How this calculator works
Noise factor is the ratio of input SNR to output SNR at a stated reference temperature. Noise figure is that factor expressed in decibels. For a matched cascade of linear stages, the Friis equation divides each later stage's added noise by the available gain preceding it. Enter power gain, not voltage gain. The model assumes compatible noise definitions and excludes frequency-converting mixer ambiguities.
Formula
F = SNRin/SNRout; NF = 10 log10(F). Ftotal = F1 + (F2-1)/G1 + (F3-1)/(G1 G2) + ... .
Worked example
A noise factor of 2 corresponds to 3.0103 dB. Cascading a first stage with F1 = 2 and gain 10, followed by F2 = 4, gives Ftotal = 2.3 and NF = 3.61728 dB.
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Frequently asked questions
Can I add the stages' noise figures in dB?
No. Convert figures and gains to linear power ratios before applying Friis. The calculator performs that conversion.
Why does the first stage matter so much?
Its noise is not reduced by earlier gain. A low-noise first stage with useful gain can suppress later stages' contributions.
Can I enter a negative gain in dB?
Yes. That describes a linear power gain below one, such as attenuation. Noise figure still needs to be nonnegative in this conventional model.