Maintaining a Local Reference Scale for Electrical Impedance by Means of a Digital Impedance Bridge

Electrical impedance is one of the most commonly measured electrical quantities and there is a wide variety of impedance meters commercially available. Electrical calibration laboratories usually use sets of artifact impedance standards to calibrate these meters. The traceability chain for electrical impedance is described with a particular emphasis on the use of impedance bridges to calibrate the impedance standards themselves. Up to now, coaxial transformer ratio bridges have been used for this purpose, but these have a number of practical disadvantages. It is shown that digital impedance bridges, which use digital techniques to provide the accurate voltage ratios for bridge balancing, offer a viable alternative to transformer ratio bridges. The principles of operation of source-based and sampling-based impedance bridges are described. The joint research project VersICaL: A versatile electrical impedance calibration laboratory based on digital impedance bridges (2018-2021) has shown that digital impedance bridges provide, even for a laboratory with limited resources and expertise, a practical means of calibrating impedance standards at the parts per million level of accuracy. A source-based digital impedance bridge, designed and constructed within the project, is described and some measurement results presented.