PHASE MODULATION (PM)

Phase modulation (PM) is the scheme in which the instantaneous amplitude of the baseband signal modifies only the phase of the carrier signal, while the carrier's amplitude and frequency are both held constant. As the modulating (baseband) signal swings positive, the amount of phase lead in the carrier increases in proportion, which has the practical effect of momentarily compressing the carrier (raising its apparent instantaneous frequency); the negative half-cycle of the baseband signal instead produces a phase lag, momentarily stretching the carrier's cycles. Because of this, a phase-modulated wave -- exactly like an FM wave -- also shows alternating compressions and rarefactions, and PM is in fact the practical technique commonly used to generate FM signals electronically; it is very similar to FM except that it is the carrier's phase, not its frequency, that is directly varied. When the modulating signal's voltage is momentarily zero, the carrier's frequency is unchanged, which is the same resting-frequency idea seen in FM. If the baseband signal used is a square wave rather than a smooth sinusoid, phase reversal occurs in the modulated signal, and FM and PM waves generated from the same square-wave input then look completely different from one another. PM's own advantages are that an FM signal derived from a PM signal is very stable, and its centre (resting) frequency is extremely st …