Reduced non-linearities and improved temperature measurements for the NIST Johnson noise thermometer**US government work, not protected by US copyright.

Abstract

The variance of temperature measurements made with the NIST Johnson noise thermometer has been reduced by the use of improved cross-correlation electronics and a unique method for measuring and reducing non-linearities. The spectral response of the voltage noise and the reproducibility of the measured temperature were improved by the use of a lower-noise preamplifier with better common-mode rejection and by removal of a systematic aliasing error through additional filtering. Even with these improvements, however, measurements with precision multi-tone comb and two-tone waveforms, which were synthesized with the quantum voltage noise source, showed that distortion generated by non-linearities in the electronics still limited the measurement standard deviation to (3–5) µK/K greater than that expected for uncorrelated white noise. The new electronics and the measurement techniques used to characterize the non-linearities are described. Synthesized waveforms that have higher tone density are shown to reduce the distortion generated by the remaining non-linearities so that the variance of the measurement is no longer limited and continues to decrease with measurement time as expected.