Enhanced broadband common-mode filter based on periodic electromagnetic bandgap structures

In this article, we present an enhanced broadband common-mode (CM) noise filter based on periodic electromagnetic bandgap (EBG) structures with transmission lines. The unit-cell of the proposed filter is composed of a mushroom-like resonator and transmission lines. We developed a lumped circuit model with transmission lines to enable analysis of the unit-cell using the Bloch-Floquet theorem. The dispersion relations derived from the Bloch-Floquet theorem have CM suppression characteristics with respect to various circuit parameters. Although optimizing the period of the EBGs by the transmission line length widens the CM noise stop band, the impedance mismatching between the EBGs and the transmission lines causes differential-mode (DM) insertion loss. We solved this problem by adjusting the characteristic impedance of the DM signals on the EBG. We then built a prototype of the proposed CM noise filter and measured its performance. The frequency range of the 20-dB CM noise stop band was measured to be 5.3-9.74 GHz. The DM insertion loss was measured to be < 3 dB from DC to 10 GHz.