Wideband and wide stopband superconducting bandpass filter using asymmetric stepped‐impedance resonators connected to open stub

A wideband wide stopband filter is designed using asymmetric stepped-impedance resonators (ASIRs) connected to a large open stub. The capacitive open stub and the parallel-coupled microstrip line are used to achieve the strong couplings for large fractional bandwidth (FBW). For a wide-stopband performance, the proposed filter uses ASIRs to improve the high-order spurious resonant frequency. The first and last resonators of the proposed filter are further optimized to suppress the spurious resonant frequency caused by open stub. The final filter has a 70% FBW centered at 4.87 GHz with 20-dB-rejection stopband up to 15.78 GHz (approximately 3.24f(0)). The measured insertion loss is less than 0.15 dB and the return loss is better than 17 dB.