Introduction to various measures to suppress and prevent electromagnetic interference in electronic equipment

This paper comprehensively discusses the electromagnetic compatibility of electronic equipment. The transmission sources and interference transmission paths are analyzed in detail, and various measures and principles for effectively suppressing and preventing interference are introduced.

1 Introduction

-- With the rapid development of electronic technology, modern electronic devices have been widely used in various fields of human life. Currently, electronic devices are in a period of rapid development, and this development process continues at an increasing rate. The widespread use and development of electronic devices inevitably leads to an increase in the level of electromagnetic fields they generate in their surrounding space. That is to say, electronic devices inevitably work in an electromagnetic environment (EME). Therefore, the adaptability of electronic devices in an electromagnetic environment must be solved. Electromagnetic compatibility (EMC) is a science about the effects of electromagnetic interference (EMI). At present, the issue of electromagnetic compatibility has formed a new discipline worldwide. The central task of electromagnetic compatibility is to study control and eliminate electromagnetic interference so that when an electronic device or system is associated with other devices, it does not cause deterioration or degradation of the performance of any part of the device or system. An ideally designed electronic device or system should not radiate any unwanted energy and should not be affected by any unwanted energy.

2 Classification of electromagnetic interference sources

- Various forms of electromagnetic interference are the main factors affecting the electromagnetic compatibility of electronic equipment. Therefore, it is an important part of the research that needs to be studied in electromagnetic compatibility design.

2.1 Internal interference

——Internal interference refers to mutual interference between various components inside the electronic device, including the following.

——(1) The interference caused by leakage caused by the distributed capacitance and insulation resistance of the working power supply; (related to the operating frequency)

——(2) the signal is coupled to each other through the impedance of the ground wire, the power source and the transmission wire, or the interference caused by the mutual inductance between the wires;

——(3) Some components inside the equipment or system generate heat, which may affect the interference caused by the stability of the components themselves or other components;

——(4) The magnetic field and electric field generated by high-power and high-voltage components affect the interference caused by other components through coupling.

2.2 External interference

- External interference refers to interference from a line, device or system by factors other than electronic equipment or systems, including the following.

——(1) External high voltage and power supply interfere with electronic circuits, equipment or systems through insulation leakage;

——(2) External high-power equipment generates a strong magnetic field in space, and interferes with electronic circuits, equipment or systems through mutual inductance coupling;

——(3) interference caused by space electromagnetic waves to electronic circuits or systems;

——(4) The temperature in the working environment is unstable, causing interference caused by changes in internal components of the electronic circuit, equipment or system;

- (5) Equipment powered by the industrial grid and interference generated by the grid voltage through the power transformer.

3 Interfering transmission routes

——When the frequency of the interference source is high, the wavelength of the interference signal is smaller than the structure size of the interfered object, or the distance between the interference source and the victim is r ′′ λ/2π, the interference signal can be considered as radiation The field, in the form of a plane electromagnetic wave, outwards the electromagnetic field energy into the path of the interfered object.

——(2) The interference signal is in the form of leakage and coupling, through the insulation support (including air) as the medium, coupled to the interfered line, equipment or system via the common impedance.

——If the frequency of the interference source is low, the wavelength λ of the interference signal is longer than the structural size of the interfered object, or the distance r between the interference source and the interference object is “λ/2π, the interference source can be considered as a stable field It enters the path of the interfered object in the form of an inductive field.

– (3) Interference signals can be introduced into lines, equipment or systems by direct conduction.

4 Basic principles of electromagnetic compatibility design

4.1 Grounding

- Grounding is a very important issue for electronic equipment. There are three grounding purposes:

——(1) Grounding makes all the unit circuits in the whole circuit system have a common reference zero potential, which ensures that the circuit system can work stably.

——(2) Prevent interference from external electromagnetic fields. The grounding of the casing can cause a large amount of electric charge accumulated on the casing due to electrostatic induction to be discharged through the earth, otherwise the high voltage formed by these charges may cause spark discharge inside the device to cause interference. In addition, for the shield of the circuit, if a proper grounding is selected, a good shielding effect can be obtained.

