A device that shifts the phase of a sinusoidal carrier signal, c(t), in proportion to the instantaneous amplitude of the information signal, s(t). Since the definition of frequency is the rate of change of phase, phase modulator and phase modulation are very closely related to frequency modulator and frequency modulation. (Integrate the frequency signal with respect to time in order to obtain the phase signal; differentiate the phase signal with respect to time to obtain the frequency signal.) The form of the unmodulated sinusoidal carrier signal is c(t) = A cos (2πf_ct), where the amplitude and frequency of the sinusoid are A and f_c, respectively.