Accurate determination of the balance point of a current comparator bridge for resistance standards measurements

In many cases of balancing a comparator-type bridge, the presence of noise on the galvanometer output obscures precision determination of the resistance ratio which makes it difficult to achieve a balance point to better than 0.1 ppm. The signal-to-noise ratio also deteriorates when increasing the nominal resistance ratio from 1 to 10. The noise causing these effects arises from low-frequency drifts below 1 Hz and the presence of interference. The drift arises probably from unstable thermal emf´s derived from the Peltier effect. In this paper, fast Fourier transforms are employed to try to reduce noise contributions to the measurements. Results show that a Fourier analysis as well as averaging reduces noise and hence improves the accuracy of the balance point measurement. A spectrum analyzer gave the best results, improving the resolution five-fold for the 10 Ω-1 Ω measurements and at least two-fold for 10 kΩ-1 kΩ measurements