Design of balanced dual-band bandpass filter with controllable bandwidths

A dual-band balanced bandpass filter (BPF) is proposed using stub-loaded resonators. Based on the theoretical analysis, it is found that the electrical performances of the differential-mode (DM), such as center frequencies and bandwidths can be controlled independently. The center frequency ratios of the two DM passbands can range from 1 to 2.6. The resonant frequencies of the common-mode (CM) are different from that of the DM, which helps to suppress the CM. Moreover, resistors are added in the middle of the resonator to enhance the CM suppression across the given frequency range. To verify the design method, a dual-band balanced BPF with controllable bandwidths and high CM suppression is designed, fabricated and measured. The measured result is in good agreement with the simulated result.