A digitally-assisted current comparator bridge for impedance scaling at audio frequencies

A current comparator impedance bridge, suitable for the comparison of four terminal-pair impedance standards having similar phase angles (e.g., resistors or capacitors) in the audio frequency range at 1:1 and 10:1 nominal ratios is here presented. The bridge is digitally-assisted: its accuracy is granted by an electromagnetic device, a ferromagnetic-core current comparator, but the voltages and currents needed to achieve both principal and auxiliary equilibria are generated by programming a polyphase direct-digital-synthesis generator. The measurement is semi-automated: after an initial setting, the equilibrium can be achieved in a few minutes. Measurements performed on calculable resistors give a base accuracy of a few parts in 10^7 at kHz frequency, with the potential for further improvement.