Compact Balanced Dual- and Tri-band Bandpass Filters Based on Stub Loaded Resonators

A stub loaded resonator (SLR) is presented in this letter, which can achieve dual-band or tri-band differential-mode (DM) bandpass response with a high common-mode (CM) suppression. By changing the length of the center-loaded stub, the resonant frequency of the CM can be varied without affecting that of the DM. This helps in simplifying the design and tuning processes of the balanced filters. In order to validate its practicalbility, two balanced bandpass filters (BPFs) with two and three DM passbands and good CM suppression are designed. Good agreement between simulated and measured results is observed. To our best knowledge, the proposed balanced tri-band BPF is the first ever reported.