Analysis on solving various electromagnetic interference problems of electronic equipment

The widespread application and development of electronic devices will inevitably lead to the continuous increase in the level of electromagnetic fields they generate in the space around them. That is to say, electronic equipment inevitably works in the electromagnetic environment (EME). Therefore, the adaptability of electronic equipment in the electromagnetic environment must be addressed. Electromagnetic compatibility (EMC) is the science of resisting the effects of electromagnetic interference (EMI). At present, on a global scale, the issue of electromagnetic compatibility has formed a new subject. The central topic of electromagnetic compatibility is to study the control and elimination of electromagnetic interference so that when electronic equipment or systems work in conjunction with other equipment, it will not cause the deterioration or reduction of the working performance of any part of the equipment or system. An ideally designed electronic device or system should neither radiate any undesirable energy nor be affected by any undesirable 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 content that needs to be studied in electromagnetic compatibility design.

2.1 Internal interference

——Internal interference refers to the mutual interference between components inside electronic equipment, including the following types.

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

——(2) Signals couple to each other through the impedance of ground wires, power supply and transmission wires, or interference caused by mutual inductance between wires;

——(3) Some components inside the equipment or system generate heat, affecting the stability of the component itself or other components and causing interference;

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

2.2 External interference

——External interference refers to the interference of factors other than electronic equipment or systems on lines, equipment or systems, including the following types.

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

——(2) External high-power equipment generates strong magnetic fields 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 of the working environment is unstable, causing interference caused by changes in parameters of internal components of electronic circuits, equipment or systems;

——(5) Equipment powered by industrial power grids and interference caused by grid voltage passing through power transformers.

3 Transmission pathways of interference

——When the frequency of the interference source is higher, 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 interfered object is r>>λ/2π, the interference signal can be considered as radiation. Field, which emits electromagnetic field energy outward in the form of plane electromagnetic waves and enters the path of the interfered object.

——(2) The interference signal takes the form of leakage and coupling, passes through insulating supports, etc. (including air) as a medium, and enters the interfered line, equipment or system through coupling of public 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 between the interference source and the interference object is r<<λ/2π, the interference source can be considered to be a quasi-stationary field. , which enters the path of the disturbed object in the form of an induction field.

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

4 Basic principles of electromagnetic compatibility design

4.1 Grounding

——Grounding is a very important issue for electronic equipment. Grounding has three purposes:

——(1) Grounding enables all unit circuits in the entire circuit system to have a common reference zero potential, ensuring that the circuit system can operate stably.

——(2) Prevent interference from external electromagnetic fields. Grounding the casing allows a large amount of charges 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 discharges inside the equipment and cause interference. In addition, if the circuit shield is properly grounded, a good shielding effect can be obtained.

——(3) Ensure safe work. When direct electromagnetic induction of lightning occurs, damage to electronic equipment can be avoided; when the input voltage of the power frequency AC power supply is directly connected to the casing due to poor insulation or other reasons, electric shock accidents to operators can be avoided. In addition, many medical equipment are directly connected to the patient's human body. When the casing carries a voltage of 110V or 220V, fatal danger will occur.

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

——There are basically three types of grounding methods for circuits, namely single-point grounding, multi-point grounding and hybrid grounding. Single-point grounding means that in a circuit, only one physical point is defined as the 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 grounding 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 equipment, or the ground wire running through the entire system. In a larger system, it can also be the structural frame of the equipment, etc. Hybrid grounding uses bypass capacitors and ground planes to connect those points that only require high-frequency grounding. However, the resonance phenomenon caused by the bypass capacitor and lead inductance should be avoided as much as possible.

4.2 Screen

——Shielding is the metal isolation between two spatial areas to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one area to another. Specifically, the shielding body is used to surround the interference source of the components, circuits, assemblies, cables or the entire system to prevent the interference electromagnetic field from spreading outward; the shielding body is used to surround the receiving circuit, equipment or system to prevent them from being affected by the outside world. The influence of electromagnetic fields.

——Because the shielding body absorbs energy (eddy current loss), reflects energy (interface reflection of electromagnetic waves on the shielding body) and offsets energy for external interference electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. (Electromagnetic induction generates a reverse electromagnetic field on the shielding layer, which can offset part of the interfering electromagnetic waves), so the shielding body has the function of weakening interference.

——The principles for selecting shielding materials are:

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

——(2) When the frequency of interfering electromagnetic waves is low, materials with high magnetic permeability should be used to limit the magnetic lines of force inside the shield and prevent them from spreading into the shielded space.

