A PXI-Based Calibration System for Low-Value AC Resistors

This paper presents a sampling measurement system designed to compare ac standard resistors within a range from 1 $\text{m}\Omega $ to 10 $\Omega $ at frequencies ranging from 40 Hz to 10 kHz and for measurement currents up to 10 A. The system allows simultaneous measurement of both components of impedance of the calibrated resistor relative to the reference resistor. Built on the basis of commercial PXI modules, complemented by modules developed by the authors, the system stands out relatively simple structure and fully automatic operation. In order to reduce the impact of parasitic impedances on the comparison accuracy, a completely automated interchange method and the original way of connecting a digitizer to the compared impedances were applied. The consistency of the measured ac resistance ratios from 1:1 to 1:10 is better than 1 $\mu \Omega /\Omega $ and phase difference better than 6 $\mu $ rad, over the entire frequency range. The results of several comparisons and the analysis of their uncertainty show that for the comparisons of resistors with a resistance ratio close to one, the relative uncertainty of the resistance ratio measurement is 2.3 $\mu \Omega /\Omega $ ( $k=2$ ) over the entire frequency range. The uncertainty of the difference between the time constants of the compared resistors is at the level of 0.2 ns ( $k=2$ ) at 1 kHz.