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Research on Johnson Noise Thermometry
Johnson noise thermometry (JNT) utilizes the fundamental properties of thermal fluctuations in conductors. Because the technique is a primary method, as opposed to a secondary or artifact method, the sensor response does not require calibration. An innovative JNT method now being developed at NIST links the measured thermal voltage directly to the as-maintained electrical units via the AC Josephson Voltage Standard. The work is part of a NIST competence initiative formed by a collaboration between the Process Measurements Division and the Quantum Electrical Metrology Division. The project also has an international collaborator, the MSL/IRL in New Zealand.
The new NIST methodology uses the AC Josephson Voltage Standard as a Quantized Voltage Noise Source (QVNS). The QVNS is capable of providing a stable and calculable pseudo noise spectrum from direct digital synthesis and pulse-quantization in a Josephson junction array. The QVNS is the noise reference for the system which compares thermal noise powers at various temperatures. This method conveys key advantages from a metrology standpoint and will extend the practical application range for JNT systems. These advantages make it well suited to the most demanding process measurement applications such as in high-temperature, high-radiation, or remote environments where artifact thermometer servicing requirements can compromise the process or add excessive costs.
The NIST JNT competence project has already demonstrated that the new methodology can achieve accuracies over the range between 273 K and 303 K sufficient for many applications. In the absolute mode, the current agreement between QVNS-derived noise temperatures and the ITS-90/SI assignments for the Ga TP and H2O TP fixed points is 150 mK/K to 300 mK/K. Efforts are now focused toward improving the accuracy to the 10 mK/K level appropriate for thermodynamic evaluations of the International Temperature Scale of 1990 and toward extending the range of measured noise powers and temperatures upward to 660 °C. Our ultimate goal is to support commercialization of the technology, through further improvements of the method, understanding of the metrology issues in applications, and the acquisition of benchmark data illustrating the uncertainties achievable with varying levels of sophistication.
Figure 1. The two auto-correlation noise spectra (red and green) and the cross-correlation noise spectrum from the NIST digital noise correlator sampling a Johnson Noise resistance probe at the gallium triple point (302.916 K).
Figure 2. Sae Woo Nam and Sam Benz (Division 817) and Wes Tew (Division 836) with the QVNS system at the NIST Boulder Labs Quantum Devices Group.
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Last Updated on: 2/25/04
