US2430126A - Phase modulation - Google Patents

Description

Nov. 4, 1947.

N. l. 'KORMAN 2,430,126

PHASE MODULATI ON Filed Aug. 25{ 1943 2 Sheets-Sheet l ATTORNEY Nov. 4, 1947.

N. l. KO'RMAN 2,430,126

PHASE MoDULAToN Filed Aug. 25, 1943 2 Sheets-Sheet 2 I TS1 Q .EL 'T1 E a Z Z 6,? fe

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THS-Z w ffm 5f [mmm/'Jima s JL ATTORNEY Patented Nov. 4, 1947 TENT OFFICE 2,430,126 russa MonULA'rIoN Nathaniel I. Korman, Camden,

N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 25, 1943, Serial No. 499,905

(Cl. 179f171.5)

10 Claims. l

This application concerns a new and improved voltage phase shifting system. lThe system of this invention is of use as a static phase shifter and as the modulator in a phase or frequency modulation system wherein the phase or frequency shift is in accordance with signals.

An object of the invention is to provide a modulator capable of greater phase deviation than is attainable in known systems, and hence to make it possible to use in the system fewer multiplier stages.

Where a large amount of frequency multiplication is used the final output has in addition to the main frequency selected for use a large number of harmonics of the oscillator frequency. An advantage of my system is the reduction of harmonies in the output of the system by producing in the modulator a greater phase deviation so that less multiplication is required in the following stages.

Another object of this invention is to provide in a modulation system of this type a greater phase deviation of wave energy in accordance with signals and to accomplish this object without causing appreciable amplitude modulation of the wave energy. This is accomplished in my system by the use of Ya differential modulation system wherein the amplitude variations are opposed to cancel,

If alternating voltage is applied to a reactance, a second reactance of opposite sign and half the value of the rst reactance and a resistance in, series a voltage is derived across the second reactance and resistance of an amplitude equal t the amplitude of the iirst voltage and of a phase which with respect to the phase of the rst voltage depends on the size of the resistance. By varying the resistance the phase of the second voltage changes and a phase change of about 180 is obtainable. I make use of this principle to provide a static phase changer or e, wave length modulator in which case the resistance is a tube.

'If a second resistance of opposite sign is put in parallel with the rst resistance the phase shift is doubled. To explain briefly how I accomplish this assume the first resistance is positive and comprises a tube. I place a second tube in parallel thereto and apply the alternating voltage to its grid in phase opposition with respect to the phase of the voltage on the first tube so that the second tube appears as a negative resistance. These tubes are now differentially modulated and the phase is shifted about twice as much as in the single tube case, i. e., about 360 degrees.

rThe principle employed above applies to other phase modulators wherein the phase shift is obtained by varying a tube resistance. In these arrangements I couple a second tube resistance in parallel with the rst tube resistance and arrange the circuits so that oney of .the tubes acts like a positive resistance while the second tube acts like a negative resistance, the tubes being differentially modulated.

It will be at once apparent, that, as in the single tube arrangements, the phase only of the voltage is varied, the amplitude remaining constant.

An advantage which flows from the use of my improved arrangement using two tubes is that less multiplication is necessary to get the final phase shift desired and this in turn reduces the harmonic components in the final output which results from the use of a large amount of multiplication.

In describing my invention reference will be made to the attached drawings wherein Figs. 1 and 3 illustrate diagrammatically simple phase shifting circuits and are used to explain the principle involved in my improved phase shifters and modulators. Figs. 2, 3a, 4 and 5 illustrate modications of my improved phase shifter and timing modulation system .as described briefly above;

while Figures 2a, 2b, 2c, 2d, 2c, 3b and 3c are basic circuits or graphs illustrating the operation of the several modifications `shown in the other figures.

