This paper presents a miniaturized quadband high-temperature superconducting (HTS) bandpass filter based on a multistub-loaded resonator. Owing to the symmetrical structure of the proposed resonator, the odd- and even-mode analysis method can be used to derive its resonance frequencies. First, the frequency ratio between passbands may be significantly changed by adjusting the impedance ratio and fold spacing of the resonator. Moreover, the proposed filter exhibits well-controlled center frequencies and bandwidths for the four passbands, resulting in a considerably enhanced design possibility of quadband bandpass filters. A low insertion loss within each band is obtained using HTS technology. Additionally, the proposed filter was etched on an MgO substrate with a thickness of 0.5 mm and combined with YBCO thin films for demonstration. The results obtained were highly consistent with those of the simulation, validating the effectiveness and viability of the proposed method.

This study proposes a high-temperature superconducting (HTS) bandpass filter with a continuously tunable bandwidth and center frequency. The proposed filter combines several gallium arsenide varactors and a dual-mode resonator (DMR). The even and odd modes of the DMR can be tuned simultaneously using a single bias voltage. The capacitive value of varactors in the circuit is tuned continuously under continuous voltage and frequency tunability. External couplings and the interstage can be realized using an interdigital coupling structure; a fixed capacitor is added to the feeder to improve its coupling strength. A low-insertion loss within the band is obtained using HTS technology. Additionally, the proposed filter is etched on a 0.5 mm-thick MgO substrate and combined with YBCO thin films for demonstration. For the as-fabricated device, the tuning frequency range of 1.22~1.34 GHz was 9.4%; the 3-dB fractional bandwidth was 12.95~17.39%, and the insertion loss was 2.28~3.59 dB. The simulation and experimental measurement results were highly consistent.