Compact tri-band bandpass filter with controllable frequency using stub-loaded triple-mode resonator

A novel microstrip tri-band bandpass filter with controllable center frequencies by applying stub-loaded triple-mode resonator is presented. The resonator is constructed by loading a stepped-impedance stub (SIS) and an open stub on a traditional stepped-impedance resonator (SIR). The novel resonator can be utilized to design tri-band BPF. In addition, four transmission zeros generated by the resonator can improve the passband selectivity greatly. Center frequency and bandwidth of the tri-band BPF can be easily adjusted by tuning the stub length and coupling gaps respectively. To verify the application feasibility, a compact planar tri-band BPF is simulated, fabricated and measured. The simulated and measured results are in good agreement and show excellent in-band performance and sharp selectivity.