US2335934A

US2335934A - Phase modulation

Info

Publication number: US2335934A
Application number: US446475A
Authority: US
Inventors: Hallan E Goldstine
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-06-10
Filing date: 1942-06-10
Publication date: 1943-12-07
Anticipated expiration: 1960-12-07

Description

plifled in the unit MA before radiation.

Patented Dec. 7. 1943 UNITED. sures- ATENT OFFICE rnasn MODULATION Hallan E. Goldstlne, Rocky Point, N. Y., assignor to Radio Corporation of America, ac'crporation of Delaware Application June It), 1942, Serial No. 446,475

This application concerns a new and improved method-of and means for phase modulating a carrier wave a controllable amount in accordance with signalling potentials of any type.

Use of mynew and improved system has numerous advantages some of which will appear from the description thereof which follows:

In describing my system reference will be made I to attached drawings wherein Figure 1 illustrates by block diagram a phase modulator arranged in accordance with my invention. Figure 2 illustrates more in detail the improved modulation circuits of a phase modulation system arranged in accordance with my invention; while Figure 3 is a vector diagram used to illustrate the manner in which phase modulation of a controllable amount is obtained by the use of my system.

Referring now to Figure 1 of the drawings, a

crystal oscillator O is shown as supplying carrier waves to two phase modulators, PM and PMI. The phase modulators are-also supplied with modulating potentials from a source MP coupled by the lines to the two phase modulators. Note that the couplings between the source.MP and the two phase modulators are reversed thereby i indicating that in the separate phase modulators the carriers of common frequency are modulated in phase in opposed sense.

The phase modulated output. is supplied to a plication factors, the factor'svdiflering bythe integral I. n The converter 6- is of the mixing tube type and in the output thereof the summation of the phase deviations appears on the selected carrier. In the system illustrated the phase deviations accomplished in the modulation PM is in one direction and this phase deviation is doubled.

In the phase modulator PMI the phase deviation is in the opposite direction and this deviation is tripled in multiplier Ml. At the output of converter C, the deviation of which is five times the deviation accomplished in one of the modulators, the difleren'ce frequency is selected and this new carrier is of the frequency of the crystal oscillator; O and, consequently, is of substantially con-- stant mean frequency. The selected carrier is again multiplied in frequency to the desired extent in the multiplier M2 and multiplied and amings similar parts in the two identical phase mod- I ulators bear similar reference numerals and only In Figure 3 the vector A represents the carrier. 'frequency'supplied to both modulators PM and PMI and the vectors Al and A2 represent a maximum. phase deviation in opposite directions of the carrier under'control in opposite directions of the modulating potentials in modulators PM and PMI.

The output of the crystal oscillator O, which is used as a frequency control of the carrier frequency, is fed to twobranches PM and PMI which are identical phase modulators. The audio modulation from MP is fed to these'modulators in opposite sense so that the phase modulation is in the opposite sense. When the phase of one of the carriers in one phase modulator is advancing,

the phase of the other carrier in .the other phase modulator is retarding as indicated in Figure 3. The output of these modulators are fed respectively to, for example, a doubler and a tripler. The doubled and tripled wave energy whereon the phase deviation (not per cent deviation) has also been doubled and tripled is fed to a converter and the'difierence frequency selected at the output, this difference frequency being equal to the crystal oscillator frequency with five times as much phase modulation as was impressed on the carrier in the modulators. Further multiplication raises the phase deviation. v I

In Figure 2 I have, shown a specific embodiment of my invention. In this figure the tube I0 is a crystal controlled oscillator of the electron coupled type comprising a grid i4 and a grid l2 coupled with a' crystal l6, biasing resistance It, charging resistance 20, and bypass condensers 22 and 23 forthe production of oscillations of substantially constant. frequency. The generated oscillations are supplied by electron coupling in the tube to the anode 24 and thence to a tuned circuit 26-28. This circuit may be tuned to the crystal frequency or a harmonic thereof. The oscillations from 26-;28 are supplied to' two identical balanced phase modulators. In the drawone thereof willbe described.

The wave energy from 26-28 is supplied by a.

coupling condenser 21 to the grid 3| of an electhe tuned circuit 31-38 is also coupled by phaseshifting circuits C. L and and Ql-Ll and RI to the grids us and us of tubes i: and n respectively. Thecathodes of said tubes are connected to ground by resistances 52 and-l8 shunted by bypassing condensers BP, thereby coupling or placing the impedances oi. the two tubes in shunt 6 to apart at least of the tuned circuit 31-; a pointon which is coupled to ground by a large condenser 30. The grids N and I. .are also coupled by chokes RFC in push-pull relation to a secondary winding or a modulation transformer 10 II coupled to-a source of modulating potentials. The phase shifters C, L and R and (ii-Li andRi are for the purpose of setting up onthe.

anodes and control grids of'the tubes 42 ll voltages displaced substantially 90 in order to l5 I grid I. of tube 42 of the described phase modobtain in a well-known manner reactive eifects of unlike sign of the tubes in the balanced modulater. In the arrangements showmth'e anodes 4e and 68 are connected to theopposite endsof' inductance 31 so that the excitation on the said anodes are in phase opposition. The phase of the excitation on the grids It and II is also displaced about 90 by the phaseshifters C, L and R and Cl-Ll and RI with respect to the voltage on the anode 44. Since the voltage on, the anode 5 46. is displaced about 180 with respect to the voltage on the anode 44, the voltage on the grids 54 and 58 is also displaced about 90 with mpect to the voltage on the anode ll. In one of the tubes the grid v'oltage'leads the plate voltage by so about 90 and in the other tube the grid voltage lags the'plate voltage by about 90. As the reactances of the tubes are varied by the control potentials diiferentially one tube will increase inductive reactance, while the other tube will de- 35 crease capacitive reactance and vice versa. In this manner the diilerential modulation results I in modulating the phase of the current supplied to the circuit '31-. Moreover, during this modulation process any amplitude modulation 4 produced in one of the tubes, say II. is balanced out or compensated or bucked out by equal and opposed amplitude modulation in the other tube, say 42.

