Over-Moded Coaxial Test Cables for 425 Gbps PAM4 SerDes Validation Test Margin

The transition from 200 GE to 400 GE -class SerDes validation increasingly pushes test-fixture and cabling bandwidth toward the PAM4 Nyquist frequency, motivating renewed scrutiny of coax test-cable tradeoffs. This paper reports an exploratory measurement-driven study of three short coax testcable candidates used to 'wrap' a stepped-loss channel emulator, creating controlled 425 Gb/s PAM4 margin channels for comparative Channel Operating Margin (COM) analysis. Two candidate test cables remain higher-order-mode free through 110 GHz, whereas the third cable (LL095) exhibits empirically observed higher-order-modes (HOM) onset near 98 GHz, i.e., below the 106.25 GHz Nyquist frequency for 425 Gb/s PAM4. The over-moded coax test cable presents modest insertion-loss deviation (ILD) at high frequency. Despite this ILD, the LL095-wrapped composite channel exhibits lower loss near 98 GHz and a larger single-symbol pulse response, correlating with a higher computed COM under identical analysis settings. These initial results suggest an opportunity to consider single-symbol response (main-cursor) preservation as a complementary screening metric for test-cable selection, while emphasizing that broader empirical validation across additional cables, fixtures, and devices is required.