This paper presents a novel design of a compact tunable dual-band bandpass filter (BPF) using varactor-loaded step-impedance resonators(SIRs). The proposed filter structure offers the possibility of two tunable passbands. Compared to uniform impedance resonator (UIR), the SIR has more excellent performance such as compact size, and adjustable harmonic resonant frequency etc. The harmonic resonant frequency of SIR is determined by the length of SIR and the ratio of the two characteristic impedances. By choosing proper ratio of the two characteristic impedances, the length of SIR can be shorter than UIR with the same resonance frequency. So the proposed filter using varactor-loaded SIRs can be compact. Both theory and experiment arc provided to validate the proposed filter. A prototype tunable dual-band filter is fabricated and measured. From the experimental results, the first passband center frequency tunable range is from 0.7 GHz to 1.2 GHz and the second passband center frequency tunable range is from 1.4 GHz to 2.15 GHz.
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