Crosstalk Impact of Periodic-Coupled Routing on Eye Opening of High-Speed Links in PCBs

Minimizing costs in printed circuit board layouts has led designers to use two signal layers between the reference planes, which introduce broadside coupling. Additionally, due to the fiber weave effect, designers route signal traces in a zig-zag fashion rather than in straight lines. This type of routing tends to be roughly periodic between the victim and the aggressor. The periodic-coupled routing creates periodic resonances in the near-end crosstalk and nulls in insertion loss (THRU) transfer functions as a consequence of Floquet modes. Due to the periodic resonances, the crosstalk is aggravated, which reduces the signal to crosstalk ratio. This study quantifies the effect of crosstalk due to periodic routing. In addition, design guidelines based on the angle of routing, length of period, and signaling speeds are formulated for designers using statistical bit error rate eye analysis.