Dual-mode dual-band balanced filter with high differential-mode frequency selectivity and enhanced common-mode suppression

A novel balanced dual-band microstrip filter based on dual-open-stub-loaded resonators (DOSLRs) is reported for the first time. The resonant frequencies of the DOSLR under differential-mode (DM) and common-mode (CM) excitation are analyzed. The presented dual-band balanced filter has inherent good CM suppression level within each DM passband because passbands of CM are misaligned with those of DM. In order to obtain a good CM suppression level in a relatively broad range and keep the desired DM response unaltered simultaneously, a stepped-impedance stub is added to the center of one DOSLR. From that purpose, the CM second harmonic of two DOSLRs are shifted away from each other. Furthermore, finite transmission zeros generated by the source-load coupling could be utilized to achieve high frequency selectivity in DM response. In addition, the positions of transmission zeros in CM response are also controllable to realize enhanced suppression within each DM passband. For validation, one balanced dual-band second-order filter working at 2.4 and 5.2 GHz is designed and fabricated, with its measured data matching well with the simulated counterparts.