Wide-Stopband HTS Dual-Band Bandpass Filter Using a Folded Step Impedance Hairpin-Ring Resonator

Abstract

This article proposes a novel folded stepped-impedance hairpin-ring resonator and designs a compact sixth-order dual-passband high-temperature superconducting (HTS) filter with a comprehensive wide-stopband response. The design employs a stepped-impedance resonator and a hairpin-ring resonator. The resonator uses a dual-feed structure to achieve external coupling for both passbands. It introduces a U-shaped secondary coupling line between the resonators to flexibly control the internal coupling, enabling independent control of the frequency and bandwidth of the two passbands. Using this method, we designed a sixth-order dual-passband HTS filter. The first passband center frequency is 1.9 GHz with a bandwidth of 40 MHz (2.1% ), and the second passband center frequency is 2.8 GHz with a bandwidth of 70 MHz (2.5% ). The filter was fabricated on a YBa₂Cu₃O₇-δ, (YBCO) HTS thin film on a MgO substrate sized 35 mm × 14.5 mm (0.57λ_g × 0.23λ_g). Measurement results showed good agreement with simulation results, showing excellent passband performance. The return loss is better than 20 dB, and the insertion loss is less than 0.04 dB. The out-of-band rejection is better than 50 dB and can reach as high as 9 GHz.