Do you know what electromagnetic compatibility is? Electromagnetic compatibility generally refers to the ability of Electronic equipment to work harmoniously and effectively in various electromagnetic environments. It can make the electronic equipment not only suppress various external interference, make the electronic equipment work normally in a specific electromagnetic environment, but also reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment. Follow the following PCB design tips to effectively improve the electromagnetic compatibility of the Circuit board!
1. Choose a reasonable wire width
Since the impulse interference produced by the transient current on the printed wires is mainly caused by the inductive components of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. Clock traces, signal lines of row drivers or bus drivers often carry large transient currents and traces should be kept as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, it can fully meet the requirements; for integrated circuits, the width of the printed wire can be selected between 0.2 and 1.0 mm.
2. Adopt the correct wiring strategy
The use of equal wiring can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a grid-shaped mesh wiring structure. The specific method is to route horizontally on one side of the printed circuit board and vertically on the other side. , and then connected with metallized holes at the cross holes.
3. Avoid long-distance equal routing
In order to suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, try to avoid long-distance equal wiring, keep the distance between the lines as far as possible, and do not cross the signal line, the ground line and the power line as much as possible. . Setting a grounded trace between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
Fourth, optimize the wiring design to avoid electromagnetic radiation generated when high-frequency signals pass through the printed wires
In order to avoid electromagnetic radiation generated when high-frequency signals pass through the printed wires, the following points should also be paid attention to when wiring the printed circuit board: minimize the discontinuity of the printed wires, for example, the width of the wires should not change suddenly, and the corners of the wires should be If the degree is greater than 90 degrees, ring-shaped wiring, etc. are prohibited.
The lead of the clock signal is the most likely to generate electromagnetic radiation interference, and the wiring should be close to the ground loop, and the driver should be close to the connector. A bus driver should be placed next to the bus it intends to drive. For those leads that leave the printed circuit board, the driver should be right next to the connector. 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 right next to the least important address lead because the latter often carry high frequency currents.
5. Suppress reflection interference
In order to suppress the reflection interference that appears at the end of the printed line, except for special needs, the length of the printed line should be shortened as much as possible and slow circuits should be used. If necessary, terminal matching can be added, that is, a matching resistor with the same resistance value is added at the end of the transmission line to the ground and the power supply end. According to experience, for the generally faster TTL circuit, the terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the output drive current and the maximum value of the sink current of the integrated circuit.
6. Use the differential signal line routing strategy in the circuit board design process
Differential signal pairs that are routed very close to each other are also tightly coupled to each other. This mutual coupling reduces EMI emissions. Usually (with some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the wiring of differential signals, especially when designing signal lines for transmission lines. This means that we must design the routing of the signal lines 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 pair, we want the two PCB lines in the differential pair to be exactly the same. This means that in practice, every effort should be made to ensure that the PCB traces in the differential pair have exactly the same impedance and that the trace lengths are exactly the same. Differential PCB traces are usually always routed in pairs, and the distance between them remains a constant everywhere along the direction of the pair. Typically, differential pairs are placed and routed as close together as possible. The above is some analysis of electromagnetic compatibility, I hope it can help you.
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