In Fig. 1, an alternating voltage c1 of carrier frequency is applied across ZZ and the tube T cathode. 2Z is a reactance which may be capacity or inductive 4in nature, while -Z is a reactance of opposite sign. The magnitude of ZZ is twice that of -Z. T which represents a variable resistance, is an electron discharge device having its anode and control grid excited by voltages of the same phase and its cathode connected to the other side of the input circuit. e2 is the phase shifted or phase modulated voltage derived from the system. In the sake of simplicity, direct current circuit have been omitted in this figure. but it will be understood that the plate of the tube T may charged through a radio frequency choke connected thereto, and to a source of direct current, while the grid may likewise be biased through a radio frequency choke, and modulating potentials may be applied through this choke to the said grid.

Fig. 1 shows a simple phase changer or modulater, Since the alternating current plate and grid voltages on the tube T are equal the admittance of the tube is readily seen to be (where g, R, and u are the transconductance,

plate resistance, and amplication factor of the tube respectively). The ratio of output to input voltage for Fig. 1 is seen to be If Z is taken as a react-.ance we see that e2 is shifted in phase from ci but that both have the same amplitude. Also, if the magnitude of g is varied (by varying the grid bias on the tube, for instance) the phase shift between e2 and ei will vary. If audio voltages are applied to the grid of the tube the carrier voltage e2` will be phase modulated in accordance with the audio voltage.

The variation in g will, at most, be from Zero to infinity. The variation in phase of e2 will therefore, at most, be from zero to 180 degrees, oriQO". If g,. however, could be varied from minus innity through zero to plus infinity the phase would be varied from minus 180 to plus 180. This can be accomplished by paralleling the tube in Fig. 1 by another tube whose grid voltage is 180 out of phase with its plate voltage. Figs. 2, 3a, 4 and 5 show circuits which accomplish this. In these arrangements the control voltages on the tube electrodes must be pushpull also.

In the actual choice o circuit elements used in these two tube modifications care must be taken that they are not such as will make the circuit oscillate.

In Fig. 2 the direct current sources have again been omitted in the sake of simplicity as have the cathode heating circuits. A second tube T1 is coupled in parallel with tube T and the control grid Gl thereof excited in phase opposition by the radio frequency oscillations. The plates and grids are supplied with direct current potentials by resistances 20 'and 22 of the proper value, resistances being preferable to choke coils in these systems.

In the arrangement of Fig. 2, 2Z is, for example, taken as a capacity C and -Z is taken as an inductance L. CI, Ll, L2 are made small so that the grid GI of tube T1 is excited by a voltage substantially 180 out of phase with respect to the voltage on the anode of this tube Ti. CC is a coupling condenser and such that the phase of the voltage of the grid G of tube T is substantially the same as the phase of the voltage on the anode of this tube T and the anode of tube Ti. This condenser CC keeps the plate potential from reaching the grid G while condenser CI in like manner keeps the direct current potential from reaching the grid Gl of tube Ti. The voltages on the grids are made equal by properly dimensioning LI, L2, CI, CC and 20.

In this embodiment the modulating potentials are applied in pushpull relation to the control grids G and Gl of the tubes by a transformer. The total phase shift obtainable in this arrangement is about twice the phase shift obtainable in the single tube arrangement of Fig. 1 or about 360 degrees.

The modulation transformer i8 has its primary supplied with modulation currents and the terminals of its secondary winding coupled to the grids G and GI by way of L2 and resistance 20 and a point on its secondary winding connected through a bias source to the cathodes of the tubes.V

The plates are supplied with potential through a resistance 22. Resistances 20 and 22 are large enough not to shunt out the radio frequency alternating current. RF bi-pass condensers BP are connected between resistances 20 and 22 and the cathodes and between the lower end of L2l and the cathode of tube T1.

Since in my improved system one tube acts as a positive resistance and the other as a negative resistance the problem of regeneration in the circuits and generation of oscillations at some frequency must be considered. The analysis may be carried out by applying the Nyquist regeneration theory as described inthe Bell Telephone Technical Journal, vol. 1l, page 126.

LC (-Z and 2Z) resonates at \/2 f (f=carrier frequency). Ll, L2, and CI are proportioned to make the grid voltages on G and GI equal. The resonance of LI, L2, Cl is greater than j. LI, L2, and

are large compared to L and For purposes of analysing for oscillations tube T may be neglected and we may therefore consider the circuit of Fig. 2a.

