Design of High-Temperature Superconducting Dual-Band Filter With Multiple Transmission Zeros

A multiorder dual-band high-temperature superconducting (HTS) bandpass filter (BPF) with multiple transmission zeros (TZs) is designed in this paper. The theoretical characteristics of the introduction of TZs by the electric and magnetic mixed coupling and the cascade quadruplet (CQ) cross-coupling structures are analyzed, respectively. These two methods can be applied to the design of the multimode compact dual-band HTS BPF with controllable central frequency, bandwidth, and TZs. For demonstration, a Y-shape tunable resonator structure, which can simultaneously implement the internal electric and magnetic mixed coupling, and the external cross coupling are constructed to achieve the proposed dual-band BPF. The external quality factors required by the two passbands and the feasibility of independently adjusting the coupling strength between adjacent resonators are analyzed. Finally, a six-order dual-band HTS BPF with the center frequencies of 1490 (bandwidth of 40 MHz) and 2340 MHz (bandwidth of 80 MHz) is designed and fabricated on the double-sided YBCO/MgO/YBCO HTS thin films with the size of 17.2 mm × 14.7 mm (0.20 λ
_g
× 0.17 λ
_g
), and there are multiple of TZs located outside the two passbands, respectively. The out-of-band rejections are greatly improved to 70 and 80 dB due to these TZs, which fully exhibits the advantages of the proposed dual-band BPF. The measured results are in good agreement with the simulated ones.