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 It is the digital descendant of FM: where FM sweeps the carrier continuously, FSK hops it among discrete tones. Two frequencies gives binary 2FSK; four gives 4FSK, the workhorse of digital land-mobile radio.

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 symbol selects one frequency offset (a deviation) from the carrier centre. In 2FSK a “space” and a “mark” tone carry one bit each. In 4FSK four evenly spaced deviations — for P25 at ±600 Hz and ±1800 Hz — each stand for a two-bit dibit, so the scheme carries 2 bits per symbol and doubles throughput for the same symbol rate. Because the transmitted waveform is always a constant-amplitude sinusoid whose only property that changes is frequency, FSK has a constant envelope: it can be amplified by cheap, efficient, saturated (Class C) power amplifiers without distortion, which is exactly why portable and mobile radios favour it over amplitude-varying schemes like QAM that demand a linear amplifier.

The quality-defining subtlety is phase continuity. Naively switching between two free oscillators produces phase jumps at each symbol boundary, which splatter energy into adjacent channels. Practical radio FSK is continuous-phase FSK (CPFSK): the same oscillator’s frequency is steered so the phase never jumps, giving a far more compact spectrum. Pushing this further with pulse shaping yields the continuous-phase modulation family — GMSK, MSK, and P25’s C4FM — which is why those schemes are best understood as filtered FSK rather than as something separate.

Variants

  • 2FSK / M-ary FSK — binary FSK is simplest; M-ary FSK uses 2, 4, 8, or more tones, trading bandwidth for bits per symbol.
  • 4FSK — four levels, 2 bits/symbol; the physical layer of DMR, NXDN, and P25 Phase 1 C4FM.
  • GFSK — Gaussian-filtered FSK used by Bluetooth and many ISM radios.
  • MSK / GMSK — the minimum- deviation, phase-continuous limit; GMSK carries GSM and AIS.
  • Audio FSK — AFSK and coherent FFSK key audio tones that then FM-modulate an ordinary voice radio.

Relevance to SDR

An FSK demodulator recovers the instantaneous frequency of the complex IQ baseband — the same phase-differentiation an FM discriminator does — then slices the result at the symbol instants into levels. The recovered symbol levels appear as three stacked openings on a symbol scope / eye diagram and as clusters on a constellation. GopherTrunk’s core decode chain is built on exactly this: it demodulates the 4FSK/C4FM physical layer of P25 Phase 1, DMR, and NXDN, recovering dibits from the four deviation levels before framing and vocoder decode.

Sources

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

See also

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