In general, electromagnetic noise is difficult to eliminate, but various measures can be taken to suppress electromagnetic noise to such an extent that electromagnetic interference is not generated. It is often difficult to solve electromagnetic noise problems in a simple way, so it is best to use several different combinations.
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First, the general requirements
The suppression of electromagnetic noise in automobiles can be carried out on the receiver side, but it is more difficult to take measures at the receiver end due to the acceptance frequency, the propagation mode of the interference wave and other various practical conditions. Since the electromagnetic noise of automotive electrical equipment can interfere with other communication equipment and various electronic equipment, it should be considered to suppress the electromagnetic noise generated by the automobile electrical equipment itself. The characteristics and levels of electromagnetic interference generated by various electrical appliances in automobiles are different, so the suppression of interference should also be consistent with its characteristics and level. The suppression of interference wave design can adopt basic measures such as damping, shielding, filtering and connection, and must meet four conditions: 1 has good suppression effect. 2 does not hinder the performance of automotive electrical equipment itself. 3 high reliability and easy to use. 4 The price is reasonable.
Second, the electromagnetic noise suppressor
At present, suppressors used in automobiles at home and abroad are basically assembled by resistors, inductors, and capacitors, so-called R, I, and C, such as resistors, shielded wires, capacitors, interference suppression inductors, and interference suppression filters.
Third, the method
shield
Shielding is the most straightforward way to establish a circuit in an electromagnetic barrier protection system between two areas that is not damaged by the electromagnetic environment.
Shields come in a variety of forms, from baffles to box closures, to cable or connector shields. The effectiveness of the shield is expressed in terms of shielding effectiveness, which is related not only to the shielding material, but also to the thickness of the material, the frequency of application, the distance from the source to the shield, and the shape and number of discontinuities in the shield.
Shielding has two main purposes: one is to limit the radiant electromagnetic energy within a specific area (called active shielding). Active shielding refers to a structure in which the interference source is internal and prevents interference waves from leaking into the external space. Grounding must be performed, and the grounding resistance is as small as possible. The second is to prevent radiated electromagnetic energy from entering a specific area (called passive shielding). Passive shielding refers to a structure in which the interference source is external and prevents interference waves from entering the shielding space.
The steps of the shielding design are: 1 to determine the interference level and energy density to be shielded; 2 to estimate the allowable signal level on the other side of the shielding layer; 3 to select the detailed shielding design in combination with the structure and housing design of the product. The design principle of shielding is: 1 high-frequency electric field shielding uses good conductive materials such as copper, aluminum and magnesium to get the maximum reflection efficiency. 2 Low-frequency magnetic field shielding uses magnetic materials such as iron and nickel-iron high-magnetic alloys to obtain maximum absorption efficiency. 3 A shield of sufficient thickness shields the electric field at any frequency and has a high shielding effectiveness. 4 Multi-layer shielding (including housing and cable) provides high shielding effectiveness over a wide frequency band, but with cost and other performance requirements (such as cable flexibility). 5 The various bonding surfaces used to seal the gap must be clean and must not have a non-conductive coating. 6 In order to maintain the shielding effectiveness of the outer casing, conductive liners, spring washers, waveguide attenuators and grids should be added to the necessary threading holes.
connection
The connection is to establish a low impedance path between the two metal faces. This path can be established between the ground reference and components, circuits, shields, and structural components. The purpose of the connection is to establish a uniform electrical structure, that is, to make the path of the RF current uniform in the structure, to avoid generating potential between the metal parts, thereby causing interference.
Ground
Grounding is the establishment of a conductive path between two points, one of which is usually the electrical component of the system and the other is the reference point.
The effectiveness of a grounding system depends on the extent to which the potential difference of the grounding system is reduced and the ground current is reduced.
Filter
The shielding is mainly to solve the radiation interference, and the filtering is mainly to solve the interference caused by the conduction path. Both involve connection and grounding techniques, and the effectiveness of EMI filters is largely influenced by source impedance and load impedance.
Fourth, technical measures for anti-electromagnetic interference design
There are a number of technical measures that can be used to reduce the susceptibility of electronic systems to electromagnetic interference, such as shielding, connections, filtering, grounding, circuit design, and component selection. Shielding, connection, filtering, and grounding have been described above. The following discussion focuses on component selection and circuit design.
Component selection and circuit design are one of the key points in anti-electromagnetic interference and electromagnetic compatibility design. By selecting components and anti-interference screening, components with high anti-interference thresholds can be obtained. This measure can increase the system's immunity to interference by 10-3dB.
The designed circuit has a high signal level and low impedance characteristics, which greatly reduces the sensitivity to interference. The general approach is to shorten the wiring of components and circuits, and use shielded twisted pairs for wiring.
For circuits, digital circuits are more robust than linear and analog circuits, and low-speed digital circuits have lower electromagnetic sensitivity than high-speed digital circuits.
When determining components and circuits, besides paying attention to their electromagnetic interference sensitivity, you should also pay attention to some components and circuits that generate electromagnetic interference. They also affect the system's morning stocks, or distorted the signal, or generate interference voltages, disturb currents, or cause system errors. Care should be taken when designing and testing.
Fifth, the circuit layout principle
1. Position and direction the components correctly. The most sensitive components should be kept away from the source of the disturbance.
2. The wiring should be reasonable. Wires of different levels for different purposes, such as input and output lines, weak current and strong power, should be kept away from each other and should not be parallel; the length of the grounding wire should be as short as possible and the cross section should be as large as possible.
3. The key components, circuits and traces should be shielded and the shield should be properly grounded.
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