Optimization of a 115 GHz waveguide mixer based on an HTS Josephson junction

A waveguide mixer based on a Josephson junction made from a high-temperatur e superconductor (HTS) has been analysed at frequencies of 90 GHz and 115 G Hz. The mixer consisted of a single-grain-boundary bicrystal junction which was integrated with a bow-tie antenna and a microstrip filter structure fa bricated on an MgO substrate. Two different designs of mixer block were tes ted. Using the first design the best noise performance was measured at a lo cal oscillator (LO) frequency of about 90 GHz. At the projected LO frequenc y of 115 GHz significant deterioration of performance was observed. A compu ter simulation of the mixer chip and substrate channel structure was perfor med using the 3D simulation program HFSS. Sufficient RF signal rejection by the filter structure was confirmed at 90 GHz. After a simulation cycle at 115 GHz the substrate channel in the mixer block was modified. Using this s econd design we measured a double-sideband mixer noise temperature of 1090 K at 10 K operating temperature which is the best performance of an HTS Jos ephson mixer to date obtained at this frequency and temperature.