Design of Waveguide NbN-Based Superconductor–Insulator–Superconductor Mixer for 650 GHz

The submillimeter band accounts for a significant proportion of the photon energy in the universe, and an atmospheric window in the 600–720 GHz band is essential for astronomical detection because it contains a large number of molecular and atomic spectral lines. The superconductor–insulator–superconductor (SIS) mixer is one of the most sensitive coherent detectors. In order to improve the accuracy of detection and sensitivity and reduce noise temperature, a novel SIS mixer is proposed. It includes the design of the SIS junction, the signal coupling circuit, and the mixing circuit. After the initial optimization of the NbN-based SIS junction's parameters, a signal coupling circuit is designed, with a return loss of less than −16 dB and an embedding impedance of 60 Ω. The mixing circuit is composed of the impedance transformer and tuning circuit. It is investigated using the SuperMix library, including the analysis of the effects of transformer width and tuner length. The return loss is less than −7.5 dB, and it is reduced by about 34% when the transformer width is decreased from 2.6 to 2.0 μm, indicating a good transmission performance. The research results can provide a reference for the development of high-frequency and high-sensitivity SIS mixers.