Circuit models for mode conversion in clock signal distribution

Clock signal paths are often routed near other signal strips over a ground plane. These situations generate configurations of strips involving coupled microstrip sections interacting with non coupled microstrip sections, and generating transitions between coupled and uncoupled microstrip sections that heavily modify the propagations of both clock and information signals. These transitions usually cause a degradation of the signal integrity. The transitions generated by typical configurations of strips are: a Microstrip-Coupled Microstrip Tee and a Coupled Microstrip-Microstrip Cross. In this paper, rigorous circuit models for these transitions are presented and applied to the prediction of the behavior of a clock distribution near a signal path. The models presented are multimodal: they split the contribution of even (common) and odd (differential) signals (modes) into different ports, and allow the analysis of circuits that present different responses to either mode. The good agreement between circuit simulation and measurements validates the models.