Thermal noise. Causes of occurrence and methods of struggle.

Thermal noise is caused by the thermal movement of charge carriers in a conductor. The existence of thermal noise was predicted by Einstein in 1906. It was discovered in 1928 by John B. Johnson who wrote about it to Harry Nyquist, the latter was able to derive patterns and a formula.

Based on the formula, it can be seen that the thermal noise depends only on the temperature of the conductor, the resistance of the conductor and the frequency band in which these noises are measured. Accordingly, to reduce thermal noise, it is necessary to reduce these parameters.

Thermal noise occurs only at the active resistance Re(Z). Inductive and capacitive reactances are not sources of thermal noise.

Reduce impedance by using signal sources with low internal impedance and by using preamplifiers with low input impedance.

You can reduce the influence of the frequency band by filtering the input signal and eliminating the spectrum of non-operating frequencies. At room temperature (300 K), the thermal noise spectrum is uniform and similar to white noise up to frequencies of 6 terahertz.

It is also possible to reduce thermal noise by reducing the temperature of the conductor (sensor). Especially effective is the use of liquid helium for these purposes, which has a boiling point of only 4.2 K. This approach is used for thermal guidance heads, telescope sensors, etc.

Below is an online thermal noise voltage calculator that allows you to estimate the effect of temperature and resistance of the conductor and the frequency band in which the noise is estimated.

It should also be borne in mind that in addition to thermal noise, resistors have current noise - due to changes in the contact resistances of the conductive particles of the resistive element. Current noise typically exceeds thermal noise and differs for different resistor technologies. Wirewound resistors have less current noise.

The current noise spectrum is uniform up to about 10 MHz and then decreases with increasing frequency.

Current noise is proportional to the voltage applied across the resistor and is measured in µV(noise)/V(applied voltage)

The inherent noise of a resistor consists of the sum of thermal and current noise and is indicated by the manufacturer in the documentation.
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