Compact wide stopband superconducting bandpass filter using modified spiral resonators with interdigital structure

In this study, we propose a novel resonator that is composed of a modified spiral with an embedded interdigital capacitor. A large ratio of the first spurious frequency to the fundamental resonant frequency is obtained, which is suitable for the design of filters with wide stopbands, and the circuit size is considerably reduced by embedding the interdigital structure in the spiral. For demonstration, a compact four-pole high temperature superconducting (HTS) filter with a center frequency of 568 MHz is designed and fabricated on double-sided YBCO film with a size of 11.4 mmx8.0 mm. The filter measurement shows excellent performance with an out-of-band rejection level better than 60.9 dB up to 3863 MHz.