Flat frequency response in the electronic measurement of the Boltzmann constant

A new quantum voltage calibrated Johnson noise thermometer (JNT) was developed at NIM to demonstrate the electrical approach that determines the Boltzmann constant k by comparing electrical and thermal noise power. A measurement with an integration period of 10 hours and bandwidth of 640 kHz results in relative offset of 0.5×10^-6 from the current CODATA value of k, and type A relative standard uncertainty of 23×10^-6. The quadratic fitting parameters of the ratio spectrum show a flat response with respect to the measurement bandwidth. This flat response is a dramatic improvement compared to the response produced by the NIST JNT system that dominated the relative combined uncertainty of previous measurements of k.