Modulation is accomplished by applying modulating potentials from any source, such as. for example, a microphone or a. scanner through choking inductances RFC to the grid electrodes 5 4 and 58 in phase opposition. Increasing the potential on the grid It increases the current through ,the tube 40. Increasing the current through the tube 40 decreases the capacitive reactance provided by the said tube. This is equivalent to adding capacity to the tank circuit.

Since the modulation is in' phase opposition at this time, the current through the other tube. say 42, is decreased thereby increasing the inductive reactance provided by tube 42. Thishas the eilect of adding inductance to the tank circuit. The added capacity and increased inductance shunt the tank winding is and alter-sine tune thereof slightly. Both the added shunt capacity and the higher shunt inductance decrease the resonant frequency of the circuit. Increasthe modulating potentials. This modulates the phase of theoscillatory energy in the tank circuit and the phase modulated oscillatory energy may be utilized directlyfrom the tank circuitthrough inductance coupledtherewith or may fverter coupled to the frequency multipliers.

be'used after ampliiicationand multiplieationte 1s 3. 11; a system, a crystal I 2 w assures the extent required in frequency multipliers and amplifiers in a unit It.

The other phase modulator comprising the tube It on the lower part of the drawings is exactly identical to the one described above and the saiddescription will not be'repeated.

Theoperation of the 'phase modulator shown on the lower part" of the figure is similar except for this important distinction. The operation here is in a sense opposite to the operation in the phase modulator in the upper part ofthe drawings. To insure this opposed phase modulation of the carrier, the leads between the modulation potential transformer Ii secondary are reversed; that is, the lead connected with the ulator is herein connected with the grid ll of tube ll, while the end of the said secondary winding coupled to the grid it of tube ll of the upper modulator is in the low modulator coupled to the grid It of the tube 42.

The phase modulated'carrier from one of the balanced phase modulators is suppliedby lead It to-a multiplier I! wherein it is multiplied, say N times. The phase modulated carrier from the other modulator, say the lower balanced modu-v lator, is supplied by a lead 04 to a multiplier wherein it is multiplied (Nil) times. 1 As described in connection with Figure 1, the multiplied phase modulated carriers are mixed in a converter II and supplied to additional multipliers and amplifiers.

My improved method and dream has the advantage of reducing distortion due to non-lin-. v earity as the phase is not shifted the same amount on each side of zero phase shift for the. same change reactance of the reactance tubes.

It-has the additional advantage of reduction in concomitant amplitude modulation due to the action of the individual balanced phase modula- -tors. These advantages considered together result in a substantially pure phase modulated wave, the wave deviation of which follows quite linearly the modulations to' be transmitted. Iclaim:

1. In a wave length modulation system, a source of oscillations of substantially constant frequency, two circuits each tuned to the frequency of said source and excited by oscillations generated by said source, a wave length modulator coupled to each tuned circuit, eachv wavelength modulator comprising two electron discharge tubes each having electron receiving electrodes coupled to said tuned circuit and having control grids coupled to said tuned circuit, the said cou-. plings being arranged to apply phase displaced voltages of the generated frequency on the electron receiving electrode and control grid of each tube, a coupling between the cathode of each tube of each modulator and the respective tunedcircult for effectively coupling the internal impedance of the tubes of each modulator in shunt to reactance of the respective tuned circuit,-a source of modulating potentials, connections coupling Y connections between .said source or modulation potentials and said tubes, a frequency multiplier; coupledtoeachofsaidtunedcircuitsandacon crate oscillations of carrier wave frequency to be phase modulated in accordance with signals, two circuits each parallel tuned to the frequency of saidgenerator coupled to said generator and v excited byoscillations generated by said crystal controlled oscillation generator, a 'phase-modulator coupledto each tuned circuit. each phase 'rhodulator comprising two electron discharge tubes each having electron receiving electrodes coupled to said tuned circuit and having control grids coupled to said tuned circuit, the said couplings being arranged to apply phase displacedvoltages-o! thegenerated irequency on the electron receiving electrode andcontrol grid of each tube, a coupling between'the' cathode of each tube of each modulator and the respective tuned circuit for effectively coupling the internal 1mpedance of the tubes or eachmodulator in shunt toreactance of the; respcctive tuned circuit, a' 1 source oi modulating potentials, connections coupling'. said source of modulating potentials dilierentially' to the tubes in one or said modulators. connections coupling-said source of modulating potentials-diiterentially to the tubes in the other 01' said modulators, said last named connections being relatively reversed with respect to said other. connections between said source oi modulating potentials and said tubes, a converter, a first frequency multiplier coupling one of said tuned circuits. to said converter and a: second frequency multiplier the multiplication factor of which diflers from the multiplication factor of the first frequency multiplier coupling the other or said tuned circuits tosaid converter.

8. A system as recited in claim 1 wherein one of said frequency multipliers has a multiplication i'actor which diflers from the multiplication iactor oi the otheror said frequency multipliers. .4. A system as recited in-claim zincluding an electron discharge til-be amplifier in each of the. k couplings between the tuned circuits and the crystal controlled generator.

HALLANE.