230 AND 492 GHZ LOW-NOISE SIS WAVE-GUIDE RECEIVERS EMPLOYING TUNED NB/ALOX/NB TUNNEL-JUNCTIONS

We report results on two full height waveguide receivers that cover the 200-290 GHz and 380-510 GHz atmospheric windows. The receivers are part of the facility instrumentation at the Caltech Submillimeter Observatory on Mauna Kea in Hawaii. We have measured receiver noise temperatures in the range of 20K-35K DSB in the 200-290 GHz band, and 65-90K DSB in the 390-510 GHz atmospheric band. In both instances low mixer noise temperatures and very high quantum efficiency have been achieved. Conversion gain (3 dB) is possible with the 230 GHz receiver, however lowest noise and most stable operation is achieved with unity conversion gain. A 40% operating bandwidth is achieved by using a RF compensated junction mounted in a two-tuner full height waveguide mixer block. The tuned Nb/AlOx/Nb tunnel junctions incorporate an ''end-loaded'' tuning stub with two quarter-wave transformer sections to tune out the large junction capacitance. Both 230 and 492 GHz SIS junctions are 0.49 mu m(2) in size and have current densities of 8 and 10 kA/cm(2) respectively. Fourier Transform Spectrometer (FTS) measurements of the 230 and 492 GHz tuned junctions show good agreement with the measured heterodyne waveguide response.