Wide range bridge-comparator for accurate impedance measurements on audio- and low frequencies

The accurate (uncertainty - better than 1 ppm, sensitivity - better than 0.1 ppm on main ranges) bridge- comparator, balanced by phase control only, is described. Bridge is aimed to measure any impedance in wide dynamic range (from 10^-7 to 10¹⁷ Ohm). It operates in wide frequency range (from 10^-2 to 10 kHz). Bridge balancing and uncertainty correction use the variational method and measurements of unbalance signals. Bridge compares the impedances with arbitrary phase angle, i.e. C ↔ C, R ↔ R, L ↔ L, C ↔ L, C ↔ R, R↔ L.