Measurement uncertainty in the presence of low-frequency noise

We discuss the measurement uncertainty in the presence of different types o f fundamental noise processes, especially 1/f noise, The treatment is based on the Allan variance of an n-sample average with two deadtime parameters relevant in metrology, Some results applicable to situations that are commo n in metrology, e.g., international comparisons, are derived, Experimentally, the output of Zener voltage standards in the frequency rang e from 10(-6) Hz to 10(3) Hz is studied and behavior closely resembling tha t of 1/f noise is found at all time scales of practical interest. The noise floor of a 10 V Zener standard was found to be 60 nV and the 1/f corner fr equency 3.5 Hz, Environmental variations cause the long-term (> 100 s) devi ations of the Zener voltage to slightly increase from the limiting 1/f nois e level.