Microwave noise standards

(engineering) An electrical noise generator of calculable intensity that is used to calibrate other noise sources by using comparison methods.

Electrical noise generators which produce calculable noise intensities at microwave frequencies, and which are used to calibrate other noise sources by using comparison methods. Noise standards are based upon the blackbody or thermal radiator and generate noise power according to Planck's radiation law. The practical realization of a blackbody in the microwave region consists of a microwave absorber with unity absorptivity. This can be achieved by using a transmission line terminated in its characteristic impedance, or in microwave terminology a matched termination. See also Heat radiation; Transmission lines.

The range of sources which require calibration and the desire to obtain low uncertainties dictate that microwave thermal noise standards are required with temperatures both above and below the ambient temperature. Sources have been developed with temperatures in the range from 4 to 1300 K (−452 to 1900°F). The low temperatures are normally achieved by immersion of the matched termination in a cryogenic liquid of which liquid nitrogen (77 K or −321°F) is the most common. Standards for measurement of high-temperature sources have the termination in a heated oven. A transition section supports the temperature gradient from the thermal termination to the ambient temperature output which connects to the measurement system. See also Microwave; Microwave measurements; Radiometry.