Tunable Tri-band Bandpass Filter Using Varactor-tuned Stub-loaded Resonators

This paper presents a novel design of a tunable tri-band bandpass filter using varactor-tuned stub-loaded resonators. The first and the third passband center frequencies of the proposed tri-band filter can be independently tuned by controlling the different values of the capacitances and different varactor diodes, whereas the second passband center frequency is fixed. A prototype tri-band filter is fabricated and measured. From the experimental results, the first passband center frequency tunable range is from 1.24 GHz to 1.50 GHz, and the third passband center frequency tunable range is from 2.26 GHz to 2.80 GHz, while the second passband center frequency is fixed at 1.80 GHz. Both theory and experiment are provided to validate the proposed filter.