The noise figure (NF) is a dimensionless number that indicates the ratio of the signal-to-noise ratio of quadripoles. It is calculated from the ratio of output noise to input noise and is expressed in decibels( dB).
The noise figure is an important parameter of low-noise amplifiers( LNA), mixers or RF receiving equipment such as LNB converters. It limits the maximum possible gain factor. Since the noise of electronic components is temperature-dependent, the noise figure is referred to the reference temperature of 290 K. It is directly related to the noise factor (F) and is calculated from the tenfold logarithm of the noise factor.
At a selected input frequency, the noise figure is the ratio of the total noise power per unit bandwidth delivered to the output to the portion introduced at the input frequency by the input. This definition assumes a noise temperature of 293 K.