Comparison of mixed-mode S-parameters in weak and strong coupled differential pairs

In this paper, it is studied how the rate of coupling (weak vs. strong) in edge-coupled differential transmission lines on a printed circuit board (PCB) affects frequency behavior of mixed-mode S-parameters. This is important for signal integrity (SI), and also may be useful from electromagnetic compatibility (EMC) point of view, since the enhancement of mode conversion (from common-mode to differential mode, and vice versa) may result in common-mode noise in high-speed digital electronics. Slightly imbalanced in length microstrip and stripline differential pairs are considered. The study is done using full-wave numerical electromagnetic simulations. Various technological features are modeled in this work: rectangular vs. trapezoid shape of a signal trace cross-section; copper foil roughness; solder mask over microstrip lines; and presence of an epoxy-resin “pocket” (EP) between the stripline traces (dielectric properties of the EP are different from the homogenized parameters of the ambient dielectric where these traces are embedded).