The stability of some Zener-diode-based voltage standards

We report on a study, carried out from the point of view of users of Josephson array voltage standards, of factors limiting the precision of Zener-diode-based electronic voltage reference standards of the type (Fluke 732A) most widely used for Josephson voltage measurements. Frequency-dependent voltage noise was studied with a very low-frequency spectrum analyzer, and was found to be the major factor limiting short-term stability to about one part in 10/sup 8/. For two of the three reference standards tested, humidity effects were the principal factors limiting the medium- and long-term stability of the 1.018 V outputs. These effects were studied by subjecting the standards to step-function changes in relative humidity. For example, increases in ambient relative humidity of 0.01 were found to provoke decreases of 0.05-0.15 /spl mu/V in the 1.018 V output of one Zener. The time lag between the humidity change and the voltage change is characterized by a time constant /spl ges/19 days.