高线性度大摆幅高速PAM4光发射机驱动电路设计

Design of High-Speed Four-Level Pulse Amplitude Modulation Optical Transmitter Driving Circuit with High Linearity and Large Swing

基于IHP 0.13μm SiGe BiCMOS工艺,设计了一款应用于超高速光通信的四级脉冲幅度调制(PAM4)光发射机驱动电路.整体电路包括两路高速非归零码(NRZ)通道(最高有效位通道和最低有效位通道)、时钟缓冲级、电流模式逻辑(CML)加法器和输出缓冲级.鉴于PAM4信号的高线性度要求,为解决传统设计中电平失配率(RLM)较低的问题,设计了带有低压共源共栅电流镜的CML加法器,避免电流镜像不精确和输出阻抗随加法逻辑变化所带来的非线性因素.同时,针对传统输出级带宽不足与摆幅过小的问题,设计了有源电感负载的f_(t)倍频器结构,在实现同等增益下更高电路带宽的同时,突破传统输出级设计中输出摆幅与阻抗匹配之间的矛盾.后仿真结果表明,在电源电压3.3 V、输入信号为两路100 mV的25 Gb/s NRZ信号的条件下,所设计的两路高速NRZ通道可实现约18.3 dB的增益和19.65 GHz的带宽,带宽范围内等效输入噪声电压小于37.6 nV/√Hz.整体电路可实现50 Gb/s PAM4输出信号,输出眼图清晰,且获得了RLM为98.6%的高线性度,输出摆幅达1.5 V.

In this paper,a 0.13μm SiGe BiCMOS-based four-level pulse amplitude modulation(PAM4)optical transmitter driving circuit was designed for ultra-high-speed optical communication application.Two high-speed nonreturn-to-zero(NRZ)channels(the most significant bit channel and the least significant bit channel),two clock buffers,a current mode logic(CML)adder,and an output buffer constituted the overall design.Due to the high linearity requirements of the PAM4 signal,a CML adder with a low-voltage cascode current mirror was designed to overcome the drawbacks of the low level mismatch ratio(RLM).This adder can avoid the nonlinear factors caused by the inaccuracy of the current mirror and the variation of the output impedance with the addition logic.A ft frequency multiplier with active inductance load was designed to avoid the narrow bandwidth and small swing,this design solves the tradeoff between output swing and impedance matching while achieving a wider circuit bandwidth with the same gain.The post simulation results showed that the two high-speed NRZ channels achieve a gain of about 18.3 dB,a bandwidth of 19.65 GHz,and an input-referred noise of less than 37.6 nV/√Hz within the-3 dB bandwidth at a power supply voltage of 3.3 V and 25 Gb/s input NRZ signals of 100 mV.The overall circuit achieved a 50 Gb/s PAM4 output with a clear output eye diagram.Moreover,a high linearity with an RLM of 98.6%was obtained,and the output swing reached 1.5 V.