US2282103A - Frequency modulation - Google Patents

Description

May 5, 1942. H. TUNlCK FREQUENCY MODULATION Filed Aug. 29, 1941 4 Sheets-Sheet 1 y 5, 1942- H. TUNICK FREQUENCY MODULATION Filed Aug. 29, 1941 4 Sheets-Sheet 2 wwm W vmm mm @8321 5:33 a wwm HARR Y TUN/ CK y 1942- H. TUNICK FREQUENCY MODULATION Filed Aug. 29, 1941 4 Sheets-Sheet 3 TN m mw/o mm Q WY mm/ w R M M H E? 0 MN m6 .90 w 9 4 w m 2 1% o d lm May 5, 1942. H. TUNlCK 2,282,103

FREQUENCY MODULATION Filed Aug. 29. 1941 4 Sheets-Sheet 4 INVENTOR HARRY TUN/ CK BY w/mm ATTORNEY Patented May 5, 1942 FREQUENCY MODULATION Harry Tunick, Rye, N. Y., assignor to Radio 001- poration of America, a corporation of Dela- ApplicatiorfAugust 29, 1941, Serial No. 408,738

7 Claims.

This is a continuation in part of my copending application Serial No. 310,495, filed December 22, 1939. Figures 1, 2, 2a, 2b, 3 and 4 herein are, respectively, original Figures 10, 2, 2a., 2b, 4 and 5 of my parent application Serial No. 310,495. This application, furthermore, is a continuation in part of my copendlng application Serial No. 369,800, filed December 12, 1940, since Figure 5 herein is Figure of my copending application Serial No. 369,800, filed December 12, 1940.

Generally, my present invention relates to angular velocity modulation and more particularly to transmitters for producing and .transmitting frequency modulated waves. In this respect, it has been proposed to employ oscillation generators having tuned grid and tuned plate circuits for the production of frequency modulated waves by varying the tuning of, say, the tuned grid circuit in accordance with the modulation. Since the grid circuit tuning is varied and the plate circuit remains fixed in tuning, the variable frequency currents are phase distorted, i. e'., shifted in phase in the plate circuit which is, of course, undesirable. One object of my present invention is to provide an improved system for frequency modulated waves which shall be substantially free of this type of distortion, namely, phase distortion. Other objects, advantages and features of my invention will appear as the more detailed description thereof proceeds.

In the drawings, Figure 1 is a wiring diagram of an oscillator having separate tuned input and output circuits andappropriate reactance tube systems for simultaneously varying the tuning of the input and output circuits to thereby produce angular velocity modulated waves substantially free of phase distortion; Figure 2 illustrates another modification of my invention in which; among other things, the plate and grid circuits of a crystal controlled oscillator are simultaneously varied to produce frequency modulation of the generated waves; Figures 2a and 2b are explanatory curves illustrating signal pro-emphasis and de-emphasis at the transmitter and receiver, respectively; Figure 3 illustrates a modification of Figure 2; Figure 4 illustrates a system for producing frequency modulation simultaneously by varying the screen grid and plate voltages of an oscillation generator; and Figure 5 is a modified form of the system shown in Figure 1.

Referring to Figure 1, tubes 26, 28 are provided with a tuned input circuit and a tuned output or plate circuit 34. Grid circuit 2|! comprises coil 22 shunted by variable tuning condenser 24 and, similarly, the tuned plate circuit 34 consists of a coil 36 shunted by a variable condenser 38. Oscillations are generated regeneratively by the cross-connected feedback condensers 30, 32. Output is taken from the, by-passing condensers 40, 42 as illustrated. Amplified signals, asexplained in my parent application Serial No, 310,- 495, are fed through transformer 6 and lead 8 to the screen grids III, I2 of reactance tubes I4, I6. In case video signals are transmitted, radio frequency by-pass condenser I8 should be made smaller so as not to by-pass the highest video frequencies fed through transformer 6. If desired, condenser I8 may be omitted.

The reactance tubes I 4, I6 act to change the tuning of the grid circuit 20, comprising coil 22 and condenser 24, an amount proportional to the amplitude of the signal fed through transformer 6 and at a rate or frequency identical to 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 3D, 32, produce frequency modulated high frequency oscillations in the tank or plate circuit 34 comprising inductance, coil 36 and condenser 38. The frequency modulated oscillations are fed through condensers 40, 42 to a power amplifier and/or limiter and/or frequency multipliers and eventually to an antenna, as explained in my parent application Serial No. 310,495.

Automatic frequency controlling voltages may be fed through lead Ib to the suppressor grids I44, I46 of reactance tubes I4, I6 and also, as illustrated, to the suppressor grids of reactance tubes RIB, RI 4. Similarly, the signal is fed from the secondary of transformer 6 to the screen grids RID, RI2 of reactance tubes RI4, RIG. Over-modulation control voltages derived as explained in my parent case Serial No. 310,495 may be inserted through resistor 234.

