US2238249A - Phase modulator - Google Patents

Description

M CROSBY muss 'uommuon Filed och-27,1938

' ADJUSTER 1 PHASE am/121m cAkR/En 5;

CARRIER .S'OURCE CARR/ER SOURCE MODULATION INPUT PHASE ADJUSTER MOD/W34 T/ON lv ur '2 s-sneet '1 OUTPUT FREQUENCY Y AND paws/z L/MITER,

MULTIPLIERS AMPLIFIER INVENTOR.

m sey ATTORNEY.

Patented Apr. 15, 1941 2.238.249 PHASE MODULATOR "Murray G. Crosby, Rlverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 27, 1938, Serial No. 237,175

7 Claims.

This application concerns a new and improved phase-modulator of the type in which two phase differentiated voltages of carrier wave frequency are relatively amplitude modulated and combined to produce a resultant which varies in phase. In one modification only one of the two voltages of carrier wave frequency is modulated and the phase and. amplitude relations of the modulated voltage and unmodulated voltage are given for linear modulation. In a second modification, frequency multiplication is introduced following the phase shifting so that the phase diiference is multiplied bythe order of the multiplication and smaller-initial phase difference is required.

In the prior art of this type of phase modulator, the carrier voltage is divided into two phase shifted branches which are differentially amplitude modulated and recombined at the same frequency. This type of phase modulator is described in my United States application #588,309,

filed January 23, 1932, now Patent #2381577 dated May 25, 1937, and my United States application #616,803, filed June 13, 1932, now Patent $1 2,065,656 dated December 29, 1936. As will be shown in the mathematical analysis which follows, this differential modulation requires aphase difierence of 90 degrees between the two carrier voltages of equal amplitude to produce the most linear modulation. By the same mathematical method, I have found that it is possible to produce the same quality and degree of modulation for the case where only one of the carrier voltages is modulated. This is accomplished by setting the carrier voltages 135 degrees apart in phase and adjusting the amplitudes oi the two carrier voltages so that the modulated carrier is equal to aver times the unmodulated carrier. Thus by properly adjusting the single modulator system, the performance of the double modulator system can be equalled for all practical purposes.

In describing my invention in detail, reference will be made to the attached drawings, wherein:

Fl 1: e 1 illustrates by block diagram a phase modulator wherein carrier wave voltages of equal amplitude are displaced in phase and difieren tially modulated in amplitude to produce outputs which are combined;

Figure 2 illustrates diagrammatically a phase modulator arranged in accordance with the present invention. In this phase modulator the carrier wave voltages which are of diflerent amplitude are displaced in phase and one of them only is modulated in amplitude. The ouputs are then combined to obtain a phase modulated resultant.

Figures 3 and 4 illustrate schematically the circuit connections of two modifications of phase modulators, as illustrated in Figure 2; while,

Figure 5 illustrates schematically the circuit elements of a new and improved phase modulator as illustrated in Figure 1.

Figure 1 shows a block diagram of the dif- 'ferential type of phase modulator. The carrier voltage from t is fed through phase adjuster 2 to modulator 3 and directly from i to modulator 4. Differential amplitude modulation is applied to modulators 3 and 4 so that 3 is modulated up at the time t is modulated down. These two modulated outputs are combined to produce a resultant output which varies'in phase between the limits imposed by the phase difierence between the two carrier voltages fed to 3 and 4. The following two amplitude modulated waves are combined:

e1=E1(1+M sin pt) sin at (l) e2=Es(1-M sin pt) sin (wt-H3) (2) where M is the percentage of amplitude modulation on the two modulators, p=21r times the modulating frequency, w=21r times the carrier frequency, and p: the phase difference between the two carrier voltages. Combining (1) and (2) 'sin By applying the binomial theorem and the relation sinlwt+45 tan" M sin pt} (5) Expanding the arc-tangent, term of (5) into Equation 6 may be rewritten:

Thus the wave is amplitude modulated with even harmonics having percentages of modulation equal to Y/Z, X/Z etc. The phase modulation produced consists of fundamental and only odd harmonics. The percentage third harmonic is equal to B/Ax 100, the fifth equal to C/Ax 100, etc.

The circuit of Figure 2 shows the arrangement of the modulator in which only one of the phase diflerentiated voltages is modulated. The carrier source is fed directly through amplifier l2 to the output and through phase shifter II to amplifier modulator l3 from which output is taken to combine with the output of amplifier l2. Only amplifier modulator I3 is modulated in amplitudel2 is an amplifier or coupling tube which serves as a one-way device to prevent modulator l3 from singing. In this case the following two voltages are to be combined:

e =E (1+M sin pt) sin (wt-H9) e E sin wt The vectorial combination produces the following resultant:

/E (1+ Msin pt) Ef-i- 2E E;(1+ Msin pt) cos S The optimum conditions are obtained when p=135 degrees and Then 0.707 1) 0.707(1 M sin pt) -0.707

tan-

Equation 13 is identical in form to 5 which is reducible to 6 and '7. Thus the two types of modulators produce the same degree of undesired amplitude modulation and the same degree and quality of phase modulation for a given degree of amplitude modulation. Since the single modulator type of circuit is simpler, an advantage is gained without the addition of disadvantages.

