Numerical evaluation of cable noise parameters under cryogenic thermal gradients

This contribution presents a very flexible tool for numerically evaluating the noise parameters of a transmission line subject to an arbitrary thermal gradient. Contrary to previous methods, the proposed approach allows straightforwardly to take into account possible variations of electrical parameters along the propagation direction, such as may be expected when temperature ranges between very different values. The main application of the proposed method is cable modeling in noise-figure measurement setups under cryogenic operation: in such circumstances, indeed, the coaxial cables (or waveguide) at the interface between the outside and the inside of the cryogenic chamber are subject to remarkable temperature excursions. As a consequence, significant de-embedding errors may arise if the cables are not correctly modeled, given the very low values of noise figure which are commonly exhibited by in cryo-cooled active devices.