Developments of ultra-narrowband high temperature superconducting bandpass filters

This paper presents the design and fabrication of an ultra-narrow band-pass HTS filter with FBW of 0.02% in C band. Novel approaches to solve the weak coupling and unwanted parasitic coupling problems in narrowband filter design were introduced. The filter shows band rejection better than 23 dB at frequencies 500 kHz away from the center frequency. The measured minimum insertion loss is 3.8 dB, corresponding to unloaded Q-factors of about 45000. The return loss is better than 17 dB and the band-edge steepness reaches over 60 dB/MHz at both the high- and low-frequency edges.