Design and Analysis of a Waveguide NbN-based SIS Mixer using a Tuning Circuit with Two Half-Wavelength Distributed Junctions for the 900-GHz Band

We designed a waveguide SIS mixer based on NbN for the 900-GHz band. The waveguide mixer block with an MgO substrate was designed using Hewlett Packard's High Frequency Structure Simulator (HFSS) and uses a waveguide-to-microstip transition. Simulation by HFSS showed that a source impedance of around 60 12 can be achieved from 786 to 988 GHz by using an MgO substrate about 30 gra thick. The tuning circuit was proposed to reduce the current density of the NbN SIS junctions without impairing broadband operation; it consists of two half-wavelength distributed SIS junctions connected by a half-wavelength microstripline. Simulated mixer performance based on the experimental I-V curve of the NbN SIS junctions showed SSB receiver noise temperatures of below 150 K from 760 to 960 GHz, assuming a junction current density of 25 kA/cm2.