Design of superconducting ultra-wideband filter with wide stopband based on quintuple-mode resonator

In this paper, a new ultra-wideband (UWB) bandpass filter (BPF) based on QMR (quintuple-mode resonator) loaded with quasi-lumped elements is proposed for wide stopband application. The method of connecting five folded U-shaped resonators with rectangular patches to construct a QMR was proposed. By adjusting the relative position of the rectangular patches, the frequency band width of the filter can be freely adjusted. Further, quasi-lumped elements were loaded to the QMR to generate three transmission zeros to enhance stopband performance. The filter designed in this paper can independently adjust both the stopband and the passband, providing greater design flexibility. The passband of the final filter is 3.18–9.43 GHz, and the FBW (fractional relative bandwidth) is 99%. The out-of-band rejection is better than −15 dB in the frequency range of 9.6–28.42 GHz. The measurement results show good performance without any tuning, and are in good agreement with the simulation results.