EMC electromagnetic compatibility rectification commonly used basic methods

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1, weaken the interference source to find the source of interference on the basis of interference sources can be allowed within the scope of the weakened, weaken the source of the method generally have the following methods:

A, between the Vcc and GND in the IC plus decoupling capacitors, the capacitance of the capacity of 0.01μF to 0.1μF between the installation of the capacitor to pay attention to the lead, so that it is as short as possible.

B, in the guarantee of sensitivity and signal to noise ratio in the case of attenuator. Such as VCD, DVD disc machine in the crystal, it is more serious impact on electromagnetic compatibility, reduce its amplitude is one of the feasible methods, but it is not the only solution.

C, there is an indirect way is to make the signal line away from the interference source.

2, wire and cable classification In electronic equipment, inter-line coupling is an important way, but also an important cause of interference, because the frequency factor can be roughly divided into high-frequency coupling and low-frequency coupling. Due to different ways of coupling, the rectification method is different, the following were discussed:

(1) low-frequency coupling is the length of the conductor is equal to or less than 1/16 wavelength of the case, low-frequency coupling can be divided into electric and magnetic coupling.

The physical model of the electric field coupling is capacitive coupling, so the main purpose of the rectification is to reduce the distributed coupling capacitance or reduce the coupling amount, using the following method:

A, increasing the circuit spacing is the most effective way to reduce the distributed capacitance.

B, additional high conductivity shield, and the shield to a single point of grounding can effectively inhibit the low-frequency electric field interference.

C, the additional filter can reduce the coupling between the two circuits.

D, reduce the input impedance, such as CMOS circuit input impedance is very high, very sensitive to the electric field interference, can be within the allowable range at the input and then connect a capacitor or lower resistance resistance.

The physical model of magnetic field coupling is inductively coupled, and its coupling is mainly through the distribution of mutual inductance to the coupling, so the main method of rectification is to destroy or reduce the coupling amount, generally can be used as follows:

A additional filter, in the additional filter should pay attention to the filter input and output impedance and frequency response.

B reduce the loop between the sensitive circuit and the source circuit, that is, as far as possible so that the signal line or carrier line with its loop near or twisted in one.

C Increase the two circuit spacing in order to reduce the mutual inductance between lines to reduce the coupling amount.

D if possible, as far as possible so that the sensitive circuit and the source circuit plane orthogonal or near orthogonal to reduce the coupling of the two circuits.

It is effective to solve the problem of magnetic field interference. It is worth noting that the magnetic resistance of the magnetic circuit will be more effective.

(2) high-frequency coupling is longer than 1/4 wavelength of the line due to the voltage and current in the circuit standing wave, will increase the coupling, can be used to solve the following methods:

A, as far as possible to shorten the ground wire, and the shell as much as possible to use the surface contact with the way.

B, re-organize the filter input and output lines, to prevent the input and output lines between the coupling to ensure that the filter filter effect is not bad.

C, shielded cable shield using multi-point grounding.

D, the connector of the plug pin to ground potential, to prevent its antenna effect.

3, to improve the ground system

The ideal ground is a zero-impedance, zero-potential physical entity, which is not only the reference point of the signal, but also the current does not produce a voltage drop. In a particular electrical and electronic device, this ideal ground is not present, when the current flows through the ground will inevitably produce voltage drop. According to the interference mechanism in the ground can be attributed to the following two points, first, reduce the low impedance and power feeder impedance. Second, the correct choice of grounding and blocking the loop, according to the ground way to sub-suspension, single-point grounding, multi-point grounding, mixed ground. If the interference of the sensitive line is mainly from the external space or the system shell, this time can be used to solve the suspension way, but the suspension of equipment prone to static accumulation, when the charge reaches a certain extent, will produce electrostatic discharge, so the suspension should not be used In general electronic equipment. Single-point grounding for low-frequency circuit, in order to prevent the power frequency current and other stray current in the signal ground between the points to produce ground potential difference between the signal ground and power and safety ground isolation, Point connection. Single point grounding is mainly applied to the frequency of less than 3MHz situation. Multi-point grounding is the only practical high-frequency signal grounding, the RF will show the characteristics of the transmission line, in order to make multi-point grounding effectiveness, when the ground conductor length exceeds the maximum frequency of 1/8 wavelength, multi-point grounding requires an equipotential Ground plane. Multi-point grounding for more than 300KHz. Hybrid ground is suitable for use since there are high frequency and low frequency electronic circuits.

4, shielding

Shield is one of the important measures to improve the electromagnetic compatibility of electronic systems and electronic equipment, it can effectively inhibit the spread of space through a variety of electromagnetic interference. Shielding by the mechanism can be divided into magnetic field shielding and electric field shielding and electromagnetic shielding. Electric field shielding should note the following:

A, choose high conductivity properties of the material, and have a good grounding.

B, the correct choice of ground and reasonable shape, it is best to shield directly grounded. Magnetic shielding is usually only refers to the DC or very low frequency magnetic field shielding, the shielding effectiveness is far less than the electric field shielding and electromagnetic shielding, magnetic shielding is often the focus of the project, magnetic shielding:

A, to use ferromagnetic materials.

B, magnetic shielding body to stay away from the magnetic components, to prevent magnetic short circuit.

C, can be double shield or even three layers of shielding.

D, the openings on the top of the shield should pay attention to the direction of the hole, as far as possible to make the long side of the seam parallel to the magnetic flux flow, so that the magnetic path length increased at least. In general, magnetic shielding does not need to be grounded, but to prevent electric field induction, or ground for the better. Electromagnetic field in the metal or through the electromagnetic field attenuation of the block, will be subject to a certain degree of attenuation, that is, the role of shielding the electromagnetic field. In the actual rectification process depending on the specific needs of what kind of shielding and shield shape, size, grounding and so on.

5, change the wiring structure of the circuit board

Some frequency points are determined by the alignment parameters on the board, which are not very useful by means of the preceding method. By changing the inductance, capacitance, and magnetic beads in the trace to change the circuit parameter structure, Limit values are required at higher frequency points. For such interference, in order to fundamentally solve its impact, we must re-wiring.

Summary: In the first few ways to improve the electromagnetic compatibility are good, but the most widely used is to change the ground structure and wire and cable sorting methods, these methods not only cost savings, but also the most effective rectification method. Although the shield will increase the cost, but it plays the shielding effect is sometimes other methods can not match. Therefore, in the actual rectification should be to change the ground structure, wire and cable classification, shielding the main method, supplemented by other methods.