Also known as: frequency modulation

Frequency modulation (FM) encodes information by varying a carrier’s frequency while its amplitude stays constant.¹ The amount of swing is the deviation.

FM varies the carrier's frequency while amplitude stays constant, which is why it shrugs off amplitude noise.

How it works

Because the information lives in frequency, not amplitude, an FM receiver can ignore amplitude noise, giving clean audio. FM also shows a capture effect — the strongest signal dominates a channel.

Relevance to SDR

FM broadcast (wide deviation) and narrowband FM two-way voice are everywhere; the latter is the analog cousin of the digital FSK voice modes GopherTrunk decodes.

Sources

1. Frequency modulation — Wikipedia, for the definition, deviation, and the capture effect. ↩

See also

• Modulation
• Amplitude modulation (AM)
• Single sideband (SSB)
• Frequency-shift keying (FSK)

Learn it step by step

• Analog modulation — AM, FM, SSB