A high-temperature superconducting filter withcontrollable transmission zero

This paper presents a novel microstrip feedline structure to introduce an extra and controllable transmission zero (TZ) with high rejection for a narrowband filter. This structure loads a reconfigurable capacitor at the end of the input feedline without changing the main structure of the filter. The capacitor is recognized by a 2-bit inter-digital capacitor array. The asymmetrical microstrip feedline structure is suitable for multiple-pole filter designs. A low-loss six-pole high-temperature superconducting bandpass filter with a reconfigurable TZ is designed and fabricated. The center frequency of the filter is 5.22 GHz with TZ at the lower stopband. The TZ can be tuned among four different states. The out-of-band rejection at the TZ frequency is higher than 90 dB, and the insertion loss is lower than 0.92 dB. The measured results are consistent with the simulations.