Reevaluating the NIST Uncertainties for AC-DC Voltage Transfer Difference

Determination of ac-dc difference of thermal voltage converters has traditionally been done by range-to-range scaling techniques, beginning at the voltage level and optimal frequency of the primary standards, and continuing until the parameter space has been completed. Range-to-range scaling propagates uncertainties of the measurement process at each step, so that the uncertainties become larger at values away from the primary standards, with the magnitude determined largely from the number of scaling steps. At the National Institute of Standards and Technology (NIST), we have recently fabricated multi junction thermal converters with exceptional properties over a large range of voltages and frequencies. Coupled with the use of an ac voltage standard based on quantum effects, we have reevaluated the NIST uncertainty matrix for ac-dc voltage transfer difference, and have made significant reductions in the uncertainties at all voltage and frequency levels.