Differential Interconnects with Integrated Equalization and Common-Mode Filtering for Broadband Signal Integrity Enhancement in High-Speed PAM-4 Signaling

In high-speed differential interconnects on printed circuit boards, signal integrity (SI) issues arise when neglecting the inherent low-pass characteristic and ubiquitous presence of common-mode noise. This work proposes a novel open-circuited stub equalizer with integrated common-mode filter in order to compensate for the low-pass characteristic while simultaneously suppressing the transmission of unwanted common-mode noise to the receiver. A theoretical analysis of the open-circuited stub equalizer is conducted, and suitable approximations are derived to facilitate equalizer synthesis. Experimental validation is provided by a test structure consisting of a 20 cm microstrip serpentine delay line for which a 4 GHz equalizer is designed. Frequency-domain measurements show a 1-dB bandwidth up to 3.97 GHz with a passband ripple of 0.38 dB for the differential mode and a significant reduction in transmission for the common mode. The resulting SI enhancement increases the possible data rate from 6 Gbps to 20 Gbps using a PAM-4 (pulse amplitude modulation 4-level) modulation scheme.