Field Guide · technology

Also known as: frequency-shift keying, FSK, 4FSK

Frequency-shift keying (FSK) is digital modulation that switches a carrier between a fixed set of frequencies, one per symbol.1 Two frequencies gives 2FSK; four gives 4FSK.

1010 each bit picks a frequency (here 2FSK; 4FSK uses four)
FSK switches the carrier between set frequencies; four-level 4FSK underlies P25 C4FM and DMR.

How it works

Each frequency offset (deviation) represents a symbol. 4FSK carries 2 bits per symbol and is the workhorse of digital land-mobile voice — C4FM in P25, and the modulation of DMR and NXDN — because it tolerates efficient, non-linear amplifiers.

Relevance to SDR

An FSK demodulator tracks instantaneous frequency; the resulting symbol levels appear on the symbol scope and as clusters on a constellation.

Sources

  1. Frequency-shift keying — Wikipedia, for the definition and the 2FSK/4FSK variants. ↩

See also

Learn it step by step

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