Dual-mode dual-bandpass filter with transmission zeros using coupling mode conversion

A dual-mode dual-band bandpass filter (BPF) with a sharp skirt is proposed. The BPF uses a coupling mode conversion technique for four clear transmission zeros (TZs) at both ends of the two passbands. The inherent capacitances in this design ensure the absence of TZs at both ends of the lower passband, and exist virtually in the outer resonator because of the inner loop resonator connected to the outer resonator via a bi-interdigital coupled line. These inherent capacitances cause inductive coupling between the degenerate modes at the lower passband. In this study, a compensative perturbation patch with a value larger than the inherent capacitance is attached to the outer corner of the outer resonator in order to eliminate the effect of inherent perturbations, which indicates a conversion from inductive coupling to capacitive coupling. This study focuses on the design process and analysis using an equivalent circuit of the proposed BPF, and provides a detailed summary of the experimental results.