Cascaded noise figure calculations for radio receiver circuits with noise-aliasing properties

A new modified form of the Friis equation for the noise figure of a cascade of stages that can be used when one or more of the stages in the cascade have noise-aliasing properties is derived. The application of the modified noise figure equation in the noise analysis of a subsampling receiver front-end is given and the equation is verified in a circuit-level simulation. The modified Friis equation gives insight into the noise behaviour of subsampling radio receiver architectures and shows that the very high noise figures often associated with subsampling mixers can be misleading in a system context.