With the changing landscape of communication technology and communication needs, there is a growing demand for high-frequency circuits in modern electronics used in various systems requiring high-speed and precise signal transmission. However, in the application process, issues related to thermal noise arise due to frequent component switching and other factors. The result is a significant degradation in the performance of high-frequency circuit systems, posing a threat to their reliability. In this article, FS Technology will delve into the issue of thermal noise to help you mitigate this threat.

What is Thermal Noise in Electronics

If you carefully observe the electronics around you, you will notice an intriguing scientific phenomenon: you may hear some noise regardless of whether you are actively using them. This intriguing phenomenon is thermal noise, also known as thermal agitation noise, which is typically the result of random voltage and current fluctuations caused by temperature.

“Electronic thermal noise” is a broad concept that encompasses both noise and interference. Generally, unwanted signals generated within a system are referred to as noise, which falls outside the scope of signal transmission. Meanwhile, electromagnetic interference originating from electronic components on circuit boards, including transistors, diodes, resistors, and integrated circuits, can be referred to as interference. These interference signals are unwanted external signals to the system.

Thermal Noise When Power is Off

In the previous discussion, an interesting issue was raised: the existence of thermal noise even when a circuit is not powered. This may seem counterintuitive, but it is indeed a real phenomenon. The generation of electric current is due to the motion of free electrons, and free electrons do not appear out of thin air; they require an applied voltage across the circuit’s terminals to create a current. In other words, when a resistor is left unconnected, and no external voltage is applied to it, the measurement result with a multimeter is 0.

Is the multimeter measurement result accurate in this case?

The answer is uncertain. Under specific conditions, such as high-temperature environments or exposure to high-energy particle radiation, electrons might acquire enough energy without the need for an external voltage (although this is less common). As for why the multimeter reads 0, it’s because the movement of electrons inside the resistor is extremely weak, and due to accuracy limitations, it cannot measure the true result. If you were to use a more precise instrument, you would find that the voltage across the resistor’s terminals is not 0 but exhibits irregular variations near the 0 line.

This irregular motion of electrons is often related to environmental temperature. In high-temperature environments, the thermal motion of electrons increases, resulting in tiny currents within the circuit. In such cases, even when no devices are operational, if your high-frequency PCB has inadequacies, it can potentially lead to thermal noise issues.

Characteristics of Thermal Noise

Depends on Temperature

You may have noticed that when the geographical location of electronic devices changes, noise issues can arise. Many people might attribute this to signal problems, and that perspective may be correct. In fact, changes in geographical location often come with variations in environmental temperature, which can potentially affect the movement of electrons in high-frequency circuits. This change is positively correlated with temperature, meaning that an increase in temperature can lead to an increase in noise.

Thermal Motion and Linear Drift Motion

First and foremost, we need to clarify the basic principles of electronic products: the various components of electronic devices require an external voltage to function, including the electronic elements on the circuit board. When we apply an external voltage to electronic devices, the electrons within them are influenced by the electric field, resulting in directed electrical current – this is the fundamental working principle of circuits. This process is independent of temperature as it is primarily driven by the external voltage.

Simultaneously, electrons within electronic components also experience thermal motion due to temperature. This thermal motion results from the irregular, high-speed movement of electrons within the materials. Thermal motion and the generation of current in the circuit are two independent phenomena that do not influence each other. Thermal motion of electrons is solely influenced by temperature and is not affected by the magnitude of current.

Therefore, increasing the current flow will not reduce the thermal motion of electrons. Regardless of the current magnitude, as long as the temperature is sufficiently high, electrons will continue to move randomly within the material due to thermal motion, resulting in thermal noise.

Random Process

Thermal motion is an irregular motion and cannot be calculated using instantaneous expressions; instead, statistical laws are used to describe electronic thermal noise. It has been tested that the thermal noise of resistors follows a normal distribution, which is a Gaussian process, hence why we also refer to it as Gaussian noise. The larger the resistance on a circuit board, the greater the noise.

When evaluating the noise performance of a circuit, it is unrelated to the PCB itself but rather dependent on the resistance and environmental temperature. To address thermal noise issues, A-Star recommends minimizing the use of resistors at the outset of high-frequency circuit design. If you don’t need them don’t use them, or reduce the number and resistance.