This paper describes methods for reducing the statistical uncertainty in measurements made by noise thermometers using digital cross correlators, and in particular, for thermometers using pseudorandom noise for the reference signal. First, a discrete-frequency expression for the correlation bandwidth for conventional noise thermometers is derived. It is shown how an alternative frequency-domain computation can be used to eliminate the spectral response of the correlator and increase the correlation bandwidth. The corresponding expressions for the uncertainty in the measurement of a pseudo-random noise signal in the presence of uncorrelated thermal noise are then derived. The measurement uncertainty in this case is less than that for true thermal-noise measurements. For pseudo-random sources generating a frequency comb, an additional small reduction in uncertainty is possible, but at the cost of increasing the thermometer s sensitivity to non-linearity errors. A procedure is described for allocating integration times to further reduce the total uncertainty in temperature measurements. Finally, an important systematic error arising from the calculation of ratios of statistical variables is described.
Pub Type: Journals
Johnson noise, Josephson junction arrays, noise thermometry, statistics, temperature, temperature measurement, pseudo noise