Dual-band bandpass filter with tunable centre frequencies

This paper presents a dual-band bandpass filter with tunable centre frequencies. The proposed filter is mainly implemented by two λ/2 resonator and three varactor diodes. By adjusting all varactors can adjust the centre frequency of the two passband, if C_v1 is fixed, only tuned C_v2 can achieve first pass band fixed while the second passband could be adjusted. When adjusted the frequency of both varactors, the first passband can be tuned from 1.2 GHz to 1.33 GHz, the return loses better than 20 dB, and the insertion loss varies from 0.33 dB to 0.64 dB at the first passband, and the second passband frequencies from 1.95 GH to 2.15 GHz. The return loss is better than 15 dB over the entire tuning range of the second passband centre. The insertion loss varies from 1.33 to 1.53 dB. For the filter with tunable second passbands and fixed first passband, its second passband frequency is tuned from 2.02 GHz to 2.15 GHz, which includes TD-SCDMA and WCDMA bands.