Using common-mode filtering structures with microstrip differential lines in a multilayer printed circuit board environment

This paper explores some aspects of the application of complementary split-ring resonators (CSRRs) for common-mode (CM) filtering of differential transmission lines in a multilayer printed circuit board (PCB) environment. Many common-mode filtering structures using CSRRs or other defected ground structures become less effective when a second dielectric layer and metal layer is present below the CSRR layer. With a second layer present, CSRR or defected ground structure resonances can couple to parallel-plane waveguide modes to limit effectiveness of CM filtering, provide a coupling path for crosstalk, and result in unintentional radiation. This paper demonstrates that vias are useful in shielding CSRRs resonances from parallel-plane resonances resulting for more effective CM filtering, an improved signal integrity environment and lower levels of electromagnetic emissions.