Two cases are possible;

(I) The resonance of LI, L2, CI is below that of LC (II) The resonance of Ll, L2, CI is above that of LC For case I:

The reactance seen by the plate of the tube is shown in Fig. 2b. The locus of the vector e/ei is shown in Fig. 2c. Since below fl, e2 leads e by at fl, e2 leads e by 90; above fi, e2 is in phase with e; the locus of the vector is shown in Fig. 2d. The locus cannot include the point I, D and therefore where the resonance of Li, L2, CI is below that of LC this circuit cannot oscillate.

For case II:

By the same sort of an argument the locus of the vector is as shown in Fig. 2e. The locus can enclose the point I, 0 and therefore under case II the circuit can oscillate.

` Thus it is seen that the circuit illustrated in Fig. 2 will be stable in operation if the resonance of Ll, L2, CI is below that of LC, and will provide a total phase change of about 360. The phase of e2 may be displaced a xed amount by properly biasing the tubes to thereby change their impedance. If modulating potentials are applied in pushpull the system becomes a phase modulator.

The principle applied above to obtain a large amount of phase change or modulation may also be applied to'phase changers of the type shown in Fig. 3.

In Roberts U. S. Patent No. 2,143,386, dated January 10, 1939, it is shown that the network of Fig. 3 has the property of shifting the phase of a voltage ei applied to the input thereof without changing its amplitude, so that at the output a voltage e2 may beV derived which is of the same amplitude as the voltage e1. The equation of operation is where Z is a pure reactance and g is the transconductance of the tube. Phase modulation may be accomplished by applying audio voltages to the grid. These audio voltages serve to vary the transccnductance g at audio rates and cause ez to be phase modulated with respect to e1.

It can be seen that with this circuit g can at most be varied from zero to infinity and hence the phase can at most be modulated about 180 peak to peak.

In my invention as illustrated in Fig. 3a, I have added another tube T1 with its plate 43 and cathode 42 in parallel with the plate 44 and cathode 46 of tube T. Condensers C5 and C6 are equal. The grid G of tube T is tapped between these condensers so that it'get-s a'voltage of a rst phase not necessarily the same as the phase of e1. The grid GI of Tube T1 gets its radio frequency voltage through C'I and L1 and this voltage is shifted by Cl and L'I nearly 180 With respect to the voltage of said first phase on'the grid G. The circuit constants are made such that the grid voltages are of like magnitilde- The effect of T is above. The effect of as given by Equation No. 3 T1 considered alone is given by the Equation No. 3 above except that when considering tube T1, g is now a negative quantity. By modulating the grids G and GI of tubes T and T1 out of phase with respect to each other, phase shifts of about 360 peak to peak are obtained. The modulation may be applied by means of a transformer I8 the secondary-winding of which couples the control grids in pushpull relation as in Fig. 2. A point on the secondary winding of the transformer I8 is coupled to the cathodes through a biasing source 5I. Resistance 20 in the modulation circuit lead to grid G has substantially the same purpose as resistance 20 of Fig. 2. Inductance Ll also completes the modulation circuit to grid GI as does L2 in Fig, 2. The grids of T and T1 may be biased by this source to cutoff, in which case the operation will be equivalent to class B or this source may be such as to bias the grids with respect to the cathodes by any voltage less than that at which cutolf is attained and then operation will be equivalent to class A, or class AB.

The phase modulated output e2 is derived from the line, i. e., across the impedance of T1 and this output may be supplied to additional circuits of apparatus such as, for example, frequency multipliers, Wave amplifiers, etc.

In this arrangement precautions must likewise be taken to assume that the circuit does not beco-me unstable and oscillate at some other frequency. The a diagram, see Fig. 3c, for the circuit above, simplified as shown in Fig. 3b, does not enclose the point I, Il and hence does not oscillate.

In a modification illustrated in Fig. 4 the grid GI is coupled to the points between the reactances and this point is also coupled by phase reversing coupled inductances 55 to the contro-l grid GI to apply to said control grid a voltage opposed in phase with respect to the voltage on the control grid G.

