Superconducting Ultra-Wideband (UWB) Bandpass Filter Design Based on Quintuple/Quadruple/ Triple-Mode Resonator

This paper analyzes the coupling modes when multiple hairpin resonators are coupled. Three kinds of current distribution and direction states exist in the coupled hairpin resonators: the current along the clockwise direction, the current along the counterclockwise direction, and no current density distribution. Different combinations of these current states lead to different coupling modes. In order to distinguish these modes, auxiliary lines are introduced between each two coupled resonators to denote each coupling mode, and this method is theoretically based on the analysis of the eigenvector of the coupling matrix. A simple and effective technique to construct multi-mode resonators (MMRs) is then proposed. An n-mode resonator can be constructed by connecting n hairpins with small patches. Resonant modes of this class of MMRs were analyzed in detail using the proposed auxiliary lines method. Single stage and four stages of quintuple-mode resonators were used to design high-temperature superconducting ultra-wideband (UWB) filters by properly allocating the first five resonant peaks. Two UWB filters based on quadruple-mode and triple-mode resonators are also given. Parallel-coupled three lines were used to provide tight external couplings, which also generated two transmission poles. Measured results are in good agreement with the simulated ones without any tuning.