Design of tri-band balanced bandpass filter with controllable frequencies and bandwidths

A tri-band balanced bandpass filter (BPF) is presented based on novel multi-stub resonators, which are modified from the conventional multi-mode resonator. From the theoretical analysis, the first three resonance modes can be flexibly controlled under differential-mode (DM) operation. The extra stub-stub couplings are introduced to increase the degrees of freedom in extracting the coupling coefficients, helping to obtain controllable bandwidths. Different elements are loaded in the middle of the resonator to enhance the common-mode (CM) suppression across the given frequency range. To validate the design concept, a tri-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.