Thermal Noise in High Frequency PCBA

Because of its excellent dielectric constant, high-frequency PCBAs are often used in various systems that require high-speed and accurate signal transmission. In addition to the common 5G base station and mini drone PCB applications, it is also used in high-frequency lines. This article analyzes the causes of thermal noise caused by high-frequency PCBAs in high-frequency circuits and provides solutions.

Definition of PCB Thermal Noise

If you look closely at the electronic equipment around you, you will notice an interesting scientific phenomenon: whether you are using it or not, you may hear some noise from it. We call this noise electronic thermal noise.

Electronic thermal noise is a relatively broad concept, which includes noise and interference. We usually refer to all unwanted signals except for signal transmission as noise, that is, unwanted signals inside the system. The noise generated by the electromagnetic disturbance on the circuit board is called interference, that is, the unwanted signal outside the system. The source of PCB thermal noise is the electronic components on the circuit board, including transistors, triodes, resistors, integrated circuits, etc. In this paper, FS Technology mainly analyzes and explains the causes of thermal noise inside the system.

High-frequency PCBA resistance causes thermal noise

The generation of current is due to the movement of free electrons, so in most people’s cognition, current does not arise out of thin air and requires a voltage to be applied across a resistor. This also means that when a resistor is left floating if we do not apply voltage to it, the current displayed by the multimeter is 0.

FS Technology hereby asks a question: When the resistance is suspended, the result of measuring the current with a multimeter is 0. Is the real value 0?

The answer is negative. Due to accuracy limitations, multimeters cannot measure resistance with no voltage applied. When we use a more accurate millivoltmeter to measure the resistance, we will find that the voltage value across it is not 0, but shows irregular changes in the 0 line.

Current Motion of Unpowered PCBA

This erratic change in the voltage indicates that there are free-moving electrons inside the resistor that are moving erratically. The irregular electronic movement of the electrons of the circuit board is because the ambient temperature at this time is not equal to absolute 0 degrees. We call this irregular motion phenomenon the thermal motion of the high frequency PCB.

As a professional PCBA manufacturer, from our point of view, the resistance of a high-frequency circuit in this environment is not just resistance, but a resistance plus a signal source. This signal source is what we call a noise source.

Characteristics of PCB Thermal Noise

The magnitude of the noise is temperature dependent

Do you find that the noise of your electronics varies by geographic location? A lot of people think it’s due to a signal problem, and that’s probably true. When our geographic location changes, the ambient temperature also changes, which affects the movement of electrons in high-frequency circuits, and this change is positively correlated with the change in temperature, that is, the higher the temperature, the greater the noise, which is why We call this the reason for thermal noise of the resistor.

Electron thermal motion and linear drift motion are independent of each other

First of all, the electronic products we use must contain circuits. This is undeniable. In other words, all components of electronic equipment require external voltage to be applied to make them work. When we input a constant voltage to the electronic device, in addition to thermal motion, the electrons in the PCB will also undergo directional drift motion under the action of the external electric field, which is the reason we often say that the current is generated. This also means that the resistance of a working high-frequency circuit contains two currents, one is the voltage applied by the external circuit, and the other is the freely moving electrons due to heat. As we continue to increase the amount of voltage applied, the amount of current flowing through the board also increases.

FS Technology raises the second question here: Is it possible to reduce the thermal motion of electrons in electronic devices by increasing the amount of current, thereby reducing thermal noise.

The answer is still no, as the title says, the thermal motion of electrons in the PCB and the linear drift motion are independent of each other, and the thermal motion is only related to temperature.

Thermal noise of a circuit board is a random process

Thermal motion is an irregular motion, so it cannot be calculated by instantaneous expressions. Usually, PCBA companies use statistical laws to represent the noise of PCBs. After testing, the resistance thermal noise has a normal distribution and is a Gaussian process, so we also call it Gaussian noise. The greater the resistance of the circuit board, the greater the noise.

When we evaluate the noise performance of a high-frequency PCB, it has nothing to do with the PCB of the circuit itself, but only with resistance and ambient temperature. To solve the problem of noise in high-frequency PCBs, FS Technology recommends reducing the use of resistors at the beginning of circuit design. If you can’t use them, you don’t need them. The smaller you can use them, the smaller they are. Adding resistors is equivalent to adding a noise source to the high-frequency PCBA. 

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