Practical Approaches to Quantifying Intra-Pair Skew Impact via Insertion Loss Deviation

TL;DR

This paper introduces reciprocal metrics, SILD and FOM SILD, to quantify intra-pair skew impact on high-speed interconnects, demonstrating their correlation with bit error rate and practical utility in data center applications.

Contribution

The paper presents two novel reciprocal parameters, SILD and FOM SILD, for accurately measuring intra-pair skew effects in high-speed interconnects, overcoming limitations of traditional methods.

Findings

FOM SILD correlates strongly with BER.

BER remains stable below 0.2-0.3 dB FOM SILD.

Most cables have FOM SILD less than 0.1 dB.

Abstract

The surge in AI workloads and escalating data center requirements have created demand for ultra-high-speed interconnects exceeding 200 Gb/s. As unit intervals (UI) shrink, even a few picoseconds of intra-pair skew can significantly degrade serializer-deserializer (SerDes) performance. To quantify the impact of intra-pair skew, conventional time-domain methods are often unreliable for coupled interconnects due to skew variations across voltage levels, while frequency-domain approaches frequently fail to address reciprocity and symmetry issues. This can result in channels that meet skew specifications in one direction but not the other, despite the inherently reciprocal nature of skew impact. To address these limitations, we introduce two new reciprocal parameters for quantifying intra-pair skew effects: Skew-Induced Insertion Loss Deviation (SILD) and its complementary Figure of Merit (FOM SILD). Measurements conducted using 224 Gb/s SerDes IP and a variety of channels with different intra-pair skews demonstrate a strong correlation between FOM SILD and bit error rate (BER). Results show that when FOM SILD is below 0.2-0.3 dB, BER remains stable, indicating minimal signal integrity degradation; however, BER increases noticeably as FOM SILD exceeds 0.3 dB. Statistical analysis across more than 3,000 high-speed twinax cables reveals that the majority exhibit FOM SILD values less than 0.1 dB, underscoring the practical relevance of the proposed metrics for high-speed interconnect assessment.