Compact High-Temperature Superconducting Filter Using Multimode Stub-Loaded Resonator

A simple multimode resonator loaded by open- and short-end stubs is proposed in this paper. Its transmission characteristics and equivalent circuit models are investigated by even-odd mode analysis. In addition, the multiband filter design theory using the proposed resonator is explained, and the desired passbands can be conveniently allocated by properly choosing the dimension parameters of the stub-loaded resonator. For demonstration purposes, a compact high-temperature superconducting (HTS) filter operating at 1.57, 2.6, and 3.50 GHz is designed and fabricated. To sharpen the passband skirts of the filter, a source-load coupling configuration with interdigital structure is arranged to produce several transmission zeros. Then, multipath coupling diagrams are constructed, and the relative phase shifts of each path are studied to explain the responses of the triple-band filter. Furthermore, a grounding technique of the short-end stub is realized by subtly connecting with the metal wall packaging instead of via holes in the substrate. In consideration of the inaccuracy in HTS fabricating and packaging, an optimization scheme is proposed. The results of the sensitivity analysis revealed that the optimization scheme is viable. Finally, measurements are in good agreement with the electromagnetic simulations and verify the circuit design methodology.