Design of fractionally-spaced feed forward equalizer based on PAM4 signal at the transmitter
In order to suppress the high frequency loss of the 18-inch FR4 backplane and improve the transmission rate,a fractionally-spaced feed forward equalizer(FFE)based on PAM4 signal was designed adopting TSMC 0.18 μm CMOS technology.The bandwidth of the delay unit was boosted by source degeneration capacitance and the group delay at low frequency was adjusted by the technology of capacitance calibration.Meanwhile,the flatness of the group delay at different process corners was improved by the calibration technology of resistance and capacitance.The core area of the equalizer chip is 0.427 mm×0.475 mm and consumes a power of 258.6 mW.The post-simulation results show that the two-way 5 Gbps NRZ signals transmitted through FFE equalizer with 3-tap and 3T/4 spaced and 18-inch FR4 backplane,can get a 10 Gbps PAM4 signal eye diagram with 95 ps horizontal opening and of 30 mV vertical opening.