In the modification of Fig. 5, one of the coupling inductances is in the cathode circuit o-f tube T and the other as in Fig. 4, is coupled to the grid GI. The modulation circuit is substantially as in Figs. 2 and 3a, but now includes a resistance 22 between the transformer I8 secondary winding and grid GI. A direct current blocking and radio frequency coupling condenser BC is in series between the secondary windingof radio frequency transformer '50 and the' grid GI.

Both of these modifications operate as described above in connection with Figs. 3 and 4a.

What I claim is:

1. In a circuit ofthe class described, two conductors `each having input and output terminals, means for impressing voltages of carrier wave frequency on said input terminals, reactance in one of said conductors, a first variable resistance of positive sign connected across said conductors at their output terminals, said connection viricluding a second reactance, and said first reactance between said resistance and the input terminal of said one conductor, a second variable resistance of negative sign connected in parallel to said rst resistance and connections for varying said resistances differentially in accordance with control potentials to correspondingly vary the voltages at said output terminals,

2. In a system of the class described, a twoconductor line having input and output terminals, means for applying voltages of carrier frequency on said input terminals, reactance in one `of said conductors, a rst electron discharge device having the impedance between its output electrodes connected across said line at its output terminals, said connection including said iirst reactance, and a second reactance between said impedance and one of said input terminals, said device having a control electrode, a second electron discharge device having a control electrode and having the impedance between its output electrodes connected in parallel to the impedance between the output electrodesl of said first device, circuits for applying voltages of said carrier frequency substantially in phase-opposed relation to the control electrodes of said devices, whereby one thereof operates as a negative impedance, while the other thereof operates as a positive impedance, and connections for varying the impedances of said devices differentially in accordance with control potentials to correspondingly vary the phase of the voltages at said output terminals.

3. In a system of the class described, a twoconductor line having input andv output terminals, means for applying voltages of carrier frequency to said input terminals, two similar reactances in one of said conductors, a iirst electron discharge device having the impedance between its output electrodes connected across said line at its output terminals, said device having a control electrode, a second electron discharge device having a control electrode and having the impedance between its output electrodes connected in parallel to the impedance between the output electrodes of said rst device, a coupling between adjacent terminals of said two reactances and the control electrodes of said devices for applying voltages of said carrier frequency substantially in phase-opposed relation to the control electrodes of said devices, whereby one thereof operates as a negative impedance, While the other thereof operates as a positive impedance, and connections for varying the impedances of said devices diiferentially in accordance with control potentials to correspondingly vary the phase of the voltages at said output terminals. y

4. In a timing modulation system, a pair of electron discharge tubes each having an anode and a control grid, a rst reactance, and a Lsecond reactance of like sign and ofsubstantially the same vmagnitude as said first reactance in azieonee series,..'connections series circuit and a point of low potential, connectionscoupling the output impedanc'es otsaid tubes inparallel between said reactances and said' point of low potential, a coupling between adjacent terminals of said reactances and-the control grid of one of said tubes, a coupling between said last named coupling and the control electrode oi the other tube, phase reversing means in said last named coupling andl circuitsfor modulating the impedances of said tubes diierentially in accordance with signals whereby voltage of the carrier wave frequency modulated as to ,timing in accordance with said signals appear at the anodes of said devices.

5. In a timing modulation system, a pair of electron discharge devices each having an anode, a cathode and a control electrode, a firstfreactance, and a second reactance of like sign and of substantially the same magnitude as said Viirst reactance in a series connection, connections for impressing alternating voltagesv the phase i of which is to `be modulated between one terminal of said series circuit and a point of low ,alternat-VV ing current potential, connections coupling the impedanc'es between the anodes and cathodes oi said devices in parallel between the other-terminal of said Series connection and .said point of low alternating current potential, high-frequency circuits lfor impressing said carrier frequency voltages substantially in phase-opposedv relation on the control electrodes of said devices,and circuits for modulating the impedances-of -said devices differentially in accordance with signals whereby voltage of the carrier wave frequency modulated as to timing in accordance with said signals appear between said other terminal of said series connection and saidipoint of low alternating current. Y.

