First of all, the product must be diagnosed according to the actual situation, and the sources of interference and the ways and methods of mutual interference should be analyzed. Based on the analysis results, targeted rectification will be carried out. The following points are for reference only.

　　1. Reduce interference sources. On the basis of finding the interference source, weaken the interference source within the allowable range. The method for weakening the interference source is related to the following aspects:

a. Add a decoupling capacitor between VCC and GND of the IC. The capacity of this capacitor is between 0.1μF and 1μF. Pay attention to the lead of the capacitor when installing, making it shorter as possible.

b. Whether the filtering, protection and isolation devices of interface signals are placed close to the interface connector, protect first and filter later; whether the power module, filter, and power protection devices are placed close to the entrance of the power supply to ensure that the input line of the power supply is as short as possible., the input and output of the power supply are separated, and the traces do not intersect each other.

c. Whether strong radiating devices or sensitive devices such as crystals, crystal oscillators, relays, and switching power supplies are far away from the single-board handle strips, connectors, input and output terminals, and PCB edges.

d. Whether the filter capacitors are placed close to the power pins of the IC, and the position and quantity are appropriate.

e. Whether the clock circuit is close to the load and is placed in Load Balancer.

f. Whether there are no other signal lines running from the input line of the power filter, and whether the input and output signal lines of the filter and other devices are not running in parallel or cross-over with each other.

　　2. Classification and organization of wires and cables In electronic equipment, line-to-line coupling is an important way and an important cause of interference. Because of the frequency factor, it can be roughly divided into high-frequency coupling and low-frequency coupling. Due to different coupling methods, the rectification methods are also different:

(1) Low-frequency coupling: When the length of the guide line is equal to or less than 1/16 wavelength, low-frequency coupling can be divided into electric field and magnetic field coupling. The physical model of electric field coupling is capacitive coupling, so the main purpose of rectification is to reduce distributed coupling capacitance or reduce the coupling amount, the following methods can be used:

a. Increasing circuit spacing is the most effective way to reduce distributed capacitance.

b. Add a highly conductive shield and make the shield single-point grounding to effectively suppress low-frequency electric field interference.

c. Adding a filter can reduce the amount of coupling between the two circuits.

d. Reduce the input impedance. For example, the input impedance of CMOS circuits is very high and is extremely sensitive to electric field interference. You can connect a capacitor or a resistor with a lower resistance at the input end within the allowable range. The physical model of magnetic field coupling is inductive coupling, which is mainly coupled through the distributed mutual inductance between lines. Therefore, the main method for rectification is to destroy or reduce the coupling amount. The following methods can be generally adopted:

– Add a filter. When adding a filter, pay attention to the input and output impedance of the filter and its frequency response.

– Reduce the loop area between the sensitive loop and the source loop, that is, try to make the signal line or carrier line close to or twist into its loop line.

– Increase the spacing between the two circuits to reduce the mutual inductance between the lines and reduce the amount of coupling.

– If possible, try to reduce the coupling between the two circuits by making the sensitive loop and the source loop plane orthogonal or nearly orthogonal.

– Using highly magnetic permeable materials to wrap sensitive wires can effectively solve the problem of magnetic field interference. It is worth noting that a closed magnetic circuit must be formed, and reducing the reluctance of the magnetic circuit will be more effective.

(2) High-frequency coupling: refers to traces longer than 1/4 wavelength, which will increase the coupling amount due to the presence of standing waves of voltage and current in the circuit. The following methods can be used to solve this problem:

a. Shorten the ground wire as much as possible, and use surface contact as much as possible to ground the housing.

b. Reorganize the input and output lines of the filter to prevent coupling between the input and output lines and ensure that the filtering effect of the filter remains unchanged.

c. The shielding layer of the shielded cable adopts multi-point grounding.

d. Connect the floating pin of the connector to the ground potential to prevent its antenna effect.

　　3. Improve the ground wire system The ideal ground wire is a physical entity with zero impedance and zero potential. It is not only a reference point for the signal, but also a voltage drop will not occur when current flows through.

In specific electrical and electronic equipment, this ideal ground wire does not exist, and a voltage drop will inevitably occur when current flows through the ground wire. Based on this, it can be attributed to the following two points based on the interference formation mechanism in the ground wire. First, reduce low impedance and power feeder impedance. Second, correctly select the grounding method and block the ground loop. According to the grounding method, there are floating ground, single-point grounding, multi-point grounding, and mixed grounding. If the interference of sensitive lines mainly comes from the external space or the system enclosure, it can be solved by floating ground at this time. However, the floating ground equipment is prone to electrostatic accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the floating ground should not be used for general electronic equipment. Single-point grounding is suitable for low-frequency circuits. In order to prevent power frequency current and other stray currents from producing ground potential differences between points on the signal ground wire, the signal ground wire is isolated from the power supply and safety ground wire, and the power line is connected to a single point at the large location.

　　4. Shielding is one of the important measures to improve the electromagnetic compatibility performance of electronic systems and electronic equipment. It can effectively suppress various electromagnetic interference propagating through space. Shielding can be divided into magnetic field shielding, electric field shielding and electromagnetic shielding according to the mechanism. The following points should be noted in electric field shielding:

a. Select a material with high electrical conductivity and a good grounding.

b Properly select the ground point and reasonable shape, it is best to directly connect the shield. Magnetic field shielding usually only refers to shielding DC or very low frequency magnetic fields. Its shielding effectiveness is far less than electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering. When shielding:

a. Ferromagnetic materials should be selected.

b. The magnetic shield should be kept away from magnetic components to prevent magnetic short circuits.

c. Double or even triple shielding can be used.

d. Pay attention to the direction of the opening for the opening on the shield, and try to make the long side of the slit parallel to the flow direction of the magnetic flux to minimize the increase in the length of the magnetic circuit. Generally speaking, magnetic shielding does not need to be grounded, but in order to prevent electric field induction, it is better to be grounded. When the electromagnetic field passes through a metal or a barrier that attenuates the electromagnetic field, it will be attenuated to a certain extent, that is, it will produce a shielding effect on the electromagnetic field. In the actual rectification process, the type of shield and the shape, size, grounding method, etc. of the shield will be selected according to specific needs.

　　5. Change the wiring structure of the circuit board

Some frequency points are determined by the distribution parameters of the traces on the circuit board. The above method is not useful. Such rectification changes the circuit parameter structure by adding small inductors, capacitors, and magnetic beads to the traces, so that it can be moved to frequency points with higher limit requirements. For this type of interference, if you want to fundamentally solve its impact, you must rewire it.

　　summary

In short, the previous methods are all beneficial to improving electromagnetic compatibility, but the most widely used methods are methods of changing the structure of ground wires and the classification and arrangement of wires and cables. These methods not only save costs, but also are the most effective rectification methods. Although shielding increases costs, its shielding effectiveness is sometimes unmatched by other methods. Therefore, in the actual rectification, the main method should be to change the ground wire structure, the classification, sorting, and shielding of wires and cables, supplemented by other methods.