Evaluation of AC-DC Transfer Errors for Thermal Converter-Multiplier Combinations

In the build-up calibration of precise coaxial ac-dc thermal voltage converters, separate thermal converter and multiplier modules are combined and intercompared. For routine calibration or testing to specification, ac-dc transfer errors associated with the thermal converter modules can usually be neglected. However, for the calibration of standards to the highest accuracy, where the uncertainty may be < 5 ppm at frequencies in the range 25 Hz to 100 kHz, thermal converter errors must be included. An expression is derived for ac-dc transfer error in a thermal converter-multiplier combination. Multiplier transfer error is subject to loading effects by the thermal converter module, and the sensitivity of multiplier transfer error to changes in converter loading is assessed. Experimental results are presented to demonstrate the contribution of thermal converter voltage and current transfer errors to the error of the combination.