HTS 4-pole bandpass filters with closed loop resonators and coupled lines using YBCO thin films for PCS

We designed, fabricated and characterized new type high temperature superconducting (HTS) 4-pole bandpass filters. We prepared epitaxial YBa₂Cu₃O_7-x (YBCO) thin films on MgO substrates by using a hollow cathode sputtering method. To reduce the physical size of the filter pattern, we used closed-loop resonators and parallel coupled-lines on the basis of a microwave circuit design theory. We observed that the HTS bandpass filters exhibited a center frequency of 3.9 GHz, a low insertion loss of about 0.42 dB, and an high skirt selectivity of 200 dB/GHz. The value of third order intercept measured at 80 K and at center frequency of 3.9 GHz was about 95 dBm