Narrowband and wideband modulation used for mobile communications and broadcast radio (mainly at VHF frequencies).
The illustration shows how frequency modulation is achieved: the top trace shows an unmodulated radio frequency carrier; the middle trace shows the audio signal to be modulated; the bottom trace shows the transmitted waveform in which the frequency of the carrier depends on the amplitude of the modulating waveform.
This modulation method results in a much wider bandwidth transmission than AM or SSB but offers better performance when the signal level is relatively high and there are few extraneous propagation effects. There is no need for accurate tuning or high frequency stability in the receiver. As a result, FM is widely used in mobile communications at VHF and UHF. It is also used for broadcasting (using wideband FM) for similar reasons.
FM detectors show the "capture effect" in which competing signals are greatly suppressed in favour of the strongest. This is a useful feature which can help to reduce noise. FM is much less affected by impulse noise than AM techniques since amplitude variations are removed in the receiver before detection takes place.
