180-425 GHZ LOW-NOISE SIS WAVE-GUIDE RECEIVERS EMPLOYING TUNED NB/AIOX/NB TUNNEL-JUNCTIONS

We report recent results on a 20% reduced height 270-425 GHz SIS waveguide receiver employing a 0.49 mum2 Nb/AlO(x)/Nb tunnel junction. A 50% operating bandwidth is achieved by using a RF compensated junction mounted in a two-tuner reduced height waveguide mixer block. The junction uses an ''end-loaded'' tuning stub with two quarter-wave transformer sections. We demonstrate that the receiver can be tuned to give 0-2 dB of conversion gain and 50-80% quantum efficiency over parts of it's operating range. The measured instantaneous bandwidth of the receiver is almost-equal-to 25 GHz which ensures virtually perfect double sideband mixer response. Best noise temperatures are typically obtained with a mixer conversion loss of 0.5 to 1.5 dB giving uncorrected receiver and mixer noise temperatures of 50K and 42K respectively at 300 and 400 GHz. The measured double sideband receiver noise temperature is less than 100K from 270 GHz to 425 GHz with a best value of 48K at 376 GHz, within a factor of five of the quantum limit. The 270-425 GHz receiver has a full 1 GHz IF passband and has been successfully installed at the Caltech Submillimeter Observatory in Hawaii. Preliminary tests of a similar junction design in a full height 230 GHz mixer block indicate large conversion gain and receiver noise temperatures below 50K DSB from 200-300 GHz. Best operation is again achieved with the mixer tuned for 0.5-1.5 dB conversion loss which at 258 GHz resulted in a receiver and mixer noise temperature of 34K and 27K respectively.