Glossary definition of Frequency Modulation

Also known as: FM

Frequency Modulation (FM) is a technique used to mix (encode) an information-carrying signal onto a much higher sine wave carrier frequency, so that the signal can be transmitted over long distances as a radio wave. Time varying changes in the information signal cause the instantaneous value of the carrier frequency to shift from its nominal or centre frequency, by an amount proportional to the amplitude of the signal ' this results in positive and negative deviations in frequency. It is these shifts of frequency that are detected in the receiver, and which are demodulated to reveal the original signal.

Analogue or digital signals can be modulated onto a carrier wave using this type of frequency modulation, and in the case of analogue signals the frequency deviations will vary in a continuous process. Digital FM is implemented differently, and in this case the carrier frequency is shifted abruptly to any of a number of new fixed carrier frequencies, based on a binary (to the power of 2) system. The number of different levels of digitisation that are required for digital FM, i.e. the number of discrete frequencies used, will depend on the bandwidth needed for the information to be transmitted.

Wireless communications commonly use narrowband FM, and this can cause frequency deviations from the carrier centre frequency of up to about 5 kHz. FM is often encountered by the general public in the form of FM radio broadcasting on the 88-108 MHz frequency band, which supports high fidelity sound and stereophonic reproduction. The size of FM radios has decreased to the point where it is now possible to find mobile phones that include an integrated FM radio built into the handset. FM communication is less liable to noise interference than amplitude modulation (AM), the other common type of modulation used for radio communications and broadcasting.