Evaluation of Uncertainties of the Null-Balanced Total-Power Radiometer System NCSl

National Institute of Standards and Technology, Boulder. CO 80303-3328 Standard unrertaintjes are evaluated for the nuU-balarced, total-power, heicrodyned radiometer system with a switched input that was recently developed U NIST lo calibrate thermal noise sources. Eighi significant sources of uncertainty due tw sysletoatic effects arc iiicnlirtcd. two attributable to the two noise standards, and one each lo conncdors, the Inpui n)i!iinatch, the input switch a-^ymmetiy, the isolator, the broadband mismatch and the attenuator. The combined ^tandartl uncertainty of a typical coaxial rtoise source calibration at a representative frequency of 2 GHz is about \%. A strategy for reducing uncertainties W discussed.