Uncertainties in the Measurement of AC Voltage Using a Programmable Josephson Voltage Standard and a Phase-Sensitive Null Detector

Providing the traceability of alternating-current (ac) voltage measurements by means of a stepwise-approximated sine wave (SASW) produced by a programmable Josephson voltage standard (PJVS) is an attractive alternative to the conventional ac voltage standards based on thermal voltage converters, because a programmable Josephson standard has a calculable output voltage that does not drift with time. This paper investigates two aspects of uncertainties in the measurement of ac voltage using a sine wave generated by a PJVS with a lock-in amplifier as a null detector. First, the uncertainty introduced by transitions between successive voltage levels of the SASW on the fundamental and root-mean-square values is examined. Second, the lock-in amplifier response to the harmonics of this synthesized sine wave is discussed. Experimental evaluation shows that harmonics that are not rejected by the lock-in amplifier can cause a systematic error when measuring the ac-direct-current difference of a thermal converter with the synthesized sine wave from 0.5 to 3.0 μV/V, depending on the number of samples per period of the SASW. Furthermore, it is shown that this error can be reduced to 0.3 μV/V by calibrating the lock-in amplifier using a distorted signal having a similar harmonic content to the synthesized sine wave.