Design of a Broadband Common-Mode Filter With Four Transmission Zeros

This paper presents a design methodology on the broadband common-mode filter (CMF) with four transmission zeros for solving electromagnetic interference in high-speed differential digital systems. An equivalent circuit model is established based on the mushroom structure to characterize the common-mode bandstop response, and the simplified transmission zeros condition is derived to predict the transmission zeros. Furthermore, the design criteria as well as design flow through the simplified transmission zeros condition can be employed to synthesize the four zeros CMF systematically. Finally, in order to verify the proposed design methodology, a broadband CMF with four transmission zeros is fabricated on a four-layer printed circuit board. The experimental results show that the proposed CMF can reduce the common-mode noise over 10 dB from 1.23 to 5.15 GHz with a fractional bandwidth of 123%. Compared with other literature on mushroom-like CMFs, the proposed CMF has the widest common-mode suppression bandwidth. The electrical size is only {\text{0.0427}}\lambda _{g}^{2}, where λ _{g} is the guided wavelength of the central frequency of its common-mode stopband. The insertion loss of differential mode is below 3 dB from dc to 8.5 GHz. In addition, the measured eye diagram of the proposed CMF indicates that it maintains good signal integrity for high-speed differential signals.