HTS Wideband Bandpass Filter Based on Ladder Topology Circuit and Microstrip Transformation

This article presents a novel high-temperature superconducting ultra-wideband filter with a 108% fractional bandwidth. A high-order ladder topology circuit is utilized in the proposed filter to ensure a deep out-of-band rejection. Superconducting quasi-lumped elements are applied to reduce the insertion loss brought by the high-order circuit. The proposed filter derives from a low-pass filter (LPF) with a cut-off frequency at 2.7 GHz. Microstrip transformation is adopted in this LPF to achieve a lower stopband at 0.8 GHz. The proposed transformation provides a possibility to separately tune the lower and upper cut-off frequency in a microstrip filter. The measured results demonstrate a good agreement with circuit analysis and EM simulations, and show an excellent performance.