- (3) to ensure safe work. When electromagnetic induction of direct lightning occurs, the destruction of the electronic equipment can be avoided; when the input voltage of the commercial frequency AC power source directly communicates with the casing due to poor insulation or other reasons, the electric shock accident of the operator can be avoided. In addition, many medical devices are directly connected to the patient's human body, and a fatal hazard can occur when the chassis is 110V or 220V.

- Therefore, grounding is the primary method of suppressing noise to prevent interference. Grounding can be understood as an equipotential point or equipotential surface, which is the reference potential of a circuit or system, but not necessarily earth potential. In order to prevent the damage caused by lightning strikes and the personal safety of the workers, the casing of the electronic equipment and the metal components of the equipment room must be connected to the earth, and the grounding resistance is generally small and cannot exceed the specified value.

—— There are basically three types of circuit grounding methods, namely single point grounding, multi-point grounding and hybrid grounding. Single point grounding means that in a line, only one physical point is defined as a ground reference point. All other points that need to be grounded are directly connected to this point. Multi-point grounding means that each grounding point in a system is directly connected to the ground plane closest to it so that the length of the grounding lead is the shortest. The ground plane can be the bottom plate of the device, or the ground wire that runs through the entire system. In a relatively large system, it can also be the structural frame of the device. Hybrid grounding is the combination of bypass capacitors and ground planes that require only high frequency grounding points. However, the resonance phenomenon of the bypass capacitor and the lead inductance should be prevented as much as possible.

4.2 screen

- Shielding is the separation of metal between two spatial regions to control the induction and radiation of electric, magnetic and electromagnetic waves from one region to another. Specifically, the shield is used to surround the components, circuits, assemblies, cables, or interference sources of the entire system to prevent interference with electromagnetic fields. The shields are used to surround the receiving circuits, devices, or systems to prevent them from being exposed to the outside world. The influence of electromagnetic fields.

——Because the shielding body absorbs energy (eddy current loss), reflected energy (interface reflection of electromagnetic waves on the shielding body) and offset energy for external electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. (Electromagnetic induction produces a reverse electromagnetic field on the shield that cancels some of the interference with electromagnetic waves), so the shield has the function of reducing interference.

- The principle of shielding material selection is:

——(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material with low resistivity (high conductivity) (P=I2R, the lower the resistivity (the higher the conductivity), the more power is consumed. Large), forming a counteracting effect of external electromagnetic waves, thereby achieving the shielding effect.

——(2) When the frequency of interfering electromagnetic waves is low, a material with high magnetic permeability is used, so that the magnetic lines of force are confined inside the shield to prevent diffusion into the shielded space.

——(3) In some cases, if high-frequency and low-frequency electromagnetic fields are required to have good shielding effect, different metal materials are often used to form a multilayer shielding body.

4.3 Other methods for suppressing interference

——(1) Filtering

- Filtering is an important measure to suppress and prevent interference. The filter can significantly reduce the level of conducted interference, because the interference spectral component is not equal to the frequency of the wanted signal, and the filter has good suppression of these components with different frequency of the useful signal, so that other interference suppression is difficult to play. The role. Therefore, the use of a filtering network is a powerful measure to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving devices. Decoupling and sensing decoupling networks isolate the circuit from the power supply, eliminate coupling between the circuits, and prevent interfering signals from entering the circuit. For the high frequency circuit, a CLCMπ type filter composed of two capacitors and one inductor (high frequency choke) can be used. There are many types of filters, and choosing the right filter eliminates unwanted coupling.

——(2) Correct selection of passive components

- Practical passive components are not "ideal" and their characteristics are different from ideal ones. The practical component itself may be a source of interference, so the correct selection of passive components is very important. Sometimes it is also possible to use the characteristics of the components to suppress and prevent interference.

——(3) Circuit Technology

—— Sometimes the requirements for suppression and interference prevention cannot be met after shielding, and circuit technology such as balancing measures can be combined with shielding. A balanced circuit is one in which two wires in a two-wire circuit and all circuits connected to the two wires have the same impedance to ground or to other wires. The purpose is to make the interference signals picked up by the two wires equal. The interference noise at this time is a common state signal, which can disappear on the load itself. In addition, other circuit technologies such as contact networks, shaping circuits, integrating circuits, and gating circuits can be used. In short, the use of circuit technology is also an important measure to suppress and prevent interference.

