A 650 GHz fixed-tuned waveguide SIS distributed mixer with no integrated tuning circuit

A Superconductor-Insulator-Superconductor (SIS) distributed mixer with no integrated tuning circuit has been designed for use in a fixed-tuned waveguide mixer block. This mixer employs two half-wave SIS resonators connected in series across the feed point of the waveguide circuit. By selecting appropriate dimensions for the resonators, we can obtain proper impedance matching over a reasonable input bandwidth. Since there is no lossy thin film microstrip line in front of the SIS junction, efficient power transfer from the waveguide feed to the mixer element can be achieved. A receiver noise temperature of 185 K at 650 GHz has been measured with a dual 0.33 x 16.8 mum SIS resonator. The noise temperature remains below 300 K over an input bandwidth of about 10%. This design can be used at much higher frequency where the loss of tuning circuit becomes significant.