Measurement uncertainty in the presence of low-frequency noise

We discuss the measurement uncertainty in the presence of different types of 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 common in metrology, e.g., international comparisons, are derived. Experimentally, the output of Zener voltage standards in the frequency range from 10^-6 Hz to 10³ Hz is studied and behavior closely resembling that 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 frequency 3.5 Hz. Environmental variations cause the long-term (>100 s) deviations of the Zener voltage to slightly increase from the limiting 1/f noise level