Stability measurements on noise sources

We report results of stability and repeatability measurements performed on a selection of different noise sources for selected frequencies between 12 GHz and 26.5 GHz. Measurements cover intervals classified as intermediate term (about 1 week) and long term (about 1 year or more). Noise sources measured include a commercial diode source, a gas-discharge source constructed by NIST, a specially modified commercial diode source, and a variable-temperature FET-based source. All sources exhibit excellent stability, typically consistent with zero drift in noise temperature within the uncertainty of the tests.