The feed line resonator (FLR) method and it’s application to superconducting wideband filter design with extreme sharp skirt

This paper proposes the novel concept and method of the feed line resonator (FLR). By introducing the external quality factor adjusting line, the feed lines can be transformed to resonators, and can be used not only for providing external coupling, as well as generate extra transmission poles in the passband. The conditions for generating transmission poles by using the FLR are systematically investigated. The key advancement of the FLR method is that extra transmission poles are introduced, without increase the number of conventional resonators, the filter bandwidth can be extended and the selectivity also improved. The triple-mode FLR was proposed which can realize six extra transmission poles in the pass band and n transmission zeros out-of-band. A five-section multi-mode resonator proposed for wideband filter. Single-, two-, three- and four-stage wideband superconducting filters were designed to demonstrate the FLR method. These filters exhibit extreme sharp skirts, the attenuation slop at lower and higher band can reach 273.9 dB GHz⁻¹ and 523.7 dB GHz⁻¹, respectively. The passband is from 2.26 GHz to 5.54 GHz with the fractional bandwidth of 95%. The measure results agree well with the simulated ones.