Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
1. Choose a reasonable wire width
Since the impact interference generated by the transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be reduced as much as possible. The inductance of a printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the traces should be kept as short as possible. For discrete component circuits, the width of the printed wires is about 1.5 mm, which can fully meet the requirements; for integrated circuits, the width of the printed wires can be selected between 0.2 and 1.0 mm.
2. Adopt the correct wiring strategy
The use of equal wiring can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a well-shaped mesh wiring structure. The specific method is to wire one side of the printed board horizontally and the other side vertically. Then use metallized holes to connect at the intersection holes.
3. In order to suppress crosstalk between printed circuit board wires , long-distance equal wiring should be avoided as far as possible when designing wiring, and the distance between lines should be opened as much as possible. cross. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
4. In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires , the following points should also be paid attention to when wiring printed circuit boards:
(1) Minimize the discontinuity of the printed wires, for example, the width of the wires should not change abruptly, the corners of the wires should be greater than 90 degrees, and ring routing is prohibited.
(2) The clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they should be close to the ground loop, and the driver should be close to the connector.
(3) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be next to the connector.
(4) The wiring of the data bus should sandwich a signal ground wire between every two signal wires. It is best to place the ground return next to the least critical address lead, which often carries high frequency currents.
(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.
5. Suppress reflection interference
In order to suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. If necessary, terminal matching can be added, that is, a matching resistor of the same resistance value is added to the end of the transmission line to the ground and the power supply end. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be used when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output driving current and the sinking current of the integrated circuit.
6. The differential signal line layout strategy is adopted in the circuit board design process
Differential signal pairs that are routed very close to each other will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal line very carefully to ensure that the characteristic impedance of the signal line is continuous and constant throughout the signal line.
During the layout and routing process of the differential line pair, we hope that the two PCB lines in the differential line pair are exactly the same. This means that in practical applications, we should try our best to ensure that the PCB lines in the differential line pair have exactly the same impedance and the length of the wiring is also exactly the same. Differential PCB lines are usually always routed in pairs, and the distance between them remains constant at any position along the direction of the pair. Typically, the layout and routing of differential pairs is always as close as possible.
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