Unintended effects of asymmetric return vias and via array design for reduced mode conversion

Return path discontinuities have been shown to have significant impact on high speed signals. Along with gaps in reference structures, return via placement can adversely affect signal integrity and radiated emissions. Of specific concern is mode conversion, often considered an indicator of radiated emission problems. Placing return vias around connectors is complicated by the presence of many signal pairs in a small area. Return vias are generally considered benign, but placing return vias can decrease the mode conversion for some pairs and increase it for others. This paper describes methods for efficiently optimizing via arrays containing many signal pairs. Genetic algorithms and a fast via solver are used for optimization studies to analyze mode conversion over regions around signal vias.