Common-mode filtering in multilayer printed circuit boards

Common mode (CM) conversion in differential communication links represents serious signal integrity (SI) and electromagnetic compatibility (EMC) hazards. Two types of common-mode filtering structures are investigated in this paper. The first employs complementary split ring resonator (CSRR) structures placed in the reference layer surrounded by via fences, and the second uses a composite right-/left-handed (CRLH) structure formed by placing metallic strips between the microstrip traces. Via fences were employed to surround the individual CSRR filtering structures, which allows cascading to either filter at multiple frequencies or else to broaden filtering bands. Via fences also prevent filtered CM energy from exciting parallel-plane waveguide (PPW) modes which would degrade the board's EMC and SI environments.