Signal-to-noise and distortion ratio (SINAD) is a measure of the quality of a signal from a communications device, often defined as:

$\mathrm{SINAD} = \frac{P_\mathrm{signal} + P_\mathrm{noise} + P_\mathrm{distortion}}{P_\mathrm{noise} + P_\mathrm{distortion}}$

where $P$ is the average power of the signal, noise and distortion components. SINAD is usually expressed in dB and is quoted alongside the receiver RF sensitivity, to give a quantitative evaluation of the receiver sensitivity. Note that with this definition, unlike SNR, a SINAD reading can never be less than 1 (i.e. it is always positive when quoted in dB).

When calculating the distortion, it is common to exclude the DC components.[1]

1. The ratio of (a) total received power, i.e., the signal to (b) the noise-plus-distortion power. This is modeled by the equation above.[citation needed]
2. The ratio of (a) the power of original modulating audio signal, i.e., from a modulated radio frequency carrier to (b) the residual audio power, i.e., noise-plus-distortion powers remaining after the original modulating audio signal is removed. With this definition, it is possible to have a SINAD level less than one. This definition is used in the calculation of ENOB for an ADC.[2]

Regardless of the exact definition, it is always true that a lower SINAD value means worse performance of the system. As the received RF signal becomes weaker it becomes progressively lost in the noise and distortion generated by receiver, demodulation and audio output drive circuits. By convention, the minimum acceptable SINAD level that will not swamp intelligible speech is 12dB for a narrow band FM voice radio system.[citation needed]

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A typical example, quoted from a commercial hand held VHF or UHF radio, might be: