US2316155A - Phase modulation - Google Patents

Description

April 13, 1943. M. e. CROSBY 2,316,155

PHASE MODULATION I Filed July 26, 1940 MOOULAT/A/G POTENUALS INVENTOR MURRAY OSBY Qmy ATTORNEY CARR/ER SOU 06' I0 i 3 CARR/ER W VE SOURCE Patented Apr. 13, 1943 2,316,155 PHASE MODULATION Murray G. Crosby, Rlverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 26, 1940, Serial No. 347,587

4 Claims.

This application concerns a new and improved phase modulation means of the type wherein an element of a phase shifter is varied in value to produce a variable phase shift and consequently produce phase modulation. A phase shifter utilizing inductance and resistance is employed so that the resistance may take the form of a tube plate resistance which may be modulated in accordance with the signal voltage. Phase modulation has been accomplished heretofore by controlling the value of a circuit element at signal frequency to thereby control the phase of current therein. In my system Iuse a mid-tapped inductance and a resistance and vary the resistance. My circuit will produce a greater phase shift for a given amount of resistance variation than certain known systems of this nature. My system also makes a greater range of total phase variation possible. One of its most outstanding advantages is the fact that the phase shift is accomplished without amplitude change. This means that the phase modulation will not be accompanied by concomitant amplitude modulation which must be limited oil by means of a limiter following the modulator stage.

In describingnry invention, reference will be made to the attached drawing wherein;

I Figs. 1 and 2 each show a modification of a phase modulator arranged in accordance with my invention; while Fig. 3 illustrates the essential elements of the phase shifter and is used with the vector diagrams of Fig. 4 to illustrate the operation of my system.

In the phase modulator of this disclosure a phase shifter such as shown in Fig. 3 is utilized. This phase shifter shifts the phase of voltage El by an amount depending upon the magnitude of the reactances of Ll and L2 and resistance R. The vector diagram of this type of phase shifter is shown in Fig. 4. The voltage to be phase-shifted is applied as El. El causes a current to flow through Li and R so that a reactance drop XLlI will appear across points O--A of Fig. 3 and a resistance drop RI will appear across terminals O--C of Fig. 3. A reactance drop XmI will appear across terminals -3 of Fig. 3 by virtue of the mutual inductive coupling which exists between LI and L2. Voltage El is the resultant of the reactance and resistance drops XLII and RI. The output voltage of the phase shifter E2 is the resultant of the mutual reactance drop XmI and the resistive drop RI. It can be seen that El is different in phase by an amount 0 depending upon the-- relative values of the resistive voltage drop RI and the reactive voltage drops XLlI and XmI. Consequently, by varying the value of R, the phase may be varied between zero and almost 180 degrees.

From the-vector diagram of Fig. 4, it can be seen that as R is varied between zero and a value equal to the reactive impedance, the phase shift, 0, will be varied between zero and degrees and the amplitude of the output voltage, E2, will not vary since E2 is always equal to El so that there is no variation of the attenuation with modulation. Consequently, phase modulation is possible without the introduction of concomitant amplitude modulation.

The phase shifter of Fig. 3 is adapted to use as a phase modulator by utilizing a resistance at R which can be controlled at a modulation potenial rate. If the phase of the current is to be keyed or modulated, the value of the resistance R may be varied by keying signals which shunt a portion of R or, all of R by a second resistance to change its value in accordance with the signal. In a preferred embodiment the resistance R is comprised of the resistance of a vacuum. tube as illustrated in Figs. 1 and 2. Hence, by applying modulating potentials to the grid of the vacuum tube, the phase of the voltage transmitted through the phase shifter is varied in accordance with the modulating potentials since, as is well known, the plate resistance of a vacuum tube may be varied byvarying the grid potential. Circuits having these features are shown in Figs. 1 and 2..

In the circuit of Fig. 1 constant carrier energy is supplied from source Ill by transformer II to the grid ll of amplifier 20. The output circuit connected with the anode 22 of the ampliilertube Zll comprises a tuned circuit 4 wherein the amplified wave appears, and from which it is fed to the phase shifter consisting of inductance 26 and the resistance between the plate 30 and cathode 34 of tube 36. The phase-controlled output of the phase shifter is fed to the control grid ill of the amplifier tube 42 and thence to tuned circuit 44. Energy from tuned circuit 44 is fed to power amplifier l8 and antenna 9. Power amplifier 48 may contain frequency multipliers and limiters as is known to the art of phase modulation.

