Thermometry noise

The main problem encountered in low-temperature noise thermometry is the very low level of signals. For example, with R = 1 kfl and B = 1 kHz ... 

In the past, except for the low-temperature range, the uncertainties of noise thermometry were not comparable to those of the gas thermometry due to the non-ideal performance of detection electronics. Up to now, the most successful technique is the switched input digital correlator proposed by Brixy et al. in 1992 [89], In this method, the noise voltage is fed via two separate pairs of leads to two identical amplifiers whose output signals are multiplied together, squared and time averaged (see Fig. 9.10). 

Thermal electric noise thermometry 1. Josephson junction point contact 2 Conventional amplifier 0.001-1 4-1400 Mean square voltage fluctuation Nyquist s law oc fegT Other sources of noise serious problem for T > 4 K... 

T. V. Blalock and R. L. Shepard, A Decade of Progress in High-Temperature Johnson Noise Thermometry, in Temperature Its Measurement and Control in Science and Industry, vol. 5, pt. 2, American Institute of Physics, New York, 1982. 

As we have seen in Section 9.5.3, in the case of resistance thermometry, the signal produced by a low-temperature thermometer is very low (microvolt range). Low-pass filters are not sufficient to narrow the detection bandwidth in order to get a suitable signal to noise ratio (S/N). Bandpass filters are needed. The most commonly used method is the synchronous demodulation, usually simply called lock-in technique, as shown in the block diagram of Fig. 10.7. 

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