Definition
The noise factor F of a system is defined as:
where SNRin and SNRout are the input and output signal-to-noise ratios, respectively. The SNR quantities are power ratios. The noise figure NF is defined as:
where SNRin, dB and SNRout, dB are in decibels (dB). The noise figure is the noise factor, given in dB:
These formulae are only valid when the input termination is at standard noise temperature T0, although in practice small differences in temperature do not significantly affect the values.
The noise factor of a device is related to its noise temperature Te:
Attenuators have a noise factor F equal to their attenuation ratio L when their physical temperature equals T0. More generally, for an attenuator at a physical temperature T, the noise temperature is, giving a noise factor of:
If several devices are cascaded, the total noise factor can be found with Friis' Formula:
where Fn is the noise factor for the n-th device and Gn is the power gain (linear, not in dB) of the n-th device. In a well designed receive chain, only the noise factor of the first amplifier should be significant.
Read more about this topic: Noise Figure
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