The Beauty of Symmetry: Common-mode rejection filters for high-speed interconnects and balanced microwave circuits

Abstract

Common-mode rejection filters operating at microwave frequencies have been the subject of intensive research activity in the last decade. These filters are of interest for the suppression of common-mode noise in high-speed digital circuits, where differential signals are widely employed due to the high immunity to noise, electromagnetic interference (EMI) and crosstalk of differential-mode interconnects. These filters can also be used to improve common-mode rejection in microwave filters and circuits dealing with differential signals. Ideally, common-mode stopband filters should be transparent for the differential mode from DC up to very high frequencies (all-pass), should preserve the signal integrity for such mode, and should exhibit the widest and deepest possible rejection band for the common mode in the region of interest. Moreover, these characteristics should be achieved by means of structures with the smallest possible size. In this article, several techniques for the implementation of common-mode suppression filters in planar technology are reviewed. In all the cases, the strategy to simultaneously achieve common-mode suppression and all-pass behavior for the differential mode is based on selective mode-suppression. This selective mode suppression (either the common or the differential mode) in balanced lines is typically (although not exclusively) achieved by symmetrically loading the lines with symmetric resonant elements, opaque for the common-mode and transparent for the differential mode (common-mode suppression), or vice versa (differential-mode suppression).