Signal-to-noise and distortion ratio (SINAD) is a measure of the quality of a signal from a communications device, often defined as
where
When calculating the distortion, it is common to exclude the DC components.
Due to widespread use, SINAD has collected several different definitions. SINAD is commonly defined as:
- 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.
- 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.
Commercial radio specifications
A typical example, quoted from a commercial hand held VHF or UHF radio, might be:
Receiver sensitivity: 0.25 μV at 12 dB SINAD.This is stating that the receiver will produce intelligible speech with a signal at its input as low as 0.25 μV. Radio receiver designers will test the product in a laboratory using a procedure, which is typically as follows:
According to the radio designer, intelligible speech can be detected 12 dB above the receiver's noise floor (noise and distortion). Regardless of how accurate this output power is regarding intelligible speech, having a standard output SINAD allows easy comparison between radio receiver input sensitivities. This 0.25 μV value is typical for VHF commercial radio, while 0.35 μV 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.