On the Nature, Root-Cause, and Elimination of Ground Resonances in a Transmission-Line Based Connector With Guard Traces

Abstract

In this paper, we investigate the important phenomenon of “ground resonance” in a transmission-line based connector with guard traces added for crosstalk reduction. Through a modal analysis, we analyze the phase differences between the differential signal mode on the transmission line and differential ground mode introduced with the addition of guard traces that are also known as ground lines. We show that the so-called “ground resonance” is due to the phase difference between the two modes, which causes the reflection of the combined field, instead of the ground mode alone, at the ends of the connector. With this revelation, we propose three methods based on two approaches to eliminating ground resonances to reduce the insertion loss and crosstalk in a connector with multiple transmission lines. One approach is to modify the configuration of the two ends of the guard traces to reduce the phase difference between the signal and ground modes, and the other approach is to compensate for the phase difference by changing the propagation velocity of the signal mode. We demonstrate the effectiveness of these approaches through numerical simulation of two connectors: one based on the microstrip structure and the other based on the coplanar structure, with both a single differential pair and two adjacent differential pairs considered.