Communication by Phase Modulation

Practical methods of generating and receiving phase modulation are described which open up the possibility of using phase modulation as a communication system. A new receiver is described which uses an off-neutralized crystal filter and provides a simple practical receiver which has not been heretofore available for phase modulation. Other methods of reception are described and discussed. Propagation tests which were conducted between California and New York indicate that the propagation characteristics of phase modulation are substantially the same as those of amplitude modulation. The noise characteristics of phase modulation are considered and it is shown that the signal-noise ratio at the output of the phase-modulation receiver is equal to the product of the phase deviation in radians and the carrier-noise ratio. The chief advantage of phase modulation is realized at the transmitter where a power gain of about four-to-one is obtained and modulating equipment is reduced by the ability to modulate at a low level without the requirement of linearity in the stages following the modulator. The chief difficulty occurs at the receiver where the susceptibility to microphonics is increased and the circuits are slightly more complicated.