Improvements in the Boltzmann constant measurement with noise thermometry at NIM

Since 2010 we have been developing a quantum-voltage-calibrated Johnson noise thermometer at NIM to measure the Boltzmann constant k. With recent improvements in grounding and shielding of the electronics, and matching of the noise sources and transmission lines, the effects of electromagnetic interference and variations of fitting parameters with different bandwidths were greatly reduced. By combining 14 measurements with about 10 hours integrating period for each measurement, the relative standard uncertainties of 6.5×10^-6 and 2.8×10^-6 were achieved from statistics and bandwidth fit variations, respectively.