6.7 A 128Gb/s PAM-4 Transmitter with Programmable-Width Pulse Generator and Pattern-Dependent Pre-Emphasis in 28nm CMOS
Kai Sheng, Weixin Gai, Zeze Feng, Haowei Niu, Bingyi Ye, Hang Zhou
Abstract
The ever-growing demands for high-bandwidth communications continuously push wireline links to operate at higher speeds. Recently reported transmitters (TXs) have achieved a data rate of more than 100Gb/s [1–6]. PAM-4 modulation, which doubles the data rate at the same symbol rate, has been widely adopted to make use of the link bandwidth more efficiently. However, the complex transitions introduce greater data-dependent jitter, decreasing the horizontal eye-opening. In addition, the transitions between non-adjacent levels bring about twice or three times inter-symbol interference (ISI) compared with transitions between adjacent levels, resulting in reduced vertical eye-opening. Although a feed-forward equalizer (FFE) can be used to mitigate these issues, it is usually implemented in a de-emphasis manner in PAM-4 TXs, which reduces the output swing and lowers the signal-to-noise ratio. The proposed TX incorporates a pulse generator with programmable width for optimizing transition edges and a pattern-dependent pre-emphasis scheme that performs equalization without sacrificing output swing.