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:

which is stating that the receiver will produce intelligible speech with a signal at its input as low as 0.25μV. Radio receiver designers will test the product in a laboratory following a procedure. A typical example procedure is as follows:

• With no signal present on the input, the noise and distortion of the receiver are measured at a convenient level
• A signal is applied to the input which causes the output to increase 12 dB
• The level of the signal needed to produce this is noted and is found to be 0.25 microVolts.

According to the radio designer, intelligible speech can be detected 12dB above the receiver's noise floor (noise and distortion). Regardless on how accurate this output power is regarding intelligible speech, having a standard output SINAD allows for easy comparison between radio receiver input sensitivities. This value is typical for VHF commercial radio while 0.35μV is probably more typical for UHF. In the real world lower SINAD values (more noise) can still result in intelligible speech but it is tiresome work to listen to a voice in that much noise.