De-mystifying the impact of Intra-pair Skew on high-speed SerDes Interconnect

Abstract

At higher data rate (50 Gbps+), there is a need to understand the different interconnect impairments that lead to margin degradation. This paper presents the impact of intra-pair skew from a magnitude and polarity perspective on the interconnect electrical characteristics. Presented is the results of differential insertion loss, mode conversion and differential far-end crosstalk along with margin degradation through channel operating margin in operating at 106.25 Gbps PAM4 signaling. Simulated is a differential microstrip structure using a full-wave electromagnetic wave simulator. Results show that magnitude of intra-pair skew leads to degradation in differential insertion loss due to energy getting converted to common-mode. Insertion loss and intra-pair skew share a cosine relation and hence the impact of intra-pair skew on differential insertion loss is dependent on the magnitude of skew alone. In the case of differential far-end crosstalk, the impact of intra-pair skew may be constructive or destructive depending on the magnitude and polarity of intra-pair skew. Intra-pair skew leads to propagation time difference which in turn impacts the crosstalk and thus impact of intra-pair skew on crosstalk is dependent on the magnitude along with the polarity of skew. Presented is a overview on the importance of capturing the impact of common-mode to differential-mode conversion on margin degradation, which is not captured by many channel simulation tools. This work is an attempt at presenting the importance of limiting the intra-pair skew in the interconnect when targeting higher data rates (50 Gbps+).