Improvements in the NIST Johnson noise thermometry system

We have developed a Johnson noise thermometry system that is calibrated by precision waveforms synthesized with a quantized voltage noise source (QVNS). Significant improvements to the QVNS and the cross-correlation measurement electronics have dramatically reduced systematic errors and measurement uncertainty. We describe some of these improvements and discuss the potential for this system to contribute to the redefinition of the Boltzmann constant.