A compact tunable balanced tri-band bandpass filter with high selectivity using stub loaded resonator

In this paper, a balanced tri-band band-pass filter (BPF) with the third independently tunable passband and the other two fixed ones based on a novel multi-stub loaded resonator (MSLR) is presented. Due to the symmetrical circuit structure, the even-and odd-mode methods are applied to analyze the differential-mode (DM) and the common-mode equivalent circuits of the proposed resonator, and the correspondingly resonant characteristics can be obtained. With the approximate design of the MSLR, the first three DM resonant frequencies are employed to form the three DM passbands. Then, the external coupling structure can be controlled by coupled lines to meet the required specifications. Furthermore, the frequency tuning of the third DM passband can be realized by adjusting the varactors loaded at the end of the resonators. Finally, the source and load coupling is introduced to improve the frequency selectivity. For verification, a balanced tri-band BPF with the tunable third DM passband was designed, manufactured and tested. The fabricated filter has central frequencies of the first two fixed bands located at 2.4 and 2.95 GHz, the third one tuning from 4.17 to 4.41 GHz, and the correspondingly fractional bandwidths (FBWs) are 12.3%, 3.5% and 1.9% +/- 0.89%. The experimental results are in good agreement with the simulated ones.