ABSTRACT
The phase modulation is similar to frequency modulation, as a change in phase of the carrier also causes a change in frequency of the carrier at the same instant. In phase modulation, phase angle of the carrier voltage is changed in accordance with the instantaneous value of the modulating signal voltage, the amplitude and frequency remaining the same. The FM and PM both belong to the general class of Angle modulation or Exponential Modulation. It can be mathematically verified that if the modulating signal is integrated, before it is fed to a phase modulator the output is an FM signal. The instantaneous phase of the modulated signal varies in accordance with the modulating signal. In other words, derivative of the instantaneous phase varies as per the modulating signal. Since the rate of change of phase is frequency, we can say that the instantaneous frequency varies as per the modulating signal, which implies that output is an FM signal.
