EBG Common-Mode 20-GHz Microstrip and Stripline Filters: Sensitivity to Design Parameters

In this article, design and characteristics of 20-GHz electromagnetic bandgap (EBG) common-mode (CM) filters for microstrip (MS) and stripline (SL) differential pairs are considered. CM filter notch (resonance) frequency, depth, and width at the -15 dB level are calculated from the mixed-mode S-parameters. The agreement between the measured and modeled S-parameters of the baseline structures validates the models and allows for further numerical experiments. The systematic analysis of the trends for both MS and SL filter characteristics as functions of various geometrical and material design parameters is presented. Sensitivity to various technological features (conductor surface roughness, trapezoid shape of the traces, and line length imbalance) is also analyzed. It is shown that the MS EBG structure is more sensitive and less monotonic to most of the design parameters than those for the SL. This is attributed to the less electromagnetic field containment and more complex mode structure in the MS structures as compared to the SL. Systematic sensitivity analysis allows for an optimal design of the EBG CM notch filters for any printed circuit board layer where a differential pair runs. Though the analysis is presented for 20-GHz EBG filters, most of the trends would be applicable to the filters for the other frequencies.