'6.- In a timing modulation system,a pair of electron discharge devices each having an anode, a cathode and a control electroda-arstreactance, and a second reactance of like sign and of substantially the same magnitude as said first reactance in a series connection, connections for impressing alternating voltages Athen-phase of which is to be modulated between said series-Acircuitand a point of low alternating currentfpotential, connections coupling the. impedancebetween the anode and cathode of said devices V'in parallel between one end of said series connection-and said point of low-alternating current potential, a coupling between the control electrode of said devices and a point intermediate said reactances and connections for modulating `for impressing .voltagescthe the impedances of said devicesV diierentially in accordance with signals whereby voltage-ofv the carrier wave frequency` modulated as to timing in accordance with said signals at the anodes-oi said devices` Y* 7. In a phase modulation system, a pair of electron discharge devices each having an anodeQa cathode and a control electrode, a rst reactance, and a second reactance of like sign and of substantially the same magnitude as said rst reactance in a series connection, connections-for impressing alternating voltages the phase of which is to be modulated between said series circuit and a point of low alternating current potential, connections co pling the impedances between the anodes and cathodes of said devices inparallel between one end' of said series connection and said point of low alternating current potential, a coupling between the control elec- 8 trode of one device and a point intermediatesaid reactances, a phase-reversing coupling between said flastcoupling. and the `controlelectrode of the other device, and Vconnections for modulating the impedances of said devices diierentially in accordance withrsignals whereby voltage of the carrier wave frequency modulated in phase in accordance with said signals appears across the impedances between the anodes and cathodes of said devices. Y

8. Ina phase modulation system, a pair of electron discharge tubes each having an anode, a cathode and a control electrode, a first reactance, and a second reactance of like sign and of .substantially the same magnitude as said first reactance in a series connection, connections for impressing alternating voltages the phase of which is to be modulated between-said series circuit andl a point of low alternating current potential,-connections coupling the impedances betweenthe-anodes and cathodesof said' tubes lin parallel between one terminal of said series connection 'and said point-of low alternating potential, a coupling between a point intermediate said reactances and the control electrode of one tube, a phase reverser coupling the cathode of said one tube to the control electrode of the other tube, and connections for modulating the impedances ofsaid tubes differentially in accordance with signals whereby. voltage or the carrier wave frequency modulated in phase in accordance with said signals appears across said tube output im- *pedances.- Y

9.V In a phase modulation system, a pair of electron discharge devices each having an anode, a cathode and a control electrode, a first condenser, and a second condenser of vsubstantially the same-magnitude as said rst condenser in a, seriesconnection, connections for impressing alternating voltages the phase of which is to be modulated between one end of said-series connection and a point of low alternating current potentiaLconnections coupling the impedances between the anodesand cathodes of said devices in parallel between the other end of 'said seriesconnection and said point of low alternating current potential, a coupling between the control electrode of one device anda point intermediate said condenser, a phase reversing coupling between said'last coupling 'and the control electrode of the other device, and connections for modulating the impedances' of said devices differentially in accordance with signals whereby voltage of the carrier wave frequency modulated in phase in accordance 'with said signals appears across the impedances between the anodes and cathodes of said devices.

10. In a phase modulation system, a pair of electron discharge tubes each having an anode, a cathode and a control electrode, a iirst inductance, and a secondinductance of substantially'the, same magnitude as said rst inductance in a Iseries connection, connections for impressing alternating voltages'the phase of which is to be modulated between one end of said series connection and a point of low alternating current potential, connections couplingthe impedances between ,the anodes and cathodes of said tubesV in parallel Ybetween the other terminal of said series connection andsaid point of low alternating potential, a coupling between a point intermediate said inductances and the control electrode of.one tube, a phase reversing coupling between said irst mentioned coupling yand the 10 contro1 electrode of the other tube, and connec- REFERENCES CITED tions for modulating the mpedances of said tubes The following references are of record in the differentially in accordance with signals whereby me of this patent. voltage of the carrier wave frequency modulated in phase in accordance with said signals appears 5 UNITED STATES PATENTS across said tube output impedances. Number Name Date NATHANIEL I. KORMAN. 2,143,386 Roberts J an. 10, 1939

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