Superconducting microstrip line models at millimeter and sub-millimeter waves and their comparison

Paper i proceeding, 2003

Performance of superconductor — insulator - superconductor (SIS) tunnel junction mixers and their instantaneous input RF bandwidth are mostly depending on the integrated tunin g circuit used to resonate out SIS junction capacitance; Nb-based SIS mixers operate in the frequency range of about 80 - 1000 GHz and typically use microstrip-based integrated tuning circuitries. One of the major challenges in designing the tuning circuitry for SIS mixers is accuracy of models for superconducting microstrip line (SML). Modeling gives the only tool to solve the problem of designing SML-based circuits because for such high frequencies no direct measurements of a superconducting transmission line can be made with required high accuracy. However, creating an accurate model for such a superconducting transmission line is a challenge by itself. In the SML, produced usually by thin-film technology, the magnetic field penetration depth is comparable with the thicknesses of the dielectric and superconductors comprising the line. As a result the electromagnetic wave is propagated not only in the dielectric media but also inside the superconducting strip and ground electrodes constituting the SML. This creates dramatic changes in the transmission line behavior that should be carefully accounted by including the superconducting material properties into the modeling. Nb superconductor, as the most commonly exploited material, was used in this study for modeling of the superconducting microstrip though the same approach would work for any different BCS superconducting material. The purpose of this paper is to introduce a new model for SML and compare it with previously suggested models and results of SML numerical simulation.