Theory of Noise Measurement
P. Heymann, Matthias Rudolph · 2021
This chapter looks at the definitions of the noise factor, which are most important for the different measurement methods. The simplest way to describe the noise of a two-port is to introduce an equivalent noise resistor at the input. This comes from vacuum-tube technology and is in many cases sufficient for field effect transistors in the MHz-range. The next step in describing the noise behavior is to know the two sources at the input, which we initially assume to be uncorrelated. This is a good approach for operational amplifiers. The 3 dB method uses a noise generator with variable power. This is usually a noise source with an attenuator. The 3 dB method is based on a fixed ratio of output power and the input power tuned to it. This is a disadvantage for the automatic measurement of many values, at different frequencies, which is overcome by the Y-method.