5 Specifications and standards for electromagnetic compatibility issues

- There are a series of international agreements and constructive rules that countries must abide by in terms of electromagnetic compatibility. Electromagnetic compatibility related organization - International Electrotechnical Commission (IEC) International Committee on Radio Interference (CISPR), mainly studies the measurement of interference noise in radio systems. In 1976, CISPR began to develop EMI standards for electromagnetic interference. In October 1900, the reprinting standard was announced on the basis of several revisions. The committee then reviewed with the International Wireless Communications Advisory Committee to develop data requirements and specific methods for the detection of electromagnetic compatibility of electronic products. "Permissible values ​​of interference characteristics of industrial, scientific and medical radio instruments and their measurement methods" (Standard No. 11) for the noise of information technology devices; "The radio interference characteristics of vehicles, motor boats and spark ignition engines" Measurement method and permissible value" (Standard No. 12); "Measurement method and allowable value of radio interference characteristics of radio and television receivers" (Standard No. 13). Until the middle of 1992, the international EMI standard was finally improved. The tolerances recommended by CISPR have been adopted by many countries in the world and serve as the basis for their national regulations.

- The radio transmitter power level is an important factor affecting the surrounding radio electronics and generating interference levels. Therefore the radio transmitter power level should be limited. For example, according to Recommendation No. 357-1 of the Radiocommunication Advisory Committee, in the (5800-8100 MHz) frequency band used by the satellite communication system and the terrestrial microwave relay communication line, when the power given to the antenna does not exceed 13 dBW, the microwave should be limited. The value of the effective radiated power of the transmitter (ie, the product of transmitter power and antenna gain) of the relay communication line is 55 dBW. It is recommended to limit the power of the ground station for satellite communications and the radiated power flux density of the communication satellites. The maximum value of transmitter power for many other radio services, such as amateur radio enthusiasts, mobile communication systems, etc., should also be limited.

—— Frequency planning has been widely adopted nationwide and worldwide. It is a way to improve the utilization of radio frequency resources and one of the important measures to ensure the electromagnetic compatibility of radio electronic equipment. Therefore, national and regional band allocations and band allocations between services should be implemented in strict accordance with international agreements (radio frequency allocation tables) and national documents. The radio channel allocation is performed according to the principle of frequency-space allocation. The frequency plan must ensure that the interference level of each radio electronic device is minimal or eliminates interference and is coordinated by the National Radio Management Committee.

——In recent years, many departments in China have carried out experimental research on electromagnetic compatibility and the formulation of relevant technical standards, and formulated a series of standards and norms. For example, the national standard GB3907-83 is the basic measurement method for industrial radio interference; GB4824.1-84 is the radio interference allowable value for industrial, scientific and medical RF equipment; GB6279-86 is the radio characteristic measurement for vehicle, motor boat and spark ignition engine drive Method and allowable values, etc. The National Radio Management Committee has made specific provisions on the frequency, bandwidth and maximum radiation field strength of electronic equipment used in industry, science and medicine. This plays an important role in ensuring the normal operation of electronic equipment and the normal life of the people and promoting the rapid development of modern science and technology.

6 Some typical electromagnetic compatibility problems are solved

——Because of the wide application of electronic technology in all walks of life, the space of human activities is full of electromagnetic waves everywhere. Therefore, electronic equipment does not solve the problem of electromagnetic interference, and it cannot be compatible. In practical applications, people have accumulated a lot of experience in researching anti-jamming technology, and constantly researched many practical methods to eliminate electromagnetic interference.

——The experiment found that the electromagnetic interference is quite prominent when the car is working, and it will damage the electronic components when it is serious. Therefore, the electromagnetic environment of automotive electronic equipment is the worst, and the electromagnetic compatibility of automotive electronic equipment has also received special attention. The high-frequency radiation generated by automobile ignition is the most prominent. In order to prevent the pollution of automobile electrical noise to the environment, the environmental protection departments of advanced countries such as Japan and the United States stipulate that only shielded wires with damping (such as carbon core) can be used as the ignition wire. Practice has shown that this is a very effective measure.

