Phase modulation is a system in which the amplitude of the modulated carrier is kept constant, while its phase and rate of phase change are varied by the modulating signal. By the definition of phase modulation, the amount by which the carrier phase is varied from its unmodulated value, called the phase deviation, is made proportional to the instantaneous amplitude of the modulating voltage. The rate at which this phase variation changes is equal oy the modulating frequency. But in the book, i see that modulating signal is a sine wave, but the curve for phase deviation is cosine wave. Why?