Tri-band high temperature superconducting filter with a wide stopband designed using a step impedance hairpin ring resonator

A high-temperature superconducting (HTS) tri-band bandpass filter with a step impedance hairpin ring resonator (SIHRR) was designed. The filter adopts the parallel mode of the loop resonator and the U-type step impedance resonator. The resonance mechanism of the resonator was studied, and the filter research process is described in detail. The three passbands can be independently adjusted by introducing an H-shaped secondary coupling structure. Finally, a third-order HTS filter with center frequencies and bandwidths of 3490 MHz (3.6%), 4290 MHz (3.4%), and 5190 MHz (5.1%) was successfully fabricated using photolithography and ion etching. The obtained filter had an out-of-band rejection of more than 37 dB up to 12 GHz, and the measurement results were in good agreement with the simulation results. Moreover, the structure of the filter is compact, with dimensions of 14 mm ×18 mm, which is equal to 0.53 λ_g × 0.3 λ_g.