——(3) In some cases, if good shielding effect is required for both high-frequency and low-frequency electromagnetic fields, multi-layer shielding bodies are often composed of different metal materials.

4.3 Other interference suppression methods

——(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 spectrum components are not equal to the frequency of the useful signal. The filter has good suppression capabilities for these components that are different from the frequency of the useful signal, thus achieving other interference suppression functions that are difficult to achieve. role. Therefore, the use of filtering networks is a powerful measure whether it is to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving equipment. Using resistor-capacitor and inductor-capacitor decoupling networks can isolate the circuit from the power supply, eliminate coupling between circuits, and prevent interference signals from entering the circuit. For high-frequency circuits, a CLCMπ filter composed of two capacitors and an inductor (high-frequency choke) can be used. There are many types of filters, and selecting the appropriate filter can eliminate unwanted coupling.

——(2) Correct selection of passive components

——Practical passive components are not “ideal” and their characteristics are different from ideal characteristics. Practical components can themselves be a source of interference, so the correct selection of passive components is very important. Sometimes the characteristics of components can also be used to suppress and prevent interference.

——(3)Circuit technology

——Sometimes the requirements for suppressing and preventing interference cannot be met after using shielding. You can combine shielding and take balancing measures and other circuit technologies. A balanced circuit is one in which the two conductors in a two-wire circuit and all circuits connected to the two conductors have the same impedance to ground or to other conductors. Its purpose is to make the interference signals picked up by the two wires equal. The interference noise at this time is a common mode signal and can disappear on its own at the load. In addition, other circuit technologies can also be used, such as contact networks, shaping circuits, integrating circuits and strobe circuits, etc. In short, the use of circuit technology is also an important measure to suppress and prevent interference.

5 Norms and standards on electromagnetic compatibility issues

——In terms of electromagnetic compatibility, there are a series of international agreements and constructive rules that all countries must abide by. The International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC), an organization related to electromagnetic compatibility, mainly studies the measurement of interference noise in radio systems. In 1976, CISPR began to formulate EMI standards for electromagnetic interference. In October 1900, the reprinted standard was announced after several revisions. Subsequently, the committee also deliberated with the International Wireless Communications Advisory Committee to formulate data requirements and specific methods for testing the electromagnetic compatibility of electronic products. The "Permissible values ​​of interference characteristics and measurement methods of industrial, scientific and medical radio instruments" (Standard No. 11) targeting noise from information technology equipment were formulated; "Permissible values ​​of interference characteristics of vehicles, motor ships and spark ignition drive devices" "Measurement methods and allowable values" (Standard No. 12); "Measurement methods and allowable values ​​of radio interference characteristics of radio and television receivers" (Standard No. 13), etc. It was not until mid-1992 that the international EMI standards were finally completed. The tolerances recommended by CISPR have been adopted by many countries around the world and serve as the basis for their national regulations.

——The power level of radio transmitters is an important factor that affects surrounding radio electronic equipment and generates interference levels. Therefore radio transmitter power levels should be limited. For example, according to Radiocommunication Advisory Committee Recommendation No. 357-1, in the frequency band (5800~8100MHz) used jointly by satellite communication systems and terrestrial microwave relay communication lines, when the power supplied to the antenna does not exceed 13dBW, microwaves should be restricted. The effective radiated power of the transmitter of the relay communication line (that is, the product of the transmitter power and the antenna gain) is 55dBW. It is recommended to limit the power of satellite communication ground stations and the radiated power flux density of communication satellites at the same time. The maximum transmitter power of many other radio services, such as amateur radio, mobile communication systems, etc., should also be limited.

——Frequency planning has been widely adopted across the country and around the world. It is a way to improve the utilization of radio frequency resources and is also one of the important measures to ensure the electromagnetic compatibility of radio electronic equipment. Therefore, national and regional frequency band divisions and frequency band allocation between services should be implemented strictly in accordance with international agreements (radio frequency allocation tables) and national documents. Wireless channel allocation is performed based on the principle of frequency-space allocation. Frequency planning must ensure the minimum interference level of each radio electronic device, or eliminate interference, and is coordinated by the State Radio Regulatory Commission.

