Electromagnetic compatibility (EMC) refers to the ability of electronic devices to function harmoniously in their electromagnetic environment without causing or being affected by electromagnetic interference (EMI). The goal of EMC design is to enable electronic devices to resist external interference while minimizing the electromagnetic interference they emit to surrounding devices.

1. Selecting the Right Wire Width

Minimizing inductance in printed wires is critical to reducing transient current interference. The inductance of a printed wire is directly proportional to its length and inversely proportional to its width. Therefore, short, wide wires are effective in suppressing interference. For signal lines that carry significant transient currents, such as clock leads or bus drivers, wire width should be carefully chosen:

• For discrete component circuits, a printed wire width of around 1.5mm is ideal.
• For integrated circuits, a wire width between 0.2mm and 1.0mm is recommended.

2. Implementing Proper Wiring Strategies

Using a parallel routing strategy can reduce wire inductance, though it may increase mutual inductance and distributed capacitance between wires. A more effective approach is to adopt a tic-tac-toe mesh structure:

• Wire horizontally on one side of the board and vertically on the other.
• Use metalized holes to connect cross-sections between layers.

3. Reducing Crosstalk Between Wires

To suppress crosstalk, avoid long-distance parallel routing, and maintain sufficient spacing between wires. Grounding strategies are key:

• Signal lines should not cross power or ground lines.
• A grounded printed line placed between interference-sensitive signals can effectively minimize crosstalk.

4. Minimizing Electromagnetic Radiation

When high-frequency signals pass through printed wires, electromagnetic radiation can be a concern. Here are key wiring practices to follow:

1. Avoid discontinuities: Keep the wire width constant, make corners greater than 90 degrees, and avoid loop routing.
2. Clock signal routing: Clock signal leads should be near the ground loop, with the driver close to the connector to reduce radiation.
3. Bus driver placement: The driver should be located near the bus it drives, especially for leads exiting the PCB.
4. Data bus routing: Place signal ground wires between every two signal lines on the data bus, with the ground loop close to the least important address lead.
5. Circuit layout: Arrange high-speed, medium-speed, and low-speed logic circuits as shown in Figure 1 (example layout).

5. Suppressing Reflection Interference

To suppress reflection interference at the terminal of printed lines, minimize line length and use slower circuits when possible. Terminal matching, with a matching resistor added to the ground or power supply, can be used if needed. As a rule of thumb:

• For fast TTL circuits, consider terminal matching if the printed line exceeds 10cm.
• The matching resistor value should correspond to the IC’s maximum output drive current.

6. Differential Signal Line Routing for PCB Design

Routing differential signal pairs close together reduces EMI by creating tight coupling between the lines. These signal pairs, often carrying high-speed signals, should follow high-speed design principles to ensure consistent impedance:

• Keep PCB lines in differential pairs identical in length and impedance.
• Maintain a constant distance between the pairs along the entire route.
• Differential pairs should always be routed closely to minimize interference.

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