A type of modulation where the phase of a carrier wave changes in response to a signal. Phase modulation is actually very similar to frequency modulation. In FM, the carrier‘s frequency is changed by the signal. When it is changing, its phase necessarily changes, but ultimately it’s only the frequency that matters. In phase modulation the frequency of the carrier necessarily changes as a result of the phase changing, but only the phase change is measured by the decoder. Variations of PM are used in a variety of advanced communications and digital encoding systems. Though it has advantages and disadvantages when compared to FM, one major advantage is that crystal oscillators can be used for the carrier since it doesn’t need to substantially vary in frequency. FM oscillators require relatively complex frequency control systems to maintain stability.