Common-mode suppressed differential bandpass filter based on open complementary split ring resonators fabricated in microstrip technology without ground plane etching

A differential (or balanced) bandpass filter based on open complementary split ring resonators (OCSRRs) coupled through admittance inverters is presented in this article. Pairs of OCSRRs are symmetrically placed in a mirror configuration between the strips of the differential line and are modeled by means of two series connected parallel resonators. For the differential (odd) mode, there is a virtual ground at the connecting plane between the OCSRR pairs, and the structure is roughly described by the canonical model of a bandpass filter, consisting of a cascade of shunt resonators coupled through admittance inverters. It is demonstrated that, through a proper design of the OCSRR stages, the common mode noise in the vicinity of the differential filter pass band can be efficiently suppressed. Due to the differential mode operation of the filter, it is not necessary to incorporate metallic vias to ground the OCSRRs. Moreover, as compared to other differential filters based on OCSRRs, defected ground structures are not present in the proposed filters. To illustrate the potential of the approach, two balanced bandpass filters are designed, fabricated, and characterized. © 2014 Wiley Periodicals, Inc.