A varactor-tuned dual-passband microstrip filter using stepped impedance resonators

A dual-passband varactor-tuned filter using stepped impedance resonators (SIRs) is proposed in this paper. A varactor is centrally loaded on the SIRs, through changing the value of the bias voltage across the varactor, the center frequency of the upper passband can be tuned from 1.49 GHz to 1.97 GHz, however, the center frequency of the lower passband is at 926 MHz and it is fixed during that process. The coupling scheme used in this paper is good at eliminating parasitic and reducing insertion loss which due to no extra blocking capacitance compared with the conventional coupling structure. The measured performances are in good agreement with the simulated perfoemances.