A new method for calibrating ac voltage waveforms based on measuring the difference relative to a synchronously synthesized waveform supplied from a programmable Josephson array is presented. In order to test the accuracy of this approach, the differences between two Josephson waveforms have been measured at the 1.2-V level for frequencies up to 5 kHz. The uncertainties achieved are at least a factor of ten lower than those from the direct sampling method. Below 400 Hz, this new method gives an uncertainty of 5 10 -8  (k = 1), while at 1 kHz, the uncertainty is about 2 10 7  (k = 1)
['Ralf Behr', 'Luis Palafox', 'Günther Ramm', 'Harald Moser', 'J. Melcher']
Direct Comparison of Josephson Waveforms Using an AC Quantum Voltmeter