A voltage source for low frequency ac waveforms with quantum traceability to a Josephson array reference using sampling and feedback

We describe a method for generating a low frequency sinusoidal calibration voltage with reference to a quantum standard, at the point of connection to the instrument under test (DUT). The voltage is made traceable to the maintained SI unit by a sampling system that compares the applied potential difference to that developed by an array of Josephson junctions. A feedback loop allows the use of a silicon DAC and active filter to source the current necessary to establish the potential difference, whilst maintaining quantum-derived accuracy with an uncertainty less than 1 mu V per volt.