A Three-Arm Current Comparator Digitally Assisted Bridge for the Comparison of Arbitrary Four Terminal-Pair Impedances

High accuracy impedance measurements are performed with coaxial transformer bridges, whose conventional design allows the comparison of like impedances - pure resistors or capacitors. Here, we present a current comparator bridge suited for the measurement of impedances of arbitrary magnitudes and phase angles. The bridge has three arms, to connect the impedance under measurement and two calibrated standards. The bridge is digitally assisted and its operation is based on a polyphase digital sine wave synthesizer. To allow the measurement of midimpedance to low-impedance magnitudes, the bridge network and the balancing procedure are designed to approximate the four terminal-pair definition of the three impedance standards. The bridge has been extensively tested with conventional impedance standards and custom designed phase standards. The relative base accuracy is in the 10^−5 to 10^−6 range at kilohertz frequency.