A broad band low noise SIS radiometer

A new type of ultra broad band SIS low noise radiometer has been developed. The SIS receiver instantaneous band of about 30% is significantly increase d compared to traditional designs. We avoid a restriction related to the li mited band of the Intermediate Frequency (IF) low noise amplifier by using a multifrequency heterodyne power source. In this regime the frequency mixi ng is performed simultaneously in a big number of the subbands and the IF s ignals are combined in a common amplifier. A low noise operation in this re gime is possible due to the quantum nature of the frequency mixing in SIS q uasi particle tunnel junctions, The SIS receiver covers 25 GHz band centered at 90 GHz with an equivalent n oise temperature of about 50 K, The receiver uses a fixed tuned SIS mixer w ith Nb/Al Ox/Nb junctions. The critical Josephson current density is about 2.6 KA/cm(2) and the junction area is of about 1.5 mu m(2). The limits to f urther improvement of the radiometer and the possibilities for extension of the technique to the sub millimeter band are discussed.