Representation of noise sources in pumped nonlinear systems

Several different methods have been used to represent physical sources of noise at a frequency ¿0 in a linear 2-port network, and this representation usually simplifies the analysis of propagation of noise through the network when it is connected to another network or a set of networks. It is shown that similar methods can be used to represent physical sources of noise in a nonlinear network when it is pumped by a strong signal and when the noise sources present in the system are small. In such a case, the device acts as a linear system to the noise and the terminal noise components are linearly related. This kind of representation simplifies the noise analysis of such systems and enables us to investigate how the noise propagates through the system. It will be shown that, at each port, for each frequency ¿0 present in the system, it is necessary to have two equivalent noise sources rather than the one that is required in circuit theory of linear noisy networks.