A LOW NOISE 410-495 GHZ Nb/Al2O3/Nb SIS WAVEGUIDE MIXER

The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/Nb superconducting tunnel junctions is measured in the 400-500 GHz frequency range. Three different arrays of two junctions in series are analyzed. An array with integrated tuning stubs has lowest receiver noise temperatures of T-rec = 120 K DSB at 480 GHz, which is to our knowledge the lowest value reported in this frequency range. We compare data of the pumped I-V curves with the Werthamer- Tucker theory and demonstrate for the first time an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs, a mixer noise temperature T-mix = 90 +/- 30 K and a DSB mixer gain G(mix) = -12.5 +/- 0.6 dB is measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows a good qualitative agreement, indicating the applicability of this theory, to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage, is 220 K at 495 GHz. This frequency is 85 % of the reduced gap frequency, indicating that Nb SIS mixers can be used up to at least the gap frequency of 680 GHz.