Design and Sensitivity Analysis of EBG Stripline Common-Mode Filters

Workflow of electromagnetic bandgap (EBG) common-mode (CM) filter design of edge-coupled differential pairs on a printed circuit board (PCB) and sensitivity of its characteristics to variations of geometrical and material parameters are discussed. A number of simple 20-GHz EBG CM notch filters for differential stripline pairs are designed using full-wave numerical electromagnetic modeling, fabricating, and testing. The cases of one and two stripline differential pairs crossing the EBG patches are considered. The modeled and measured mixed-mode S-parameters are analyzed as functions of geometrical parameters, including size and number of EBG patches, gaps between them, geometry and position of signal traces across the EBG patches, and thicknesses of dielectric layers. The agreement between the measured and modeled mixed-mode S-parameters validates the models and allows for further numerical experiments. It is shown that the EBG filter performance depends on various subtle technological features, e.g., conductor surface roughness on metal surfaces; length imbalance on a differential pair; rate of coupling of lines within each differential pair and between the pairs; and some other factors. The systematic analysis of the trends for filter characteristics as functions of various design parameters allows for an optimal design of the CM notch filter.