Effect of Feed Forward Equalization on EMI-Related Common Mode Noise in 56-Gbps PAM-4 Optical Transmitter

Electromagnetic interference (EMI) signals, which radiate in the environment and cause the malfunction of electronic devices in proximity, are generated due to the common mode (CM) noise in differential circuits. This CM noise is produced because of the imbalance in the charging and discharging paths of the driver and equalizer circuits in the transmitter. However, the effects of the equalizer on CM noise have not been examined. This paper systematically investigates the effect of different feed forward equalizer (FFE) configurations on EMI-related CM noise in a 56-Gbps four-level pulse-amplitude modulation (PAM-4) optical transmitter, aiming to provide an optimized configuration with low CM noise and power consumption, while providing high compensation. The simulation results show that an asymmetric equalizer significantly improves the compensation performance. However, the offset in differential pair of output stage should be minimized to decrease the CM noise generated from the FFE.