In phase modulation, the frequency of a carrier wave changes for a short period due to the change in phase of a carrier wave. Now it is widely used along with quadrature amplitude modulation which incorporates phase elements to its operation as well as amplitude.
This is because the relationship between phase and frequency variations is not linear;that is, phase and frequency do not vary in direct proportion. So quickly another modulation technique emerged in FM or frequency modulation. Of course, that situation is not very interesting.
However, the signal-to-noise SNR of the phase modulation is not as good as frequency modulation.
A radio frequency signal consists of an oscillating carrier in the form of a sine wave is the basis of the signal. Each phase angle that is, each shift from one phase state to another represents a specific digital input data state.
Increase in phase lead is equivalent to an increase in frequency. Frequency modulation will result in phase modulation if the signal is differentiated first.