When the modulators 3 and 4 of Figure 1 and amplifier and modulator l2 and I3 of Figure 2 are operated as frequency multipliers, the phase difference between the two carrier voltages is multiplied by the order of the frequency multiplication. Thus, in the case of frequency doubling, for the double modulator type of circuit shown in Figure 1, the phase difference eifected by 2 would be adjusted to 45 or 135 degrees so that when this phase difference is multiplied in the modulators the resulting phase difierence would be or 270 degrees. The modulators perform the dual function of varying the amplitude in accordance with the signal wave and multiplying the frequency. Such a modulator would consist of a modulator tube having its input tuned to the carrier source frequency and its output tuned to the desired harmonic which would be the second in this example of frequency doubling. If desired, the two processes of frequency multiplying and modulating may be sepparated in units 3 and 4 so that an ordinary modulator tube is cascaded with a frequency multiplier tube. The result would be the same as though both functions were accomplished in the same tube. Difierential modulation is applied to modulators 3 and 4 by means of push-pull transformer 5 which is arranged to modulate one of the modulators up at the instant the other is be ing modulated down. The two modulated outputs are combined in a common plate circuit of the modulators in 3 and 4 or in the last stage of the cascaded frequency multipliers and modulators in 3 and 4.

For the single modulator case of Figure 2, for the same example of frequency doubling, the phase difference effected by H would be 67.5 or 112.5 degrees so that when multiplied the difference would be or 225 degrees. In this circuit amplifier l2 forms the single function of frequency multiplying whereas I3 is modulated by the signal while also performing the function of frequency multiplication. Combination of the two outputs would be effected in the common plate circuits of i2 and I3 or I2 and the last stage of i3. This will be understood when it is remembered that frequency multiplication multiplies frequency or phase deviations. This is the case of a phase diiference which would be the same as a phase deviation,

The insertion of the frequency multiplication may precede the modulators or succeed them. but it must preceed the combination point in order to accomplish the function of reducing the required phase difference between the carriers.

The circuit of Figure 3 shows a specific embodiment of the type of phase modulator in which one of two phase differentiated carrier voltages is modulated and combined with the other. Carrier source I feeds tuned circuit illl which is tuned to the carrier frequency. R and C form a phase shifter to effect a phase shift in conjunction with the 180 degree shift of push pull tuned circuit i0l so that the voltages of carrier wave frequency 'fed to grids Gr and C2 of amplifier I03 and modulator I02 are 135 degrees apart which is the optimum phase adjustment for this type of modulator. Phase shifter R, C also effects an amplitude reduction which is made such that the amplitude of the voltage ing P of transformer N6, the secondary S of.

which is connected to the suppressor grid ill of tube i102. I

Theicircuit of Figure 4 shows a specific embodiment of the type of phase modulator in which the modulator and amplifier tubes are frequency multipliers. Wave energy of carrier wave frequency is furnished from an electron discharge tube oscillator of the constant frequency type comprising a crystal PC connected with a control electrode of a tube 200 having a tuned reactive circuit 203 connected with its anode 200. The oscillations produced are fed by a phase shifter R, C to the grid Ch of modulated frequency multiplier tube 2M and through amplitude regulating condenser C1 to the control grid G: of frequency multiplier tube 202. Without frequency multiplication a 135 degree phase separation between the carrier voltages is required, but for the case of frequency doubling a 67.5 degree phase shift would be used preceding the doublers. Thus R, C is adjusted for for 67.5 degrees shift. Ci allows an amplitude adjustment of the carrier voltage supplied to tube 202 to be made to com- 0.707;]. between the multiplied outputs of the modulated and unmodulated stages 20! and 202, respectively. The modulated tube uses suppressor-gricl modulation the same as in Figure 2. Tuned circuit 204 would be tuned to the desired harmonic of the carrier frequency.

Figure 5 shows how frequency multiplication may be applied to the differential type of modulator in which both frequency multiplier stages are modulated, For the case of frequency doubling in which 300 is tuned to twice the frequency of the carrier wave from 303, phase shifter R, C would be adjusted to shift 45 degrees and the amplitudes of the two voltages could be balanced with condenser C1. The modulators are suppressor-grid modulated by modulating potentials applied to the primary winding P of push-pull transformer 306, which has its secondary winding S connected to suppressor grids Mi and iii. For the example of frequency doubling, with the voltages to G1 and Ge seadegrees apart, the modulated voltages appearing in the plate circuits of 3M and 302 would be degrees apart by virtue of,the frequency multiplication of the phase shift. The differential modulation applied by transformer 30 therefore modulates the. combined output appearing in tuned circuit 304 between the 90 degree limits and either side of the mean between the 90 degree difference.

