Dual-Band High-Temperature Superconducting Bandpass Filter Using Quint-Mode Stub-Loaded Resonators

Two dual-band high-temperature superconducting (HTS) bandpass filters (BPFs) using quint-mode stub-loaded resonators are presented. The quint-mode resonator is formed by loading four open-circuit stubs in shunt. By properly adjusting the lengths of the resonator, three transmission zeros separate five resonant frequencies into two groups. The first three resonant frequencies form the first passband. The two other frequencies are combined to create the second passband. In addition, a novel approach is proposed to extract the external quality factor Q_e based on the group delay of S₂₁. A magnetic coupling is introduced to adjust external coupling to meet the desired external Q_e. The first filter prototype is initially designed and implemented to verify the theory. Afterward, the second filter is constituted by introducing an additional half-wavelength resonator, resulting to improve the isolation between two desired passbands. Finally, two HTS BPFs are fabricated, and measured results agree well with theory.