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 waveg uide receiver employing a 0.49 mum2 Nb/AlO(x)/Nb tunnel junction. A 50 % operating bandwidth is achieved by using a RF compensated junction m ounted in a two-tuner reduced height waveguide mixer block. The juncti on uses an ''end-loaded'' tuning stub with two quarter-wave transforme r 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 side band mixer response. Best noise temperatures are typically obtained wi th a mixer conversion loss of 0.5 to 1.5 dB giving uncorrected receive r and mixer noise temperatures of 50K and 42K respectively at 300 and 400 GHz. The measured double sideband receiver noise temperature is le ss than 100K from 270 GHz to 425 GHz with a best value of 48K at 376 G Hz, within a factor of five of the quantum limit. The 270-425 GHz rece iver 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 indi cate large conversion gain and receiver noise temperatures below 50K D SB from 200-300 GHz. Best operation is again achieved with the mixer t uned for 0.5-1.5 dB conversion loss which at 258 GHz resulted in a rec eiver and mixer noise temperature of 34K and 27K respectively.