The relative amplitudes of the two voltages to be combined in any of the above circuits could be adjusted by varying the element voltages on the modulators, amplifiers or frequency multipliers as well as by the variable condenser method as shown in Figures 4 and 5.

It will be understood that there are many more possible combinations of frequency multiplication and phase shift that could be used with these circuits. For instance, in the case of Figure 5, frequency tripling could be used and R, C adjusted for 30 degrees phase shift. In this circuit the plate circuits could be combined in parallel as shown or'in push-pull since this would merely change thephase of combination from 90 to 270 degrees which is immaterial as far as distortion and depth of modulation are concerned, but might improve the efiiciency of the frequency multipliers. Another alternative for the double modulator type would be the use of the phase shifting circuit employed in Figure 3 andoperate the modulators as frequency doublers. This phase shifting circuit applies a degree phase shift which would be multiplied to the optimum 270 degrees.

What is claimed is:

1. In a phase modulation system the combination of a source of wave energy of carrier wave frequency, an output circuit, a pair of electron discharge devices having input electrodes and having output electrodes coupled to said output circuit, means forimpressing voltage of a first phase'from said source on the input electrodes of one of said devices, means for impressing voltage of a phase differing from the phase of said aforesaid voltages by about 135 degrees on the input electrodes of the other of said deviceameans for producing a ratio of substantially .707 to'i between the amplitudes of said voltages impressed on said input electrodes, and means for controlling the impedance of that one of said devices, on which voltage of the least amplitude is impressed, at signal frequency.

2. In a'phase modulation system, a source of wave energy of carrier wave frequency, an output circuit, a pair .of electron discharge tubes having input electrodes and having output electrodes coupled to said output circuit, means for operating said tubes as frequency multipliers, means for impressing voltage of a first phase from said source on the input electrodes on one of said tubes, means for impressing voltage of a phase didering fromthe phase of said aforesaid voltage by about 135 degrees from said source on the input electrodes on the other of said tubes, means for producing a ratio of substantially .7 7 to 1 between the amplitudes of said voltages impressed on said input electrodes and means for modulating the impedance of that one of said tubes, on which voltage of the least amplitude is impressed, at signal frequency.

3. A system as recited in claim2 wherein the impedance of theother of said tubes is modulated at signal frequency and in phase displaced relation with respect to the impedance modulation of said one tube.

4. In a phase modulation system, an oscillation generator of the substantially constant frequency electron discharge tube type, a pair of electron discharge devices each having input electrodes and output electrodes, one of said devices atleast having an additional control electrode, a high frequency circuit including phase displacing means coupling the input electrodes of one of said devices to said generator, a high frequency circuit including amplitude reducing means coupling the input electrodes of the other of said devices to said generator, a high frequency output circuit connected to the output electrodes of said devices, and means connected with said additional control electrode and the cathode of said one of said devices only for impressing modulating potentials on the additional control electrode and cathode of said one of said devices only.

5. In a phase modulation system, an oscillation generator of the substantially constant frequency electron discharge tube type, a pair of electron discharge devices each having input electrodes and output electrodes, one of said devices at least having an additional control electrode, a high frequency circuit including phase displacing means coupling the input electrodes of one of said devices to said generator, 9. high frequency circuit including amplitude regulating means coupling the input electrodes of the other of said devices to said generator, an output circuit tuned to a harmonic of the frequency of the oscillations generated by said generator connected to the output electrodes of said devices, and means connected with said additional control electrode and the cathode of said one of said devices only for impressing modulating potentials on the additional control electrode and cathode of said one of said devices only.

6. The method of phase modulating wave energy at signal frequency which includes the steps of, producing voltages of frequencies related harmonically to the frequency of said wave energy and of difierent phases and amplitudes, varying the amplitude of said produced voltage having the smallest amplitude in accordance with modulating potentials, and combining said voltage the amplitude of which is varied at signal frequency with another of said produced voltages the amplitude of which has not been so varied to produce a resultant voltage of varying phase.

7. The method of phase modulating wave energy in accordance with signalling potentials which includes the steps of, producing ,voltages characteristic of said wave energy, relatively phase displacing the said produced voltages, producing a difference in the respective amplitudes of said produced voltages, relaying said phase displaced voltage having the greatest amplitude, amplitude modulating only said phase displaced voltage having the smallest amplitude in accordance with signal potentials, and combining the relayed voltage with the amplitude modulated voltage to produce wave energy modulated in phase at signal frequency.

MURRAY G. CROSBY.

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