Johnson noise thermometry has an extensive history [12-15], but precision applications were made possible only more recently by NIST's development of a superconducting, quantum-accurate, pseudorandom, voltage-noise source (QVNS) as an exact synthetic noise reference.
The small magnitude of Johnson noise, only 0.123 nV [[ohms].sup.-1/2] [Hz.sup.-1/2] at 273.16 K, poses a challenge for accurate measurement.
Section 3 describes the use of the complex ratio of the measured noise sources in the context of a measurement of the undesired correlated noise in the JNT system, such as possible current noise from the amplifiers that could cause frequency-dependence in the measured Johnson noise signal.
Since the magnitude of the Johnson noise is similar to that of typical amplifiers, two separate channels of amplifiers, low-pass anti-aliasing filters, and digitizers are used (discussed in Sec.
3) with that of the connection leads and sense resistors of the Johnson noise source.
However, when the amplifiers are connected to the Johnson noise source, there is a low-ohmic connection between the terminals of the two amplifiers (Fig.
In the earlier configuration, we added a common-mode ferrite choke around each twisted pair input from the Johnson noise voltage source immediately before the signal relay circuit board and added Ramp = 10 [ohms] between the circuit board and each terminal of the amplifier.
The noise thus consists of a frequency-independent component due to Johnson noise and a peaked component due to Brownian noise.
This suggests that the performance of the transducer is limited by the combination of Brownian and Johnson noise, rather than amplifier noise and interference.
Frequency Q Brownian Noise Johnson Noise Noise Measured Measured Predicted Predicted Measured [kHz] [mV] [mV] [mV] 126 1.4 12 5 14 351 4.8 12 5 22 500 5.5 12 5 19
So instead of having a 100K resistor in the path, you have only a 5K resistor, which reduces the level of Johnson noise produced by the resistor.
Bias current noise can also add to the Johnson noise, increasing the noise level slightly.