Reduction of Common-Mode Noise in Bent Differential Interconnects for PAM4 Signaling

A wideband common-mode (CM) suppression filter for bend discontinuities in differential signaling is proposed. A known approach, using tightly coupled microstrips and providing CM reduction in the frequency range from DC till 6 GHz, is revisited and adapted to interconnect structures with a signal bandwidth (BW) up to 50 GHz. The design's effectiveness in reducing unwanted mode conversion and its flexibility in terms of the applied signal BW is assessed by means of simulations and measurements. Furthermore, the potential of the filter design for usage with 4-level pulse amplitude modulation (PAM4) signaling schemes is discussed. It is demonstrated that the high frequency implementation of this filter reduces CM noise by 62% and that it can successfully be applied in high speed interconnects using PAM4 signaling for data rates up to 70 Gbps.