| the ratio of the exchangeable power spectral density of the noise appearing at a given frequency at the output of a given linear two port device, to the spectral density which would be present at the output if the only source of noise were the thermal noise due to a one-port device connected to the input and which is assumed to have at all frequencies a noise temperature equal to the reference thermodynamic temperature fixed by convention around 290 K
NOTE 1 – The spot noise factor F ( f ) is related to the equivalent spot noise temperature T ( f ) as follows:
where T0 is the thermodynamic reference temperature.
NOTE 2 – The value of the ratio F ( f ) may by expressed in decibels. In English, the term "noise factor" is generally employed when the ratio is expressed arithmetically, and "noise figure" is employed when the ratio is expressed in decibels.
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