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 Joseph son 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, th e 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 lif nature and is characterised by using the power spectral density and Allan variance, a statistic used in time and frequency metrolog y. The uncertainty in the measurements of the 10V outputs of 13 Zeners is l imited by 1/f noise to a 'floor' value ranging between two parts in 10(9) a nd eight parts in 10(9) of the nominal output value. Methods are proposed f or estimating the Allan variance of a Zener using either a standard cell, a JAVS, or a small group of similar Zeners.