Noise Factor is the measure of degradation of the signal to noise ratio in a device. It is the ratio of the Signal to Noise Ratio at the input to the Signal to Noise Ratio at the output. Since the signal to noise ratio at the output will always be lower than the Signal to Noise ratio at the input, the Noise Factor is always less than 1. The Lower Noise Factors results in better performance of a devices.

Where, 

S_in = Signal level at input

N_in = Noise level at input

S_out = Signal level at output

N_out = Noise level at output

The Noise Figure is noise factor expressed in decibels (dB). It can be calculated by the following equation:

The Noise Figure is the parameter that is widely used to represent the noise level in RF systems and devices. Since it is represented in a logarithmic scale it is easier to use than the Noise Factor.