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The definition, classification and propagation path of electromagnetic interference.

October 13, 2020


Definition of electromagnetic interference

Electromagnetic interference (EMI) refers to electromagnetic harassment behavior that causes deterioration of equipment, transmission channels, and system performance. It is electronic noise generated by electromagnetic radiation sources such as motors and machines, which can interfere with cable signals and reduce signal integrity. Because electromagnetic interference is an electromagnetic phenomenon that often occurs in people's daily life, people have a certain understanding and understanding of electromagnetic interference at the same time that the electromagnetic effect phenomenon is discovered. In 1881, the famous British scientist Heaviside published an article "On Interference", which marked the beginning of the study of electromagnetic interference phenomena. In 1888, the German physicist Hertz invented the antenna and used experiments to prove the existence of electromagnetic waves, which opened the prelude to human experimental research on electromagnetic interference. In the following year, the British Post and Telecommunications Department studied the problem of interference in communications, pushing human research on electromagnetic interference to engineering and industrialization. So far, the study of electromagnetic interference phenomena has formed a rigorous scientific system.


Elements and classification of electromagnetic interference

According to scientific research, no matter whether the electromagnetic equipment is complex or simple, electromagnetic interference must meet the following three conditions: the source of interference, the path of interference, and the object being interfered. These three points also constitute the three elements of electromagnetic interference. .

According to the physical properties of interference, it can be divided into two forms: radiation and conduction.

According to the interference source, it is divided into natural interference source and man-made interference source. Natural interference sources are noises from the atmosphere or the outer space of the earth, such as thunder and lightning, solar electromagnetic wind and other phenomena generated by nature itself. They are a basic part of the electromagnetic environment of the earth, but they are also the main sources of interference to radio communications and space technology. Natural interference sources are divided into thermal noise sources, antenna noise sources and atmospheric interference sources inside components. The celestial noise comes from the outer space of the non-solar system, and it interferes with the launch of ballistic missile carrier rockets and the operation of artificial satellites and spacecraft. There are two forms of man-made interference. One is a functional interference source intentionally emitted by humans to achieve a certain function; the other is a useless non-functional interference source.

According to the signal spectrum width, electromagnetic interference signals can be divided into broadband interference sources and narrowband interference sources. The two kinds of interference sources are divided according to the bandwidth of the designated susceptor. The interference bandwidth is greater than that of the susceptor, which is called broadband interference, and vice versa, narrowband interference.

Transmission path of electromagnetic interference

Any form of electromagnetic interference is accompanied by energy and energy channels. For example, light is also an electromagnetic wave. Sunlight comes to the earth from 150 million kilometers away, and energy is transmitted in the form of radiation. Space is cosmic space. In real life, there are two ways of propagation of electromagnetic interference, one is conduction transmission; the other is radiation transmission. Similarly for the disturbed sensor, the two ways of electromagnetic interference can be called radiation coupling and conduction coupling.

The radiation coupling method is spread in the form of electromagnetic waves, which emits to the surrounding space in the form of energy fields to interfere with the sensors. There are three common radiation couplings: ①Unintentionally received by another non-receiving antenna after the electromagnetic signal is sent; ②Induction of the electromagnetic field between the wire and the passing interference source, also known as field-to-line coupling; ③Between two parallel wires The high frequency signal induction is called line-to-line inductive coupling.

Effective preventive measures for electromagnetic interference

In order to effectively prevent electromagnetic interference, people have introduced the concept of electromagnetic compatibility. The purpose is to make various electronic devices or electronic systems work normally in the same electromagnetic environment without causing unbearable electromagnetic interference to other electronic devices or electronic systems. There are two specific requirements for electromagnetic compatibility: ①The electromagnetic interference intensity generated by the equipment during normal operation must not exceed the highest standard required in the environment; ②The equipment in the environment should have a certain degree of anti-interference ability, that is, electromagnetic sensitivity. From the perspective of electromagnetic compatibility, it is generally necessary to do the following: first, when multiple lines are connected in series, the line impedance value should be reduced as much as possible; second, when grounding, the grounding points in the system should be connected together, and then Connect with the safety grounding grid; finally, the points of the safety grounding grid should be as consistent as possible.

When people use a computer, they often crash or blue screen for no reason, and there is no failure after restarting. In fact, this situation is often due to the computer power supply. Computer power is an important way for electromagnetic interference to enter the automated weak current system. The power supply is connected to the hard disk, motherboard, CPU and other parts of the computer, and the power supply is inevitable with the interference source. Therefore, the interference source can easily interfere with various parts of the computer through the computer power supply, causing crashes, blue screens and other phenomena. Therefore, the grounding of the computer power supply is very important. In practice, the computer power supply and the chassis are often disconnected, so as to reduce the surge current flowing through the power supply caused by interference, increase the anti-interference ability, and significantly improve the reliability and safety of the computer. However, this method has disadvantages, and it is easy to increase the coupling capacitance between the chassis and the system components. In order to avoid this situation, the ground wire can be shortened or thickened as much as possible, and the trace length of the key parts of the circuit board can be reduced as much as possible.

The computer power circuit is the most important part of preventing electromagnetic interference. The following effective preventive measures can be adopted: First, a filter can be installed on the computer power supply to filter high frequency interference and higher harmonics from interfering AC power. At this time, two issues should be paid special attention to: ① Install a suitable filter on the power supply. ②The filter should be installed at the front end of the power supply, in order to prevent the interference electromagnetic wave from coupling with this section of the line and entering the device. If conditions permit, it can be directly connected to the case. Secondly, install an isolation transformer on the input side of the power supply, and the output of the isolation transformer will directly supply power to the microcomputer; finally, in order to more effectively prevent interference in the low-frequency normal state of the power grid, the power supply of the computer system can be changed to a UPS power supply.

Concluding remarks

Perform systematic analysis of electromagnetic interference to find out the cause of mutual interference, and take precautions so that various electronic equipment and electronic systems in the same electromagnetic environment can work normally without mutual interference to ensure the safe, reliable and stable operation of the weak current system.