Compact Superconducting Bandpass Filter Using Dual-Mode Loop Resonators

A compact superconducting bandpass filter using dual-mode loop resonator is developed for wireless local area networks (WLAN) in this paper. Shunt capacitive loading structure (SCLS) is applied to enhance the self-capacitance and electrical length of the loop resonator. Compared to the conventional resonator without SCLS, the central frequency of the proposed resonator is reduced by 27%. Furthermore, a four-pole superconducting bandpass filter with central frequency of 5.2 GHz for WLAN application is designed by cascading two resonators with SCLS. Also, a non-resonance-node resonator is added to realize the coupling between degenerate modes of resonators. Finally, the proposed filter is fabricated by depositing YBCO films on a 0.5-mm-thick MgO substrate. Its size occupies only 20 mm × 15 mm. Measured results demonstrate an insertion loss of less than 1.0 dB and a return loss of better than 16 dB. Measured results agree well with the simulated results.