With the development of science and technology, more and more electrical and electronic equipment have entered every corner of society, and electronic technology has penetrated into every aspect. As the density of electrical and electronic equipment increases sharply, the transmitting power of equipment becomes increasingly large, and the radio spectrum becomes increasingly crowded, the electromagnetic environment in limited space deteriorates day by day. Therefore, electromagnetic compatibility is a key problem to be solved. However, due to the late start of electromagnetic compatibility in China, many electrical and electronic products are not qualified due to insufficient consideration of electromagnetic compatibility, and the re-design is obviously not large and practical, so the change of electromagnetic compatibility is quite important. Below the author in the actual work of the electromagnetic compatibility of the rectification of a summary.

First of all, the product should be diagnosed according to the actual situation, and the source of its interference and the way and means of mutual interference should be analyzed. Then according to the analysis results, the targeted rectification. Generally speaking, the main rectification methods are as follows.

1. Reduce interference sources

On the basis of finding interferon source, the allowable range of interferon source can be weakened. Generally, the methods of attenuation source can be as follows: a. B. Add a attenuator to ensure sensitivity and SNR. For example, the crystal vibration in VCD and DVD players has a serious impact on electromagnetic compatibility. Reducing its amplitude is one of the feasible methods, but it is not the only solution. C. another indirect method is to keep the signal line away from the interference source.

2. Classification and arrangement of wire and cable

In electronic equipment, interline coupling is an important way and an important reason for interference. Because of frequency factors, it can be generally divided into high frequency coupling and low frequency coupling. Due to different coupling modes, the rectification methods are also different. The following are discussed respectively:

(1) low frequency coupling

The length is equal to or less than 1/16 of a wavelength of low frequency coupling is guidelines, and can be divided into low frequency coupling electric field and magnetic field coupling, physical model is capacitively coupled to electric field coupling, so the main purpose of the rectification is distributed coupling or decrease coupling, can adopt the following methods: a. spacing increases circuit is the most effective way to reduce the distributed capacitance. B. The high conductivity shielding is added, and the single point grounding of the shield can effectively inhibit the low-frequency electric field interference. C. The additional filter can reduce the coupling between the two circuits. D. Lower input impedance. For example, the input impedance of CMOS circuit is very high and extremely sensitive to electric field interference. The input terminal can be connected with a resistor with a low capacitance or resistance within the allowable range. The physical model of magnetic field coupling is inductively coupled, and its coupling is mainly coupled through the distributed mutual inductance between lines. Therefore, the main method of modification is to destroy or reduce the coupling amount. Generally, the following method can be adopted: a. B. Reduce the area of the loop between the sensitive circuit and the source circuit, i.e., make the signal line or the carrier line as close or twisted as possible to its return line. C. Increase the spacing between the two circuits so as to reduce the mutual inductance between lines and reduce coupling. D. Whenever possible, minimize coupling between the sensitive circuit and the source circuit by making them planar or nearly orthogonal. E. Wrapping sensitive lines with highly conductive materials can effectively solve the problem of magnetic field interference. It is worth noting that closed and magnetic circuit should be constructed, and efforts to reduce the magnetic resistance of magnetic circuit will be more effective.

(2) high frequency coupling

High frequency coupling refers to that the grounding line of more than 1/4 wavelength is strengthened by the presence of standing waves of voltage and current in the circuit, which can be solved by the following methods: a. B. Rearrange the input and output lines of the filter to prevent coupling between the input and output lines, so as to ensure that the filtering effect of the filter is not poor. C. Multi-point grounding is adopted for the shielding layer of shielded cable. D. Connect the suspension pin of the connector to the ground potential to prevent the antenna effect.

3. Improve the ground wire system

The ideal ground wire is a physical entity with zero impedance and zero potential, which is not only the reference point of the signal, but also does not generate a voltage drop when the current flows through it. This ideal ground wire does not exist in a specific electrical and electronic equipment, and when the current flows through the ground wire, a voltage drop is inevitable. According to the formation mechanism of interference in ground wire, it can be summarized as the following two points. First, reduce the low impedance and feed impedance. Second, correct selection of grounding mode and blocking ground loop. According to the ground mode, there are suspension ground, single point ground, multi-point ground and mixed ground. If the interference of sensitive lines mainly comes from the outer space or system shell, it can be solved by suspending ground. However, the equipment in suspension ground is easy to generate electrostatic accumulation. When the charge reaches a certain degree, it will generate electrostatic discharge, so the suspended ground is not suitable for general electronic equipment. Single point grounding is applicable to low frequency circuits. In order to prevent the ground potential difference between the power frequency current and other stray currents in the signal ground wire, the signal ground wire is isolated from the power source and the safe ground wire, and it is connected at a single point in the power line. Single point grounding is mainly used in cases where the frequency is less than 3MHz. Multi-point grounding is the only practical grounding method for high frequency signals. Transmission line characteristics will be presented in rf. In order to make multi-point grounding effective, when the length of the grounding conductor exceeds 1/8 wavelength of the highest frequency, multi-point grounding requires an equipotential ground plane. Multi-point grounding is suitable for over 300KHz. Hybrid grounding is suitable for electronic circuits with high frequencies and low frequencies.

4、shielding

Shielding is one of the important measures to improve the electromagnetic compatibility of electronic system and electronic equipment. Shielding mechanism can be divided into magnetic field shielding, electric field shielding and electromagnetic shielding. Electric field shielding should pay attention to the following points: a. Select materials with high conductivity and good grounding. B. Choose the correct connection location and reasonable shape, preferably direct grounding of the shielding body. Magnetic field shielding usually refers to the shielding of dc or VLF magnetic fields. Its shielding effectiveness is far less than that of electric field shielding and electromagnetic shielding. B. Magnetic shield should be kept away from magnetic elements to prevent magnetic short circuit. C. Double-layer shielding or even three-layer shielding may be adopted. D. The hole on the upper side of the shielding body should pay attention to the direction of the hole opening, so as to make the long edge of the joint parallel to the flux direction as far as possible, so as to minimize the increase of the magnetic circuit length. Generally speaking, the magnetic shield does not need to be grounded, but it is better to be grounded in order to prevent electric field induction. When the electromagnetic field passes through the metal or the barrier body which has the attenuation effect on the electromagnetic field, it will be attenuated to a certain extent, that is, it will have the shielding effect on the electromagnetic field. In the actual rectification process, the shielding and the shape, size and grounding mode of the shielding and shielding body shall be chosen according to the specific needs.

5. Change the wiring structure of the circuit board

Some frequency points are determined by the routing distribution parameters on the circuit board. The above method is not very useful. This kind of modification can change the circuit parameter structure by adding small inductors, capacitors and magnetic beads to the circuit board, so that it can be moved to the frequency points with high limit value. For this kind of interference, to fundamentally solve its impact, it is necessary to rewire.

Summary: in a word, the previous methods are beneficial to improving electromagnetic compatibility, but the most widely used methods are to change the structure of ground wire and the classification and arrangement of wire and cable. These methods not only save costs, but also are the most effective correction methods. Shielding, while costly, is sometimes more effective than other methods. Therefore, in the actual rectification, the main method should be to change the ground wire structure, the classification and arrangement of wire and cable, shielding, supplemented by other methods.