Balanced dual-band superconducting filter using square ring loaded resonators with ultra-low insertion loss and common-mode noise suppression

Abstract

In this paper, a multi-mode square ring loaded resonator (SRLR) comprising a full-wavelength ring resonator and six open stubs is introduced. The resonant characteristics are discussed and analyzed using differential-mode (DM) and common-mode (CM) equivalent circuits. Compared to the conventional design, the proposed SRLR involves two advantageous modifications for high-order balanced structure. First, two pairs of open stubs on the four corners of the ring resonator provide more flexible control of the internal coupling between two SRLR units under DM excitation. In addition, the shorted H-shaped resonator is compact, which makes proposed structure suitable for high-order filter design with desired operating frequency and bandwidth. Second, one pair of open stubs is added to the ring resonator along the horizontally symmetric plane. We found that the proposed SRLR provides more design freedom for CM noise suppression without extra components or defected ground structure. In addition, the proposed SRLR topology structure is folded to avoid large circuit occupation. Based on our analyses, a fourth-order balanced dual-band bandpass filter was designed with two passbands operating at 2.2 GHz and 3.5 GHz with corresponding in-band insertion loss of 0.1 dB and 0.12 dB, respectively. The filter was fabricated using high-temperature superconductor (HTS) YBCO thin films on an MgO substrate. Good agreement between simulated and measured frequency responses was observed, which verifies the proposed structure and design method.