High-Order Dual-Band High-Temperature Superconducting Bandpass Filter Using Asymmetric Input/Output (I/O) Feeding Technique

In this article, a high-order dual-band high-temperature superconducting (HTS) bandpass filter (BPF) is proposed, integrating a novel dual-mode hairpin ring resonator (HRR) and an asymmetric input/output (I/O) feeding technique. The proposed HRR features two independently controllable resonant modes, providing enhanced design flexibility for precise frequency and bandwidth control. By leveraging the dual-mode HRRs and optimized coupling schemes, a miniaturized, high-isolation, sixth-order dual-band HTS BPF is designed. The two passbands are centered at 1.42 GHz with a 7.04% fractional bandwidth for deep-space exploration and 1.9625 GHz with a 3.31% fractional bandwidth for 5G mobile communication. To further improve filter performance, an asymmetric I/O feeding technique is introduced, enabling independent control over transmission zeros (TZs). This approach results in a more compact design, greater flexibility, and eliminates the need for additional space. In addition, this technique allows independent tuning of the external quality factors for both passbands, providing greater design flexibility in optimizing filter performance. Four TZs are excited, i.e., two between the two bands, one in the lower stopband and one in the upper stopband, improving the skirt selectivity and stopband performance of the HTS filter. The working mechanisms of TZs are analyzed in detail. Finally, the designed filter is fabricated on an MgO substrate with YBCO thin films. The measured results agree well with the simulated ones, demonstrating maximum insertion losses of 0.25 dB and 0.21 dB in the two passbands, respectively.