Ultra-wideband Balanced-Line-Based Common-mode Rejection Filter with Dumbbell-Shaped Conductors for Over 64 Gb/s Digital Transmission

Ultra-high-speed data transmission over 100 Gb/s necessitates an ultra-wide frequency bandwidth that is several times the Nyquist frequency, posing numerous tough challenges in conventional digital circuits. Among them, a common-mode noise can cause degradation of the differential signal quality and originate electromagnetic (EM) interference. Various commonmode rejection (CMR) filters have been reported, but the rejection bandwidths are insufficient for ultra-high-speed data transmission. In this paper, a balanced-line-based CMR filter with significant amount of CMR level at exceeding 40 GHz is proposed. Balanced lines (BLs) are categorized into two types: coplanar stripline (CPS) and parallel stripline (PSL), both having ultra-wideband CMR characteristic. In addition, the CMR level can be significantly enhanced by using a pattern composed of conductors alongside the BL. A PSL-based CMR filter with dumbbell-shaped conductors has been designed, and the performance of the implemented filter has been measured. The proposed CMR filter demonstrates the capability to significantly reject common-mode noise, achieving suppression level of 10 dB from 5.8 GHz to 40 GHz. Also, with a 64 Gb/s PAM-4 PRBS digital data, the proposed CMR filter exhibits a substantial reduction in ultra-wideband common-mode noise levels.