waves derived from said generator in accordance to reduce the .with said variable control voltages, a frequency controlling circuit arrangement for frequency stabilizing lsaid generator: a rectifying system, operative to produce rectified current only when the frequency modulated waves swing beyond desired frequency limits, to produce a voltage representative of the excessive frequency swing, and

said generator operating at a substantially constant mean frequency, and a second discriminator-detector system responsive to over-modulation of said generator for reducing said overmodulation. i

16. In a system for controlling the maximum l deviation of frequency modulated oscillations irrespective of changes in the average intensity of said signals, means for producing rectied current only when the deviations of the modulated oscillations exceed a selected frequency, the intensity of said` rectified current being proportional to the extent by which the frequency of Vsaid modulated oscillations exceeds said selected frequency, and means excited by the rectified current produced for changing the mean frequency of said oscillations as a function of .said

rectified current, said last-mentioned means be ing operative only when said maximum deviation -is exceeded.

17. The method of controlling the peak deviations` of oscillations modulated in frequency by voltages the average value `of which may vary slowly which includes the steps of producing, only from undesired, excessive frequency deviations, additional voltage the value of which is a function of the extent which the peak deviation of said modulated oscillations .exceed a selected value during modulation, and changing the mean frequency of said oscillations in accordance with said additional voltage.

18. In a frequency modulator including an oscillation generator, a reactance tube modulator coupled thereto and means for controlling the impedance of said reactance tube by signals,

means for deriving from said modulated oscillations a potential solely from peak swings thereof which go beyond a selected frequency, and means for additionally controlling the impedance of said reactance tube in accordance with said derived potential.

19. In combination, a generator of carrier waves, a source of control voltages, means to v frequency modulate carrier Waves from said generatorv in accordance with voltages from said source, a frequency to amplitude converting circuit, a rectifier coupled thereto, means for feeding frequency modulated waves to said converting circuit andthence to said rectifier, anda circuit coupling the output of said rectifier to said generator, said converting circuit and rectiiier operating to reduce the degree of frequencyi modulation of the carrier waves only when the degree of modulation exceeds predetermined limits.

20. In combination, a generator of carrier waves, a source of modulating waves, means to frequency modulate waves derived from said generator with waves from said modulating wave source, a'frequency to amplitude converting and detecting system responsive only to over-modulated waves for producing detected current only when over-modulation occur's, and a circuit utilizing said detected currents for reducing the de- 5 gree of frequency modulation of said carrier waves when over-modulation occurs.

HARRY TUNICK.