Phase modulation (PM) is a form of modulation that represents information as variations in the relative phase of a carrier signal. Mathematically, phase modulated signal is written as

where kp is the phase deviation constant and  is the message signal. Instantaneous phase of phase modulated signal is given by

this clearly shows that phase changes linearly with message signal m(t). An example speech signal and its corresponding phase modulated signal are shown in Figure 1.

Figure 1: Phase modulation of a segment of speech signal

At the end of this experiment, students are encouraged to observe various phenomena relating phase modulation and frequency modulation. There are some questions to enable students to develop a better understanding of these concepts.