Using power spectra and Allan variances to characterise the noise of Zener-diode voltage standards

The differences between the voltage outputs of Zener-diode based electronic voltage standards (Zeners) and standard cells, between Zeners and a Josephson array voltage standard (JAVS) and between pairs of Zeners are measured. The data are analysed as time series. They are serially correlated so that the use of the usual expression for the standard deviation of the mean. The experimental standard deviation divided by the square root of the number of measurements, to characterise the dispersion is not justified. Noise in Zeners is of a 1/f nature and is characterised by using the power spectral density and Allan variance, a statistic used in time and frequency metrology. The uncertainty in the measurements of the 10 V outputs of 13 Zeners is limited by 1/f noise to a `floor´ value ranging between two parts in 10 ⁹ and eight parts in 10⁹ of the nominal output value. Methods are proposed for estimating the Allan variance of a Zener using either a standard cell, a JAVS, or a small group of similar Zeners