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 /spl Omega/-1 /spl Omega/ measurements and at least two-fold for 10 k/spl Omega/-1 k/spl Omega/ measurements.