Impedance Mismatch Effects in Microstrip and Stripline EBG Common-Mode Filters

In this paper, impedance mismatch effects on the characteristics of common-mode (CM) electromagnetic bandgap (EBG) filters are studied using 3D full-wave numerical simulations. Herein, the terminations are fixed at 50 Ohms, and the effect of the differential line impedance variations are studied. Two types of CM EBG filters are considered in this work, both are designed using standard printed circuit board technology. The first group contains microstrip (MS) differential pairs running above the EBG plane, and the second group contains stripline (SL) differential pairs running on one of the layers next to the EBG plane. It is shown that the lines impedance mismatch in MS and SL has different effects on the EBG CM filter behavior. Overall, the considered variations in the widths of the traces and edge-to-edge separation distances in the MS and SL structures do not drastically deteriorate the performance of the EBG CM filters. Notch frequencies do not vary much, especially in the MS case, and the filter bandwidth at -15 dB just increases as mismatch increases, which is the favorable effect. Below -15 dB, the CM notch depth itself does not matter from the CM mitigation point of view as soon as the filter bandwidth at the required frequency is achieved.