SINAD
SINAD stands for Signal-to-noise and distortion ratio. It is a measure of the quality of a signal from a communications device, often defined as:
where 
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]
Due to widespread use, SINAD has collected a few different definitions. SINAD is calculated as one of:
- 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 now possible for SINAD to be less than 1. This definition is used when SINAD is used in the calculation of ENOB for an ADC.
Regardless of the exact definition, it is always true that a lower SINAD value means worse performance of the system.
In a real world example 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.
Convention is that in a narrow band FM voice radio system a Signal, Noise and Distortion level of 12dB SINAD is the maximum acceptable level of noise that will not swamp intelligible speech.
A typical example quoted from a commercial hand held VHF or UHF radio;
- Receiver sensitivity: 0.25μV at 12dB SINAD
The example above is stating that the receiver will produce intelligible speech with a signal at its input as low as 0.25μV. This value is typical for VHF commercial radio and 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.
See also
References
- ^ Glossary of Frequently Used High-Speed Data Converter Terms, Maxim Integrated Products, 2001-12-17, http://www.maxim-ic.com/app-notes/index.mvp/id/740, retrieved 2011-11-05
External links
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).