Novel Compact Quad-Band Bandpass Filter with Controllable Frequencies and Bandwidths

This letter presents a novel compact quad-band bandpass filter (BPF) with controllable operating frequencies and bandwidths. The proposed filter utilizes a novel second-order multi-stub-loaded resonator with four modes. Theoretical analysis indicates that the four modes can be controlled individually, resulting in controllable operating frequencies. Meanwhile, by controlling external quality factors and coupling coefficients, the bandwidths can also be adjusted individually. To validate the proposed idea, an experimental filter is implemented. Good agreement between the predicted and measured results demonstrates the proposed idea.