A High-Output-Swing 64-Gb/s PAM-4 Transmitter with a 4-Tap Hybrid FFE in 28-nm CMOS

Abstract

This article presents a high-output-swing 64-Gb/s four-level pulse amplitude modulation (PAM-4) transmitter, in which a 4-tap hybrid feed-forward equalizer (FFE) employing both fractionally-spaced pre-emphasis (FS-PE) and baud-spaced de-emphasis (BS-DE) is proposed. The PE technique enlarges the output swing by directly stacking pre-distortion pulses, and naturally consumes less power as it boosts only the desired symbols. The FS-based pre/post1 taps equalize the high-frequency loss, provide a wide compensation range beyond the Nyquist frequency, and realize a flexible equalization capability, while the BS-DE-based post2 tap compensates for the low-frequency channel loss. Fabricated in a 28-nm CMOS process, the 64-Gb/s transmitter obtains a differential output swing of 1.4 V_ppd with an energy efficiency of 1.53 pJ/bit.