Tri-band superconducting filter with controllable frequencies and bandwidths

A four-pole tri-band bandpass filter (BPF) is designed with stub-loaded resonators (SLRs). The SLR is constructed by a double spiral structure (DSS) loaded with two folded T-shape stubs at the midpoint. A tri-feeding structure is proposed to meet the required external couplings of the three passbands simultaneously. The center frequencies and bandwidths of the three passbands can be independently controlled. The filter is successfully designed and fabricated on a MgO substrate with a compact size of 42. 88mm× 13. 40mm. The three passbands are centered at 1. 00, 1. 15, and 1. 30 GHz, respectively, and the fractional bandwidths (FBWs) are all set to be1%. The measured results exhibit high performance and match well with the simulations.