Design Of A Four-Channel Johnson Noise Thermometer System

Johnson noise thermometer (JNT) is one of the few techniques available to determine the thermodynamic temperature. One of the main challenges in the measurement is that the amplitude of the thermal noise is so small that requires extremely long integration periods to eliminate the effect from the background noise. In this paper, a four-channel JNT system is developed, which can acquire thermal noise and reference quantum voltage noise simultaneously and thus reduces the integration periods twofold. We present the main four aspects of the system in detail as follows.(1) A switchboard with four input ports and four output ports is developed. The connections on the board are symmetrical and designed with low distributed capacitance. Magnetic latching relays on the board are controlled by a field-programmable gate array (FPGA) which is in off-power mode during the measurement period.(2) A high-speed data acquisition system is developed which consists of analog-to-digital converter (ADC) circuits, digital waveform analyzers and FPGA-based clock circuits. The system is capable of four-channel acquiring synchronously and plays a key role in the four-channel JNT system.(3) The data acquisition and measurement control is automated by Lab VIEW software.(4) The thermodynamic temperature of triple point of water (TPW) was measured. Measurement with an integration period of14hours and bandwidth of650kHz resulted in relative offset of4.9mK from ITS-90value, and combined relative standard uncertainty of28.2μK/K.