EM emission of differential signals across connected printed circuit boards in the GHz range

In this paper it is shown that when differential signals pass between printed circuit boards (PCBs) through connectors, common mode signals can be induced due to e.g. imbalanced current paths or asymmetrical line configurations. In the GHz range connectors are electrically large. Therefore, we describe connector pins and traces as transmission lines in order to analyze differential mode to common mode signal conversion. Two conversion mechanisms, conductor length mismatches and asymmetrical ground pin configurations, are investigated and validated against Method of Moment (MoM) simulations using the CONCEPT-II package. The common mode signal can return through ground pins or as displacement current between PCBs. Both of the mechanisms can cause EMI problems. Radiation from a daughter card on a motherboard structure are then studied using MoM for various configurations. The variation of board configurations can have little effect in the GHz range while the ground pin configuration has a much greater impact on the EMI performance.