Electromagnetic Compatibility Design of Mobile Phone PCB (2)

4. 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:

(1) Minimize the discontinuity of printed wires, for example, the width of the wires should not be abruptly changed, the corners of the wires should be greater than 90 degrees, and ring routing is prohibited.

(2) The lead of the clock signal is most likely to generate electromagnetic radiation interference. The wiring 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 right 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 right next to the least important address lead because the latter often carry high frequency currents.

(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged as shown in Figure 1.

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. The differential signal line routing strategy is adopted 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.