Frequency deviation

Frequency deviation (Δf) is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency. The term is sometimes mistakenly used as synonymous with frequency drift, which is an unintended offset of an oscillator from its nominal frequency.

The frequency deviation of a radio is of particular importance in relation to bandwidth, because less deviation means that more channels can fit into the same amount of frequency spectrum. The FM broadcasting range (88-108 MHz) uses a channel spacing of 200 kHz, with a maximum frequency deviation of 75 kHz, leaving a 25 kHz buffer above and below the center frequency to reduce interaction with other channels.[1] AM broadcasting uses a channel spacing of 10 kHz, but with amplitude modulation frequency deviation is irrelevant.

FM applications use peak deviations of 75 kHz (200 kHz spacing), 5 kHz (25 kHz spacing), 2.25 kHz (12.5 kHz spacing), and 2 kHz (8.33 kHz spacing).[2]

Frequency deviation (Δf) is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency. The term is sometimes mistakenly used as synonymous with frequency drift, which is an unintended offset of an oscillator from its nominal frequency. The frequency deviation of a radio is of particular importance in relation to bandwidth, because less deviation means that more channels can fit into the same amount of frequency spectrum. The FM broadcasting range (88-108 MHz) uses a channel spacing of 200 kHz, with a maximum frequency deviation of 75 kHz. The theoretical bandwidth of an FM signal is infinite but Carson’s rule estimates that in practice, the bandwidth is limited to 2 * (frequency deviation + modulating frequency). For a frequency deviation of 75 kHz and a maximum modulating frequency of 15KHz this gives a total bandwidth of 180KHz leaving a 30 kHz (180KHz - 150KHZ) buffer between channels to avoid interference between adjacent channels. AM broadcasting uses a channel spacing of 10 kHz, but with amplitude modulation frequency deviation is irrelevant. FM applications use peak deviations of 75 kHz (200 kHz spacing), 5 kHz (25 kHz spacing), 2.25 kHz (12.5 kHz spacing), and 2 kHz (8.33 kHz spacing).[2]

See also

• Frequency modulation

References

•  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).