A Novel Dual-Function Circuit Combining High-Speed Differential Equalizer and Common-Mode Filter With an Additional Zero

A new circuit topology combining differential-mode equalizer and common-mode filter is proposed to apply in high-speed digital channel. A differential shunt stub (DSS), which is required in passive equalizer in improving the eye pattern quality, provides an additional transmission zero for the common-mode band-stop filter. This additional zero can be tuned dynamically by adjusting the DSS length and thus enhance the stop-band depth and bandwidth. This topology is implemented on a three-layer PCB. The eye opening is significantly improved for 8 Gb/s differential signals. In addition, the common-mode fractional bandwidth is enhanced from 13% to 46% by introducing an additional zero. Good agreement between simulation and measurement will be seen.