Generation of common mode in non-uniform differential interconnections

This work investigates the mechanism of generation of undesired common-mode (CM) currents in differential-line interconnects with slight non-uniformity owing to imperfect production process. To this end, three different non-ideal configurations are considered, obtained by perturbation of an ideally-uniform differential interconnect composed of two microstrips. By resorting to modal analysis, the governing transmission line equations are written as a perturbation of those of the ideal structure. This allows circuit interpretation of the mode conversion phenomenon in the analogy with crosstalk. The role played by each geometrical parameter (varying along the line) on the frequency response of the common- and differential-mode quantities induced at the line terminals is shown and discussed.