Multimodal circuit model for the analysis of asymmetric shunt impedance transitions

In PCB circuits, signal traces are often routed close to other signal traces. This situation originates the appearance of coupled strip sections. Due to the fact that coupled strip sections behave as multimodal transmission lines, several electromagnetic modes propagate simultaneously. Any asymmetry in the circuit will generate an energy exchange among the propagating modes which will lead to signal integrity problems. In this paper, a new multimodal circuit model for a Three-Line-Microstrip Asymmetric Shunt Impedance Transition is presented. The model allows a rigorous and a quantitative analysis of the transition. The model is successfully applied to the analysis of a PCB configuration where a clock signal trace is routed close to two other signal traces. The good agreement between measurements and circuit simulations validates the proposed multimodal circuit model.