Dual-mode dual-band filter with flexible frequency ratio using degenerate modes of ring resonator

Abstract

A novel dual-mode dual-band bandpass filter design is introduced by virtue of modified ring resonator. According to qualitative analyses on the field distributions of a conventional ring resonator, T-junctions are utilized instead of ring transmission lines to control the frequency ratio of passbands. Meanwhile, each individual passband is formed using a pair of degenerate modes of the same order resonance. Stepped-impedance perturbations are adopted to split the degenerate modes and control the bandwidths. A design example is given to validate the design approach. The measured results agree well with the performances predicted by analyses.