Reduced non-linearities and improved temperature measurements for the NIST Johnson noise thermometer

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) νK/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.