Constraints on a synthetic-noise source for Johnson noise thermometry

Conventional Johnson noise thermometers based on switching correlators have conflicting matching requirements for the sensing resistance. To mitigate distortion effects in the correlator, the RT products of the two sensors must be the same, and to mitigate frequency-response errors in nominally identical input circuits, the two sensing resistances should be the same. A noise thermometer using synthetic noise for the primary reference signal overcomes this conflict because the output voltage and output resistance are independent. This paper presents the rationale and design constraints for a noise thermometer using a synthetic-noise source based on Josephson junctions. The quantized voltage noise source developed at NIST produces tunable waveforms with a spectral density composed of a comb of frequencies of equal amplitude and random phase. In addition to the conventional noise-power and impedance constraints, it has additional constraints relating to the number of tones and the tone spacing.

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