Also known as: SINAD, signal-to-noise-and-distortion ratio
SINAD (signal-to-noise-and-distortion ratio) is the ratio, in decibels, of total output power — signal plus noise plus distortion — to the power of the noise and distortion alone.1 It is the standard way to specify analog receiver sensitivity, most familiarly as the “12 dB SINAD” figure quoted for FM radios and scanners. Unlike a plain signal-to-noise ratio, SINAD folds in harmonic and intermodulation distortion, so it reflects the quality a listener actually hears.
How it works
SINAD is measured at the receiver’s audio output while feeding a modulated test signal — conventionally a 1 kHz tone. The total output power is measured, then a sharp notch filter removes the 1 kHz tone, leaving only noise and distortion; the ratio of the two measurements, in dB, is SINAD. Because the denominator includes distortion as well as noise, SINAD is always slightly worse than the pure SNR of the same signal, and it is closely tied to total harmonic distortion plus noise (THD+N): SINAD ≈ −(THD+N) when expressed as a power ratio.
Receiver sensitivity is then defined as the RF input level required to produce a reference SINAD at the output. For analog FM land-mobile and broadcast gear the reference is 12 dB SINAD, roughly the point at which speech becomes comfortably intelligible; for some standards 20 dB (quieting) is used instead. A radio spec of “0.25 µV for 12 dB SINAD” means it takes just a quarter-microvolt at the antenna to reach that quality — a lower number is a more sensitive receiver.
In practice
- 12 dB SINAD is the near-universal benchmark for comparing analog FM receiver sensitivity across radios and scanners.
- SINAD is an analog metric, tied to recovered audio. Digital modes are instead graded by bit error rate at a reference RF level, because their output is bits, not audio — there is no gradual audio degradation to measure.
- A SINAD test exercises the whole chain — front-end noise figure, IF filtering, demodulation, and audio stages — so it captures distortion that a mid-chain SNR measurement would miss.
Relevance to SDR
SINAD remains the yardstick for the analog side of the radio world: broadcast FM, marine VHF, amateur FM, and conventional analog land-mobile receivers are all specified in SINAD, and it is how you compare the front-end quality of an SDR used for analog reception. It does not, however, describe GopherTrunk’s digital decode path — P25, DMR, and the other trunked digital modes GT targets live or die by BER and constellation error (EVM), not by recovered-audio SINAD. Where GT does touch analog audio — decoded vocoder output — the perceptual quality is set by the codec and the upstream BER, so SINAD is not a meaningful measure of it. Think of SINAD as the metric for the analog receivers GT often shares a band with, rather than for GT’s own decoders.