Design of Quad-Band HTS Bandpass Filter Using Quadruple-Mode Square Ring Loaded Resonator

In this paper, a quadruple-mode square ring loaded resonator (SRLR) is proposed to design a quad-band HTS bandpass filter (BPF). To investigate the resonant characteristics of SRLR, the resonator is analyzed by even-and odd-mode method. Then, two design graphs for the relationship of the parameters of electrical length and resonant frequencies were built. To construct a quad-band BPF, two quadruple-mode SRLRs are coupled with a pseudo-interdigital coupling structure and such that two poles are created in every passband. The filter is fabricated on a 2-in-diameter 0.5-mm-thick MgO wafer with double-sided YBCO thin films and measured at the temperature of 77 K. Measured results agree well with the simulated results and show that the insertion losses in passbands are less than 0.87 dB, which exhibit superiority in transmission loss.