A type of modulation in which the phase of the carrier wave is varied about its unmodulated value by an amount proportional to the amplitude of the signal wave and at a frequency of the modulating signal, the amplitude of the carrier wave remaining constant. If the modulating signal is sinusoidal the instantaneous amplitude, e, of the phase-modulated wave may be written:
where Em is the amplitude of the carrier wave, F the unmodulated carrier wave frequency, β the peak variation in the phase of the carrier wave due to modulation, and f is the modulating signal frequency. The peak difference between the instantaneous phase angle of the modulated wave and the phase angle of the carrier is the phase deviation.
If the modulating signal is not sinusoidal but is made up of discrete levels, the resulting modulation is known as phase shift keying (PSK). Any number of discrete-amplitude signal levels can be used. In the general case where M discrete levels of the modulating signal are used, the modulation is called M-ary PSK. If only two levels are used, the result is binary PSK (BPSK).
Modulation of a phase-modulated signal may or may not make use of a phase reference to determine the phase of the modulating signal. Instead of a reference just the difference between the phases can be used to modulate the carrier. This form of phase modulation is called differential phase modulation or, in the case of phase shift keying, it is abbreviated to DPSK.