Dual-Band Superconducting Bandpass Filter Using Embedded Split Ring Resonator

In this paper, a superconducting YBa₂Cu₃O_y (YBCO) filter employs two sets of octagonal half-wavelength split ring resonators, which are designed to generate two passbands with compact size. The center frequency of the lower passband can be tuned by the outer resonators with loading spirals, and the center frequency of the upper passband can be adjusted by the inner resonators with interdigital structure. Both of them provide sufficient degrees of freedom to control the center frequencies and fractional bandwidths of the two passbands, independently. In addition, a pair of side-coupled stubs is applied to control the bandwidth of the lower passband whereas the interdigital structure is used to adjust the coupling strength between the inner split ring resonator (SRR). Based on the above-mentioned design methodology, a dual-band superconducting elliptic-function-response bandpass filter operating at 2.45 and 3.5 GHz is proposed in this paper. Simulated results show that the insertion losses in both passbands are less than 0.05 dB. Four transmission zeros are created close to the passband edges, resulting in high skirt selectivity. For fabrication, high-T_c superconducting YBCO films are deposited on double-side-polished 0.5-mm-thick MgO substrates. The overall size of this filter is 12.1 × 8.4 mm (about 0.25 × 0.17 λ_g, where λ_g is the guided wavelength at the center frequency of the first passband).