Modulating potentials are applied to Jack 3i and then through transformer 32 to the grid 35 oftube 36. Bias is furnished to the grid 35 of tube 36 by means of battery 31.

It is well known that the variation of the grid voltage on a tube varies its plate resistance. Consequently, the application of modulating poten tials to the grid 35 of tube 38 will vary the plate resistance of tube It and, in turn, vary the phase In the circuit r Fig. 2 the resistance between the anode 50 and cathode 52 of a diode tube 54 is used for the variable resistance of the phase shifter. This diode resistance is varied by applying the modulating potentials through Jack 3|, transformer 32 and the radio-frequency choke RFC. Condenser II is a blocking condenser to keep the plate voltage from amplifier tube 20 from being applied to diode 54. Battery 5 1 biases the diode to a mid-position on its resistance-voltage characteristic so that the diode resistance may be modulated at a higher or lower value than this mid-position value, It can be'easily seen; how the elements of the circuit of Fig. 2 can be substituted for the corresponding elements of Fig. 1, and further description of this modification is believed unneces- SB 'Y.

When the frequency of the carrier to be phase modulated is high, so that the plate capacity of tube36 becomes a low-impedance shunt in comparison to the plate resistance, tuned circuit TC may be added to tune out the" capacity. This tuned circuit is tuned to the carrier frequency so as to be resistive to that frequency. TC is addedby closing switches S and SI.

Asdescribed in my United States application Serial No. 36,473 filed August 16, 1935, now United States Patent #2,104,318 issued January 4, 1938, several of these phase modulators may be cascaded in order to produce an increased or more faithful degree of phase modulation.

What is claimed is:

1. 'Means'for shifting the phase of alternating current including an inductance, a variable resistance having one terminal connected to a.

point on said inductance and a second terminal connected to a point of low alternating current voltage, said variable resistance having inherent reactance, connections for impressing voltages the phase of which is to be shifted on said injductance and resistance, connections'for deriving voltages, the phase of which may be varied by varying the value of said resistance, from said 1 inductance and resistance, and connectionsincluding the inherent reactance ocf said resistance in a circuitparallel tuned to the frequency of said voltages.

2; In a phase modulation system a source of wave energy the phase of which is to be modulated in accordance with control potentials, an

inductance, a variable impedance having one terminal coupled to a point on said inductance and a second terminal coupled to ground or equivalent potential, said impedance having inherent reactance, a circuit coupling said source to one terminal oi said inductance and to said second terminal of said impedance, connections for deriving phase shifted current from said inductance and said impedance, a control circuit for varying the value of said variable impedance in accordance with said control potentials, and connections including the inherent reactance of said impedance in a circuit parallel tuned to theifrequency of said wave energy.

3. In a phase modulation system, a source of wave energy the phase of which is to be modulated in accordance with modulating potentials, an inductance, an electron discharge device having an anode, a cathode and a controlgrid, a connection between the anode of said device and a point on said inductance, means for impressing wave energy from said source between a second point on said inductance and the cathode of said device, means for deriving phase modulated wave energy from a third point on said'inductance and the cathode of said device, a source of modulating potentials, means for impressing modulating potentials from said source on the control electrode and cathode of said device, and a circuit tuned to the frequency of said wave energy connected between the anode and cathode of said device.

4. In a. wave length modulation system, a source of alternating current the wave length of which is to be modulated, a source of modulating potentials, an inductance, anelectron discharge device having an anode electrode and a cathode electrode, said device having inherent reactance which appears between said electrodes, a lead coupling one of said electrodes to a point on said inductance, couplings for impressing current from said source to be modulated on said inductance and the other electrode of said device, a circuit for modulating the impedance of said device in accordance with modulating potentials from said source, connections for deriving modulated current from said inductance and said other electrode of said device, and connections including the reactance inherent in said device between its anode and cathode in a circuit parallel tuned to the frequency of the current of said source of current.

' MURRAY G. CROSBY,

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