Tri-band bandpass filter using dual-mode resonators

This letter presents a compact tri-band bandpass filter (BPF) with controllable bandwidths. The proposed filter employs dual-mode resonators, i.e., crossed open-short stub loaded resonators and tri-section stepped impedance resonators. The center frequencies and bandwidths of the three passbands can be conveniently controlled by tuning the dimensions of two resonators. Moreover, on each side of each passband, there is at least one transmission zero, resulting in high skirt selectivity. To validate the design and analysis, a prototype of tri-band BPF is fabricated. Good agreement can be found between the measured and simulated results.