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 artefact 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 with outstanding accuracy 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 needed 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. A joint research project whose aim is to show 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 is introduced. A source-based digital impedance bridge, designed and constructed within the project, is described and some preliminary measurement results presented.