Waveguide-type all--NbN SIS mixers on MgO substrates

We present the first experimental results on waveguide-type all-NbN SIS receivers fabricated on MgO substrates designed in a frequency range from 780 to 960 GHz. The receiver has a resonant distributed SIS junction one wavelength long at 870 GHz as the tuning circuit and a diagonal feed horn. The mixer chip was fabricated with accuracies of less than 10 im in width and +/5 jam in thickness by optimizing the cutting and polishing conditions for MgO substrates and was successfully mounted in a waveguide-mixer block. Using a conventional Y-factor method for receiver noise measurement, a minimum DSB receiver noise temperature of 700 K was obtained at an LO frequency of 770 GHz. The contribution to noise made by the input system was dominant in receiver noise and the input noise temperature was more than 300 K over the measured band.