Built-in Ring Transmission Line Structure for Signal Integrity Optimization of PAM4 Signaling in PCBs

In this article, we propose a built-in ring differential transmission line for four-level pulse amplitude modulation (PAM4) signal integrity optimization. An optimized ring structure, which comes from the simplistic idea to leverage the voltage produced by near-end crosstalk inside the ring, is attached at the end of the transmission line to improve the eye diagram. Additionally, we introduce the comprehensive design guideline with analytically derived formulas that is beneficial to designers to obtain an accurate design without significant effort. The proposed structure is implemented on a 100 mm transmission line, six-layer printed circuit board. The proposed structure can achieve an improvement of up to 32.9% eye height with a 4 Gsps PAM4 signal.