Product Phase Modulation and Demodulation

A type of phase modulation system is described in which modulation of the phase angle of a carrier wave is brought about through an amplitude modulation process. The modulating voltage e
m
is first impressed in parallel on a "sine phase converter" and a "cosine phase converter," which produce respectively, at their outputs, the functions sin ke
m
and cos ke
m.
The first of these modulating functions is used to modulate, in a balanced modulator, the carrier wave, cos ωt. The second modulating function modulates in a similar balanced modulator the same carrier wave displaced in phase by 90 degrees, or sin ωt. The outputs of the two modulators are added to produce a phase-modulated wave in accordance with the trigonometric identity cos ke
m
cos ωt+sin ke
m
sin ωt= sin (ωt -ke
m
). The "phase converters" proposed consist of oscilloscope tubes in which the phosphor has been replaced with an anode for collecting electrons, and a mask is interposed in the electron beam in order to produce, at the anode, a voltage proportional to the sine or cosine of the linear beam deflection, which in turn is proportional to the modulating voltage applied to the deflecting plates. It is indicated that phase deviations as high as ±25π radians may be obtained with this system.