RESEARCH OF KEY TECHNIQUES ON PAM-4 SINGLE-ENDED HIGHSPEED TRANSMITTER

With the development of high-speed communication in recent years, the world has entered the era of big data and cloud computing. The solutions for faster and higher-capacity connectivity have become a hot research topic today. 112 Gb/s PAM-4 communication link serves as a mainstream researching objective in recent years, while 224 Gb/s PAM-4 is an emerging and state-of-the-art technology. Despite the bottlenecks of data rate and semiconductor technology, the transmission density is the main limitation of high-speed communication system. Most of the chips for high-speed communication interface are differential. However, the single-ended schemes can double the transmission density in backplane communication. Therefore, the main research content of this thesis is the design of single-ended high-speed and high-data-density transmitter (TX) drivers.

According to the background, this thesis discusses the research status at home and abroad, analyzes the critical technologies of 112 Gb/s PAM-4 and 224 Gb/s PAM-4 TX or transceivers (TRX), and points out the differences of the schemes under different processes and architectures. Afterward, it introduces the relevant theories of wireline communication systems and compares the advantages and disadvantages of different signal modulation methods. After analyzing the influence of noise, jitter, crosstalk, and channel attenuation on signal transmission, the equalization thechnologys of high-speed links have been studied. Also, the evaluation indexes of communication circuit, such as signal-to-noise ratio (SNR), eye diagram and bit error rate are discussed.With the above analysis, two chips have been fabricated in IHP SiGe 130 nm process.

The 112 Gb/s PAM-4 TX single-ended driver consists of input termination matching network, input buffer, pre-emphasis, and differential to single-ended output driver (D2S). The pre-emphasis circuit is based on CTLE, and the resistor-capacitor series branch is added to the emitter degradation to adjust the mid-frequency gain. The measured bandwidth with −8 dB @ 28 GHz channel is 38 GHz. The equalization capacity of the pre-emphasis is 7.9 dB – 15.2 dB. For 56 Gbaud signal, the SNR of NRZ eye diagram is 12.45, and the ratio of level mismatch (RLM) of PAM-4 is 0.865. Further more, in 112 Gb/s PAM-4 transmission, the output swing is 230 mV, and the power efficiency is 1.06 pJ/b.

The 224 Gb/s PAM-4 TX single-ended driver consists of input termination matching network, pre-emphasis, emitter follower, and differential to single-ended output driver. Passive attenuation is introduced to the input matching network. A feed-forward path is added to the pre-emphasis, which enriches the regulation range of equalization. The output driver uses high-frequency negative feedback to improve the high-frequency bandwidth of the output node. The post-simulated bandwidth of the driver is 69.6 GHz with the channel of −6.8 dB @ 56 GHz. The pre-emphasis can cover the channel attenuation from 2.5 dB to 6.7 dB. For 224 Gb/s PAM-4, the swing of output eye diagram is 160 mV, and the power efficiency is 0.619 pJ/b.

随着通信技术的发展，5G时代已经来临，世界也走进了大数据和云计算的时代。112 Gb/s PAM-4技术是现今的研究热门，为了进一步提高通信速率，224 Gb/s PAM-4技术也终于步入研究人员的视野。目前主流的高速通信接口电路主要为差分电路，如果把原来的差分通道用来进行两通道的单端通信，那么通信接口的信息传输密度就是原来的两倍。因此，本论文的主要研究内容为设计单端高速高密发射机驱动器。

本论文结合研究背景，回顾了国内外的研究现状，对国际上112 Gb/s PAM-4和224 Gb/s PAM-4发射机或收发机的关键技术进行分析，指出了不同工艺和不同架构下的方案的差异。本文接着介绍了有线通信系统的相关理论，对比了不同信号调制方式的优缺点，分析了噪声、抖动、串扰和信道衰减对信号传输的影响，研究了发射机的主要信号均衡调制方式，还介绍了通信电路的评价指标，如信噪比、眼图和误码率等。结合以上分析，本论文基于IHP 130 nm锗硅工艺完成了两个芯片设计。

112 Gb/s PAM-4发射机单端驱动器由输入端接匹配网络、输入缓冲器、预加重电路和差分转单端输出驱动器组成。预加重电路以CTLE为基础，在发射极退化加入了阻容串联进行中频调节。在使用−8 dB @ 28 GHz的信道条件下，芯片的测试数据达到38 GHz的−3 dB带宽和7.9 dB到15.2 dB的预加重电路均衡能力调谐范围。在56 Gbaud数据率下，NRZ眼图的信噪比达到12.45，摆幅为140 mV；PAM-4眼图的电平失配比为0.865，摆幅为230 mV。在112 Gb/s PAM-4的传输环境下，实现了1.06 pJ/b的高能效比。

224 Gb/s PAM-4发射机单端驱动器由输入端接匹配网络、预加重电路、射极跟随器和差分转单端输出驱动器组成。输入匹配网络加入了无源衰减，预加重电路添加了前馈支路丰富了均衡调节范围。输出驱动器使用了高频负反馈，提高了输出节点的高频带宽。在使用−6.8 dB @ 56 GHz信道的条件下，该发射机单端驱动器后仿真结果达到69.6 GHz的−3 dB带宽，预加重电路的均衡能力可覆盖2.5 dB – 6.7 dB损耗。在112 Gbaud数据率下，眼图摆幅160 mV，能耗比0.619 pJ/b。

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