DESIGN AND ANALYSIS OF A WAVE-GUIDE SIS MIXER ABOVE THE GAP FREQUENCYOF NIOBIUM

We present the design and experimental data of an SIS waveguide mixer for frequencies from 760 to 820 GHz. We use a Nb-Al2O3-Nb junction wit h an integrated niobium tuning structure. The waveguide mixer block co ntains no adjustable tuning elements. Design criteria for lossy tuning structures, differing from the impedance matching techniques used in the lossless case, are described. We separate the influence of the int rinsic mixing properties of an SIS junction from the effects of the po wer coupling to the signal source on the overall noise. This allows us to derive the contributions of the optics, the losses in the striplin e and the noise generated in the junction to the total receiver noise from the measurements. We achieve double sideband receiver noise tempe ratures of around 850 K at frequencies from 780 to 820 GHz and 4.2 K o perating temperature of the mixer. Cooling the mixer to 2.5 K results in an improvement of the receiver noise temperature by 150 to 200 K. T he bandwidth is presently limited by the local oscillator. The mixer w as successfully used in a dual channel receiver (440 to 490 GHz and 78 0 to 820 GHz) at the Submillimeter Telescope Observatory (SMTO) on Mou nt Graham, Arizona.