Predrag Acimovic, Senior Signal Integrity Engineer, PMC-Sierra

High-speed digital signals are usually carried over PCB boards, backplanes, and cables using differential lines. Unfortunately, in addition to the desired differential mode propagating along differential lines, a common mode can propagate as well. The common mode can present a source of radiation that can cause equipment to fail EMI requirements. This paper presents a novel structure that can be used to devise a true common-mode filter based on analog signal processing, which will affect only the common-mode signal and not attenuate the differential mode signal. It is well suited for use with multilayer PCB and does not require any extra components. There can be a number of implementations of this structure, but one effective and compact implementation uses strip-line (microstrip-line) to slot-line transitions. A second type of common-mode filter, based on loaded parallel stub implementation, that acts simultaneously as both common-mode filter and equalizer is also presented.