In phase modulation, the amplitude of the message signal does not directly affect the phase modulation itself. Phase modulation is a modulation technique where the phase of a carrier signal is varied in accordance with the instantaneous amplitude of the message signal.
The message signal affects the phase modulation by causing variations in the instantaneous phase of the carrier signal. These variations result in the shifting of the carrier signal's phase in proportion to the amplitude of the message signal at any given point in time.
The amplitude of the message signal, however, influences the amount of phase shift induced by the message signal. A higher amplitude of the message signal will cause a larger phase shift in the carrier signal, while a lower amplitude will result in a smaller phase shift.
It's important to note that phase modulation is primarily influenced by the instantaneous amplitude of the message signal and not the amplitude of the carrier signal or the amplitude of the modulating signal itself. The carrier signal's amplitude remains constant in phase modulation, and the amplitude of the modulating signal primarily affects other modulation techniques like amplitude modulation (AM).