Frequency modulation (FM)

What it is

In frequency modulation (FM) the carrier's strength never changes; its frequency swings up and down with the audio. The loudness of the sound sets how far it swings (the deviation), and the pitch sets how fast. FM is the standard mode for VHF and UHF voice and repeaters.

How it works

An FM transmitter bends its oscillator's frequency in proportion to the audio voltage, for example with a varactor diode. See Oscillators. With no audio the carrier sits still on its channel. Talk louder, or move closer to the microphone, and the swing grows. Too much and the signal spills over into adjacent channels (over-deviation), which is why radios limit it.

The receiver's limiter squashes every amplitude variation flat, then a discriminator turns frequency changes back into voltage. Because FM keeps nothing in its amplitude, noise that rides on the amplitude is thrown away with it.

Sidebands and bandwidth

A tone makes a carrier plus a whole family of sidebands, spaced by the audio frequency. Their heights follow Bessel functions of the index β. Small β gives one pair of sidebands, much like AM. Large β spreads the power over many pairs, and the carrier line itself can shrink toward nothing.

There is no sharp edge, but Carson's rule is a handy rule of thumb: roughly 98% or more of the power lies within 2 × (Δf + fm).

	Deviation	Audio	β	Carson width
Ham voice, narrow FM	±5 kHz	3 kHz	1.7	16 kHz
FM broadcast	±75 kHz	15 kHz	5	180 kHz

Broadcast FM trades width for quality. Ham voice FM is deliberately narrower.

In practice

• Capture effect: when two FM signals share a channel, the stronger one (even by just a few dB) tends to win outright and the weaker one disappears, rather than the two mixing as in AM.
• Quieting: above a threshold, FM is much quieter than AM. Below it, noise rises suddenly and the signal falls apart.
• Pre-emphasis: noise in an FM receiver is worst at high audio frequencies, so the transmitter boosts highs and the receiver cuts them back. Net response is flat, with less hiss.
• FM is far less suited to weak-signal HF work: it needs a strong signal to quiet, and it is wide. See FM voice operation and Single sideband (SSB).

Related

• What modulation isPutting information onto a carrier wave.
• Amplitude modulation (AM)Varying carrier strength: sidebands, bandwidth and efficiency.
• FM voice operationRepeaters, simplex and the "local" mode.
• Repeaters and how to use themOffsets, tones and etiquette.
• Squelch, CTCSS and DCSKeeping the speaker quiet until it matters.
• Time domain, frequency domain and FourierSeeing a signal as its sine-wave ingredients.