——In order to solve the micro-electric technology, especially the application and promotion of computer in the automobile, according to the needs and actual requirements, a filter circuit with good effect can be designed, and the interference noise that enters the system due to conduction can be placed in the front stage. Elimination at the entrance of the circuit system; isolation circuits, such as transformer isolation and opto-isolation, can be set to solve the conducted interference entering the circuit through the power line, signal line and ground line, while preventing interference caused by common impedance and long-line transmission; The energy absorption loop can be set to reduce the noise energy absorbed by the circuit and the device; or the influence of interference can be reduced by selecting components and arranging the circuit system reasonably.

——Software anti-interference of microcomputer equipment is mainly to stabilize the memory data and guarantee the program pointer. The microcomputer is a programmable control device, and the software can support and enhance the anti-interference ability of the hardware. If the random memory RAM in the microcomputer system is mainly used for temporary storage of data during measurement and control, the memory space is small. For the stored data, if the collected data is averaged as the sampling result, the acquisition can be avoided. The data is destroyed due to interference; if the data stored in the random memory is lost due to interference or the data changes, the check flag can be set in the random memory area; in order to reduce the damage to the random memory area, the random data can be randomly A trigger device is added to the write signal line of the memory chip, and is sent only when the CPU writes data. There are many measures against software interference, such as digital filtering programs, delay programs against narrow pulses, and authenticity of logic states. Sometimes, you must use a combination of software and hardware to suppress interference. A common method is to set a timer to protect the program from running normally.

——In recent years, electronic instruments have developed toward “light, thin, short, small” and multi-functional, high-performance and low-cost. Plastic cases, plastic parts or panels are widely used in electronic instruments, so external electromagnetic waves can easily penetrate the outer casing or panel, which can cause harmful interference to the normal operation of the instrument, and the electromagnetic waves generated by the instrument are also easily radiated to the surrounding space. , affecting the normal operation of other electronic instruments. In order to make this electronic instrument meet the requirements of electromagnetic compatibility, in practice, people have studied the process of plastic metallization, such as sputter galvanizing, vacuum plating (AL), electroplating or electroless copper plating, and bonding metal foil ( Cu or AL) and coating conductive coatings and the like. After metallization, the fully insulated plastic surface or the plastic itself (conductive plastic) is reflected as metal (such as a cell phone). Absorbs, conducts, and attenuates the characteristics of electromagnetic waves, thereby shielding electromagnetic waves. In practical applications, conductive coatings are used as shielding coatings with excellent performance and reasonable price. Where a shield is required, a closed conductive housing is formed and grounded to isolate the two different electromagnetic waves inside and outside. Practice has shown that it is practical and feasible if the shielding material can achieve the shielding effect of attenuation above 30~40dB.

——Because of the wide application of electronic technology and the complicated radiation of various interference devices, it is impossible to completely eliminate electromagnetic interference. However, according to the principle of electromagnetic compatibility, many technical measures can be taken to reduce electromagnetic interference, so that electromagnetic interference can be controlled within a certain range, thereby ensuring the compatibility of the system or equipment. For example, when the communication system is originally designed, the on-site electric wave should be strictly performed. Test, select the frequency and polarization mode in a targeted way, avoid the clutter interference such as radar and mobile communication; when selecting the path of the high-voltage line, it should try to bypass the radio station (station) or make full use of the terrain and ground object shielding of the receiving section; The receiving equipment and the industrial interference source equipment are properly configured to make the receiving equipment and a variety of industrial interference sources away from a certain distance; in the microwave communication circuit design, in order to reduce interference, the antenna high-low station mode can be used to adjust the microwave circuit reflection point, and the hill is blocked. The reflected wave is such that it does not interfere with the direct wave. In addition, the microwave tower is an independent tall building, and lightning protection measures such as grounding and shielding should be adopted.

7 Conclusion

- Ensuring electromagnetic compatibility of equipment is a complex technical task, and there is no omnipotent solution to this problem. Electromagnetic compatibility technology involves a wide range of fields, and the field of electromagnetic compatibility is also developing. It is important to master the basic principles of electromagnetic compatibility, and to carefully analyze and test, you can choose the appropriate method to solve the problem.