Reactive voltage substantially in quadrature with that existing at the upper terminal of tuned grid circuit 20 is impressed upon the grid I56 of reactance tube I4 through the action of condenser I48 and resistor I52. Similarly, a quadrature voltage with respect to the voltage at the lower terminal of circuit. 20 is impressed on grid I58 through the action of condenser I and resistor I54. The plates of the reactance tubes I4, I6 are fed with suitable plate voltage through chokes I62, I64 which are appropriately bypassed by by-passing condensers I66, I68. Condensers I10 and H2 are large coupling condensers.

As already explained, a second pair of reactance tubes R14, R18 are connected to the plate circuit 34. The suppressor grids R144 and R148 of the plate circuit reactance tubes R14, R18 are connected in parallel and to the suppressor grids I44, 148 of reactance tubes 14 and 18. Similarly, screen grids R10, R12 are connected in parallel and to the screen grids 10, 12 of the grid circuit reactance tubes 14, 18. Phase displaced voltages are taken from the plate circuit 34 and fed to the control grids R158 and R158 by means of condensers R148, R150 and resistors R152, R154.

In view of the foregoing description, it will be observed that the signal input not only varies the tuning of the grid or input circuit 20, but it also simultaneously changes the tuning of the circuit 34. In this way, phase distortion of the modulated currents is avoided. That is to say, if, for example, circuit 34 were not simultaneously tuned to the grid circuit 20, then as the circuit 20 is altered in frequency, the circuit 34 would present undesired reactance to the changed frequency of oscillation and, hence, would introduce phase shifts which are undesired. By simultaneously varying both the grid and plate circuits this defeet is circumvented.

In the transmitter of Figure 2, microphone 300 feeds an audio frequency amplifier 302 whose characteristic is as shown in Figure 2a. That is, amplifier 302 should have a fiat characteristic along the range A to B of about 10 to 500 or 10 to 1000 cycles. From points B to C the characteristic should rise, preferably linearly. B to represents the remainder of the audio frequency.

range which may be as desired from 500 or 1000 to 5000, 7500 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 complementary characteristic, namely, fiat along the frequency range from A to B and then drop off from the frequency range B to C, as shownin Figure 2B.

The output of audio frequency amplifier 302 is fed through transformer 304 and additional transformers 308, 308 to the grids of reactance tubes 310, 312. Audio frequency by-passing condensers 314, 310 are provided as are also radio frequency chokes 318, 320. Depending upon the adjustments of reactance tubes 310, 312 the crystal 322 is effectively shunted by variable capacity or inductance represented by tube 310 and the plate circuit of the crystal controlled oscillation generator tube 324 is also shunted by a variable inductor or capacity represented by reactance tube 312. Both tubes 310, 312 should be adjusted so as to add similar reactances, namely, capacitive or inductive, to the grid and plate circuits of crystal controlled generator 324. If desired, of course, the crystal may be replaced by a tuned circuit or a quarter wave resonant line. The audio frequency voltages applied to the reactance tubes 310, 312 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 is radiated by the antenna 338.

Over-modulation pickup antennae or coils 338, 340 are provided, antenna 338 feeding into a circuit 342 tuned to or slightly beyond one extreme 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

turn connected to rectifier 348. when the swing exceeds the channel allotted to the transmitter, an indication is produced in indicator 343 and to automatically prevent this condition from persisting, an automatic volume control lead 358 is provided. Lead 350 may be connected to one of the audio stages of amplifier, 302 to cut down its gain when output is produced in rectifier 348.

Similarly, when the transmitter goes beyond its extreme allotted frequency in theother direction, rectifier 352 becomes 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 358 leading to the same or another amplifier stage in audio frequency amplifier 302. If desired, the system may be operated without the automatic volume controlling feature, in which case switch 380 and/or switch 382 may be opened In the transmitter of Figure 3. reactance tubes 800, 002 are made more conductive by the action of transformer 804 associated with the cathodes of reactance tubes 800, 802. Transformer 804 is energized by signal amplifier 808 in turn connected, if desired, to a microphone 010. Condenser 812 and resistor 814 are adjusted so as to energize the grid 818 with a relatively leading voltage. Condenser 818, coil 818 and resistance 820 are adjusted so as to impress a relatively lagging voltage with respect to the voltage at point 822 upon the grid 824 of tube 802. 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 840 is identical to that within the dotted recbe adjusted so that they both draw leading cur- In the transmitter of Figure 4, the oscillator] tube 000 may have either crystal 002 connected between the plate 004 and, grid 000 or, as shown, the series tuned circuit 008 is connected by means of switch 010 between the plate 004 and grid 008. The screen grid 020 is supplied with voltage through radio frequency choke 022 and by-pass condenser 924. The plate 004 is supplied with plus voltage through choke 830 bypassed by by-passing condenser 032. Transformers 034 and 0 38 insert alternating signal voltages in the plate and screen voltage leads. These signal voltages are derived from potentiometer 040 connected to the secondary of transformer 042 whose primary is connected across the plates of push-pull amplifier 844 energized from audio or signaling amplifier 848. Since like voltage changes applied to the screen 020 and plate 004 produce opposite efiects on the frequency of oscillation of tube 000 transformers 038 and 034 are so polarized as to oppositely swing the screen 020 and plate 004 in voltage in accordance with signal voltages derived from amplifier 048. The output circuit 050 of the oscillator 000 is fed to the limiter 052 which may be a power amplifier and/or frequency multiplier and in turn 952 energizes the antenna 954.

