Design and analysis of 230 GHz all-NbN superconductor-insulator-superconductor mixer

In this work, a 230 GHz all-NbN superconductor-insulator-superconductor (SIS) mixer was designed and analyzed. The NbN/AlN/NbN tunnel junctions (with an energy gap of 5.7 meV) and NbN/MgO/NbN tuning circuits were utilized. Full-height waveguides and bow-tie waveguide probes were adopted. The signal coupling circuit was designed and optimized to be transmissible to the given band. The quantum mixing characteristics of the all-NbN mixers were optimized with different-size SIS junctions. The mixer with a junction diameter of 1 μm achieves a conversion gain close to 0 dB and a noise temperature close to 40 K. The proposed design and analysis will provide technical support for the recovery and upgrade of the heterodyne receiver for the Leighton Chajnantor Telescope (LCT).