Abstract
The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctionsand an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated atfrequencies above the gap frequency of Nb. To design the mixer, the complex refractiveindex of the MgO substrate was measured by using a THz time-domain spectrometer. Thesuperconductive properties of NbTiN films on MgO were examined in detail. Anuncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IFbandwidth of 4–12 GHz at a bath temperature of 4.2 K. By comparing the measured andcalculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimatedto be 0.98 dB at 810 GHz.
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