——In recent years, many departments in our country have been carrying out experimental research on electromagnetic compatibility and formulating relevant technical standards, and have formulated a series of standards and specifications. For example, national standard GB3907-83 is the basic measurement method for industrial radio interference; GB4824.1-84 is the allowable value of radio interference for industrial, scientific and medical radio frequency equipment; GB6279-86 is the measurement of radio characteristics of vehicles, motor ships and spark ignition engine driving devices. Methods and allowed values, etc. The State Radio Regulatory Commission has made specific regulations on the frequency, bandwidth and maximum radiation field strength of electronic equipment in industry, science, medicine and other fields. This plays an important role in ensuring the normal operation of electronic equipment and the normal life of people, as well as promoting the more rapid development of modern science and technology.

6 Solutions to some typical electromagnetic compatibility problems

——Due to the widespread application of electronic technology in all walks of life, the space of human activities is filled with electromagnetic waves. Therefore, electronic equipment cannot work compatible without solving the problem of electromagnetic wave interference. In practical applications, people have also accumulated a lot of experience in researching anti-interference technology, and have continuously developed many practical methods to eliminate electromagnetic interference.

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

——In order to solve the application and promotion of microelectronic technology, especially computers in automobiles, according to needs and actual requirements, a good filter circuit can be designed. Placing it in the front stage can prevent most of the interference noise that enters the system due to conduction. Eliminate the entrance of the circuit system; you can set up isolation circuits, such as transformer isolation and photoelectric isolation, to solve the conduction interference entering the circuit through power lines, signal lines and ground wires, while preventing interference caused by public impedance and long-term transmission; also Energy absorption loops can be set up to reduce the noise energy absorbed by circuits and devices; or the impact of interference can be reduced by selecting components and rationally arranging circuit systems.

——The software anti-interference of microcomputer equipment is mainly to stabilize memory data and ensure program pointers. 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 average of several sets of collected data is used as the sampling result, it can avoid The authenticity of the data is destroyed due to interference; if the data stored in the random memory is lost due to interference or the data changes, a check flag can be set in the random memory area; in order to reduce the damage to the random memory area by interference, a check flag can be set in the random memory area. A trigger device is added to the write signal line of the memory chip, which is only sent when the CPU writes data. There are also many software anti-interference measures, such as digital filtering programs, delay programs to resist narrow pulses, and authenticity judgment of logic states. Sometimes, a combination of software and hardware must be used to suppress interference. A common method is to set a timer to protect the normal operation of the program.

——In recent years, electronic instruments have developed in the direction of being “light, thin, short, small” and multi-functional, high-performance and low-cost. Plastic chassis, plastic parts or panels are widely used in electronic instruments, so external electromagnetic waves can easily penetrate the casing or panel, causing harmful interference to the normal operation of the instrument. The electromagnetic waves generated by the instrument can also easily radiate into the surrounding space. , affecting the normal operation of other electronic instruments. In order to make this electronic instrument meet the requirements of electromagnetic compatibility, people have developed plastic metallization processes in practice, such as sputtering galvanizing, vacuum plating (AL), electroplating or chemical copper plating, pasting metal foil ( Cu or AL) and coating with conductive paint, etc. After metallization, the completely insulating plastic surface or the plastic itself (conductive plastic) is made reflective like a metal (such as a mobile phone). It has the characteristics of absorbing, conducting and attenuating electromagnetic waves, thereby shielding electromagnetic wave interference. In practical applications, conductive paint is used as a shielding coating, which has excellent performance and is affordable. Where shielding is required, a closed conductive shell is made and grounded to isolate two different electromagnetic waves inside and outside. Practice has shown that if the shielding material can achieve a shielding effect of attenuation above (30-40) dB, it is practical and feasible.

——Due to the wide application of electronic technology and the complexity of radiation from 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 and control it to a certain range, thereby ensuring the compatibility of the system or equipment. For example, when the communication system is initially designed, on-site radio waves should be strictly carried out. When testing, select the frequency and polarization method in a targeted manner to avoid interference from radar, mobile communications and other clutter; when selecting a path for high-voltage lines, try to avoid the radio station (station) or make full use of the terrain and ground object shielding in the receiving area; The receiving equipment and industrial interference source equipment should be properly configured to keep the receiving equipment at a certain distance from various industrial interference sources; in the design of microwave communication circuits, in order to reduce interference, the antenna high and low stations can be used to adjust the reflection points of the microwave circuit and use hills to block Reflected waves so that they cannot interfere with direct waves. In addition, the microwave tower is an independent tall building, so complete grounding, shielding and other lightning protection measures should be adopted.