Update on the future BIPM on-site comparison program for Josephson ac voltage standards

BIPM has continued its program of pilot studies by investigating the possibilities to derive a protocol for Josephson Voltage Standard comparisons of ac voltages up to 7 V rms at low frequencies (<; 1 kHz) based on the differential sampling technique. We have confirmed that an agreement within a few parts in 10
⁷
can typically be achieved using a commercial calibrator as a transfer standard. We have also investigated other ac generators as possible candidates for a stable transfer standard. The sensitivity of the rms voltage measurement result to key parameters (filtering at the output of the ac source, grounding of the measurement setup, acquisition software, etc.) has been evaluated. The possibility of a measurement system (meter and software) that performs a full sampling of the ac source signal in addition to the differential sampling has also been investigated. It offers the possibility to follow the drift of the source.