A LOW-NOISE SIS RECEIVER MEASURED FROM 480 GHZ TO 650 GHZ USING NB JUNCTIONS WITH INTEGRATED RF TUNING CIRCUITS

A heterodyne receiver using an SIS waveguide mixer with two mechanical tuners has been characterized from 480 GHz to 650 GHz. The mixer uses either a single 0.5 x 0.5 mum2 Nb/AlO(x)/Nb SIS tunnel junction or a series array of two 1 mum2 Nb tunnel junctions. These junctions have a high current density, in the range 8 - 13 kA/cm2. Superconductive RF circuits are employed to tune the junction capacitance. DSB receiver noise temperatures as low as 200 +/- 17 K at 540 GHz, 271 K +/- 22 K at 572 GHz and 362 +/- 33 K at 626 GHz have been obtained with the single SIS junctions. The series arrays gave DSB receiver noise temperatures as low as 328 +/- 26 K at 490 GHz and 336 +/- 25 K at 545 GHz. A comparison of the performances of series arrays and single junctions is presented. In addition, negative differential resistance has been observed in the DC I-V curve near 490, 545 and 570 GHz. Correlations between the frequencies for minimum noise temperature, negative differential resistance, and tuning circuit resonances are found. A detailed model to calculate the properties of the tuning circuits is discussed, and the junction capacitance as well as the London penetration depth of niobium are determined by fitting the model to the measured circuit resonances.