A Novel Ultra-Wideband Common-Mode Suppression Circuit Based on Multi-Mode Transmission Line

In this paper, a novel ultra-wideband common-mode suppression circuit is proposed based on the concept of multimode transmission line. Wideband common-mode suppression is achieved by combining two coupling traces in the vertical direction. One coupling trace placed above the differential pair is the quarter-wavelength stepped impedance resonator and the other is half-wavelength resonator below the differential pair. The quarter-wavelength resonator can generate two common-mode transmission zeros while the half-wavelength resonator can generate one common-mode transmission zero. Three common-mode transmission zeros correspond to three different resonant modes. It is found that the common-mode noise can be reduced over 10 dB from 2.37 to 9.34 GHz with the fractional bandwidth of 119%. Meanwhile, the differential-mode cutoff frequency can maintain up to 10 GHz with -3 dB insertion loss, which is good enough for keeping differential signal integrity.