A portable computer-controlled Johnson noise thermometer

A Johnson noise power thermometer was implemented using existing filters and low-noise preamplifiers and main amplifiers. The interfacing circuitry needed to provide on-board computer control was designed and fabricated. Algorithms were developed for the calibration of the thermometer using a single temperature reference which included a new technique for the determination of equivalent input noise resistances of both voltage and current sensitive preamplifiers. Although several sources of systematic error were identified in repeated calibrations of the thermometer, the goal of limiting measurement uncertainty to 0.1% for 100-second integrations was never realized. Typical uncertainties for 10-second integrations were approximately 0.2% for measurements of a static ice bath using probe resistances from 21.5 Ω to 195 Ω. Data from 100-second integration measurements were less conclusive because frequent intermittences and failures in hardware made collection of enough data for statistical analysis difficult at best. It is clear, however, that significant systematic and stochastic uncertainties still exist and could be reduced with reasonable effort to reach the specified goal for accuracy.