In the transmitter of Figure 5, tubes 2, 4 are provided with tunable high frequency input and output circuits 6 and B respectively. Feed-back causing high frequency oscillation generation is accomplished by means of feed-back condensers Ill, l2.

Tubes l4 and I6 are connected symmetrically and to opposite sides of the tuned circuit 8 by means of condensers l8 and 20. It will be appreciated, therefore, as the tubes I4 and I6, which have their grids connected in parallel by means of lead 22', are made more conductive, more and more capacity is eifectively connected in shunt to circuit 8', thereby lowering the frequency of oscillation of the oscillation generating tubes 2', 4'.

Tubes 30, 32, which have their grids connected in parallel by lead 34, are connected to opposite sides of the plate tuned circuit 8' through inductors 36, 38'. Hence, as the tubes 30, 32' are made more and more conductive, more and more inductance is effectively connected in shunt with the coil of the tuned circuit 8 and, hence, this acts to reduce the total inductance in circuit and, hence, raise the frequency of oscillation.

For frequency modulating the push-pull oscillation generator 2, 4' the leads 22', 34' are connected as shown to opposite sides of the secondary of transformer 40. The primary of the latter is fed with amplified voice or tone currents from amplifier 42 in turn supplied by microphone 44.. By making the amplifier 42 have a rising characteristic for the modulating frequencies fed to its input side, the resulting output of the system taken from leads 46, 48 will be phase modulated.

The tunable grid circuit of the transmitter of Figure may be similarly varied in frequency by means of tubes 50', 52' connected to opposite sides of the tuned circuit 6 through condensers 54' and 56 and by means of tubes 80,

secondary of transformer 80'. The primary of source of modulating waves, and means for simultaneously varying the resonance of said input circuit and said output circuit in response to waves from said source to thereby vary the frequency of oscillations generated.

2. In combination, a push-pull connected vacuum tube oscillation generator having a parallel tuned plate circuit and a parallel tuned grid circuit, a pair of reactance tubes symmetrically connected to the grid circuit, another pair of reactance tubes symmetrically connected to said plate circuit, andmeans for simultaneously varying the conductivities of all of said reactance tubes in order to thereby simultaneously vary the frequency of oscillations generated by said generator.

3. Apparatus as claimed in the preceding claim, characterized by the provision of means connected to the tuned grid circuit for supplying the grids of the reactance tubes symmetrically connected to the grid circuit with phase displaced voltages relative to the voltages across the grid circuit, and being further characterized by the fact that means are provided for supplying the grids of the reactance tubes connected to the plate circuit with phase displaced voltages displaced in phase with respect to and derived from voltages across said plate circuit.

4. In combination, an oscillation generatorhaving a tuned input circuit and a tuned output circuit, a pair of reactance tubes symmetrically connected to said tuned input circuit, a second pair of reactance tubes symmetrically connected to said tuned output circuit, and means for simultaneously varying the conductivity of all of said reactance tubes and thereby varying the frequency of oscillations generated by said generator.

5. In combination, a vacuum tube oscillation generator having a resonant grid circuit and having a separate resonant plate circuit, an electron discharge device connected to said grid circuit through a reactance, a second electron discharge device connected to said plate circuit through another reactance, and means for simultaneously varying the conductivity of said tubes with control voltages to simultaneously change the tuning of said grid and plate circuits.

6. In combination, a push-pull vacuum tube oscillation generator having a resonant grid circuit and a resonant plate circuit, a pair of electron discharge devices connected to said grid circuit through reactors, another pair of electron discharge devices connected to said tuned plate circuit through another [pair of reactors, and means for varying the conductivities of said pairs of devices with control voltages so as to simul taneously vary the tunings of said grid and plate circuits.

'7. In combination, a high frequency circuit, a pair of electron discharge devices connected to said circuit through capacitive reactances, a second pair of electron discharge devices connected to said circuit through inductive reactances, and a circuit for oppositely varyin the conductivities of said pairs of devices to thereby alter the tunin! of said first-mentioned circuit.

HARRY TUNICK.

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