A 64Gb/s PAM4 Transmitter Achieving a Reconfigurable Output Swing Range from 0.333V to 1V without Stress Issue

This paper presents a quarter-rate transmitter (TX) supporting 32Gb/s non-return-to-zero (NRZ) and 64Gb/s four-level pulse amplitude modulation (PAM4). The TX incorporates a novel bias circuit structure to address the voltage stress issue when the reconfigurable differential peak-to-peak swing (VPPD) ranges from 0.333V to 1V. The key circuits include a quarter-rate pre-driver and a 7-bit digital-to-analog converter (DAC) based current-mode logic (CML) driver. The simulation results indicate that the TX operates without stress issues in both NRZ and PAM4 modes at various output swing configurations. The TX exhibits random jitter (RJ) is 131 fsrms with a 1VPPD swing and 132 fsrms with 0.333 VPPD swing, and achieves an energy efficiency of 1.35 pJ/bit at 64Gb/s PAM4 with a 1VPPD swing.