Phase modulation( PM) is an angular modulation of a carrier oscillation in which the phase angle of the modulated oscillation deviates from that of the carrier oscillation by an amount proportional to the primary sign oscillation. This deviation amount is called phase deviation.
In phase modulation, a distinction is made between phase state modulation and phase difference modulation. The former assigns a fixed phase state to a certain binary number, e.g. binary "0" corresponds to 90 degrees phase state and binary "1" to 270 degrees.
In phase difference modulation, which is used almost exclusively because of the immunity of phase modulation to interference, the binary characters are encoded by the phase difference of two successive AC pulses. In this method, several binary characters are combined into an alternating current signal and transmitted.
Phase modulation is used in data transmission technologywith modems in the form of phase shift keying( PSK) and in another variant as phase jitter modulation( PJM) with the smallest phase changes in RFID.