A Low-Temperature Microwave Noise Standard

An X-band thermal noise source is described that consists of a waveguide termination, cooled in a cryogenic environment, along with temperature and pressure monitors and controls. With liquid helium as the cryogen, the effective noise output at the room temperature flange can be set to values around the boiling point of helium (4.2/spl deg/K) with an accuracy of /spl plus mn/ O.O5/spl deg/K. With liquid nitrogen as the cryogen, the output temperature can be set to values around 77/spl deg/K with an accuracy of /spl plus mn/ 0.13/spl deg/K. This accuracy is made possible by several unique features. First, the terminating waveguide section is a vapor bulb thermometer with the absorbing load surrounded with the cryogenic liquid. Second, heat exchangers are used to permit an unusually short waveguide transition seetion between cryogenic and room temperature. Third, an absolute pressure regulator is used to control the cryogen boil-off rate resulting in a temperature stability of 0.003/spl deg/K for helium and 0.02/spl deg/K for nitrogen. At present, the useful accuracy of the standard is limited to /spl plus mn/0.1/spl deg/K because of uncertainties in the insertion loss (about 0.001 dB) of mating room temperature flanges.