We report on the first-ever use of a quantum ac source to calibrate a thermal transfer standard as part of the NIST calibration service for such devices, with reductions in calibration uncertainty of as much as an order of magnitude over traditional ac-dc transfer methods. We briefly describe the basic quantum ac calibration system, its operation and measurement results. An analysis of the uncertainties for the measurements is presented and plans for further development are described.

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Abstract-NIST recently reported the first application of a quantum ac Josephson Voltage Standard for calibration of thermal transfer standards in the 1 kHz to 10 kHz frequency range. This paper describes preliminary work on extending its frequency calibration range up to 100 kHz by correcting systematic errors due to voltage leads. A ground loop created by the dc blocks, a previously unaccounted for source of high frequency systematic error, has been identified and its effects partially mitigated.

The NIST calibration service for ac-dc difference of thermal current converters relies on multijunction thermal converters as the primary standards, and various thermal converters and thermoelements (TEs) as the reference and working standards. Calibrations are performed by comparing the ac-dc difference of a customer’s thermal current converter to the ac-dc difference of a NIST standard current converter. Typical artifacts accepted for calibration include single-junction thermoelements, multijunction thermal converters, and transfer shunts for use with TEs. This paper describes the standards on which the calibration service is based and the results of the study to characterize the NIST standards over the extended frequency range from 50 kHz to 100 kHz at currents from 1 mA to 20 A. The general method for the frequency extension at high frequency involves the use of thermoelements in the 5 mA range, with small frequency dependence, as the starting point for build-up and build-down chains to cover the whole range from 1 mA to 20 A.

We have measured the thermoelectric transfer difference of two thermal voltage converters using a Josephson source and compared the results to similar measurements made with a conventional semiconductor source. Both sources use the fast reversed dc method. The Josephson source is an array of 16384 superconductor-normal-superconductor Josephson junctions that is rapidly switched between voltage states of +0.5, 0, and-0.5 V. A marginally significant difference is detected between measurements with the two different sources.