Compact narrowband high-order dual-band superconducting bandpass filters using stub-loaded double-folded resonators

Compact narrowband high-pole dual-band high-temperature superconducting (HTS) bandpass filters (BPF) have been developed that use newly developed stub-loaded double-folded microstrip resonators. The resonators have a weak coupling property that enables the distance between resonators to be reduced and that suppresses unwanted nonadjacent coupling. Moreover, it enables independent control of the resonant frequencies of the first and second passbands. Additional microstrip lines and a dual-feeding structure are introduced to enable flexible adjustment of the internal and external couplings of the two passbands. A prototype ten-pole dual-band HTS BPF operating at 1.5 and 2.0 GHz was designed and constructed. The filter for both passbands has a 1% fractional bandwidth and was fabricated using YBa2Cu3Oy thin film on an Al2O3 substrate. The measured frequency responses of the filter were in good agreement with the simulated ones. These results demonstrate that the compact filter has high selectivity, low insertion loss, and good stop-band rejection.