CSRR Common-Mode Filtering Structures in Multilayer Printed Circuit Boards

When complementary split ring resonator (CSRR) structures are used as common-mode (CM) filters in multilayer printed circuit board (PCB) environments, their effectiveness in filtering CM signals suffers as CM energy couples to the parallel plane waveguide which can be formed by the conducting layers below the CSRR layer. Via fences can largely prevent this coupling to allow some recovery of CM filtering effectiveness in addition to eliminating the signal integrity (SI) hazard introduced by the presence of excited parallel plane waveguide modes. Simulation and measurement results in initial investigations of filtering structures indicate CM filtering between 7 and 8 GHz and differential-mode (DM) transmission effective to 16 GHz. Simulations are provided showing that the via fences allow cascading for filtering at multiple frequencies or else to broaden filtering bandwidths. Next steps show simulations of CM filtering structures in differential links with effective DM transmission at higher frequencies.