Frequency Modulation

Definition. In frequency modulation (FM), it is the carrier's instantaneous frequency, not its amplitude, that is made to vary in proportion to the instantaneous value of the message signal -- the carrier's amplitude is instead held completely constant throughout. Wherever the message signal is at a positive peak, the carrier's instantaneous frequency rises above its unmodulated (centre) value by the largest amount; wherever the message is at a negative peak, the carrier's frequency falls below its centre value by the same amount; when the message signal passes through zero, the carrier is back at its unmodulated centre frequency. The maximum amount by which the frequency swings away from the centre value is called the frequency deviation, and it is made proportional to the amplitude of the message signal (not its own frequency).

How it differs from AM. Two differences stand out immediately. First, an FM wave's amplitude never changes -- all of the message information is encoded purely in how closely or how widely spaced the carrier's zero-crossings are, never in the wave's height (this chapter's figure sets an AM waveform and an FM waveform, produced from the very same message signal, directly side by side to show this). Second, because the carrier's frequency must be allowed to swing over a range considerably wider than the message frequency itself, an FM signal occupies a substantially larger bandwidth than an AM signal carrying the same message -- for instance, a commercial FM broadcast channel is allotted a bandwidth of the order of a couple of hundred kilohertz, far more than the roughly 2-3 kHz width of the audio message it carries, whereas an AM channel need be only about twice the message bandwidth. …