  In this paper, a simple algorithm for detailed system-level thermal noise
analysis is developed, demonstrated, and verified. This method uses noise-wave
theory and noise covariance matrices to cascade noise and scattering parameters
of multiport devices at different temperatures. This method addresses the
effects of return loss, multiport isolation/coupling, and static temperature
differentials between components, and will work in cases where the noise
temperature is at or near the quantum noise limit. An ideal multidevice network
will first be demonstrated to show that this method's results are consistent
with the Friis cascade when component parameters such as return loss and
isolation are ideal. Following the ideal multidevice example, a cryogenic
experiment is conducted to demonstrate that the proposed simulation method is
successful when real data are used.
