A 691 GHZ SIS RECEIVER FOR RADIO ASTRONOMY

We report the development of a low noise heterodyne receiver optimized for astronomical observations in the 650 GHz atmospheric window, and specifically for the CO(J=6-->5) line at 691.5 GHz. The system is base d on an open structure SIS heterodyne mixer pumped by a continuously t unable solid state oscillator. A niobium SIS junction double array is placed at the end of an integrated V-Antenna. For broad band impedance matching a combination of microstrip impedance transformer and radial stub was used. Receiver noise temperatures of 550 K DSB at 684 GHz we re achieved at a 1.8 K physical temperature. The performance improved substantially when decreasing the temperature from 4.2 to 1.8 K. Compa rison of model calculations and Fourier transform direct detection mea surements of the tuning structure implies that this effect is likely d ue to the coincidence of operational frequency and the gap frequency o f the niobium.