A novel wideband bandpass filter using open stubs multi-mode square ring resonator

A novel wideband bandpass filter (BPF) using open stubs multi-mode square ring resonators is proposed in this paper. The filter simultaneously realizes a broad passband, a compact size, and two transmission zeros at upper stopbands. The common-mode resonant frequencies is mainly determined by the length of ring resonator, while differential-mode resonance frequency can be flexibly controlled by the two identical stepped-impedance open stubs. A wideband bandpass filter is designed and fabricated for demonstration, indicating good agreement with the theoretical expectation.