Design of tri-band bandpass filter with tunable lower passband

A tri-band bandpass filter (BPF) with tunable lower passband using novel tri-mode resonators is proposed in this paper. Based on the theoretical analysis, it is found that three resonance modes of the proposed resonator can be controlled independently, which are designed for the tri-band filter with controllable passband frequencies. By controlling the reverse bias voltage of the varactors, the lower passband can be conveniently tuned without affecting the other passbands. Hook-shape feed-line is introduced to add extra degree of freedom in extracting external quality factors and high selectivity is achieved. The newly designed tri-band BPF is verified by circuit implementation and comparisons between the simulated and measured results are presented.