When designing EMC for printed circuit boards, the layout should be considered first. PCB engineers must coordinate with structural engineers and EMC engineers to take both into account.

　　First of all, the structure and size of the printed circuit board should be considered, and how to arrange the device. If the devices are widely distributed, the transmission lines between the devices may be long, the printed lines will be long, the impedance will increase, the noise resistance will decrease, and the cost will increase. If the device distribution is too concentrated, the heat dissipation is not good, and the adjacent lines are susceptible to coupling and crosstalk. Therefore, according to the functional unit of the circuit, the overall layout of all components of the circuit. Factors such as electromagnetic compatibility, heat distribution, sensitive and non-sensitive devices, I/O interface, reset circuit and clock system are also taken into account.

　　In general, the overall layout should comply with the following basic principles:

　　1, when the circuit board on the high, medium and low speed circuit at the same time, should according to the logic speed of segmentation: decorate fast, medium speed and low speed logic circuit, high speed of the device (fast logic, clock oscillator, etc.) should be placed near the connector range, reduce the antenna effect, and low speed logic and memory, the range should be placed far away from the connector. This is beneficial to the reduction of coimpedance coupling, radiation and cross - interference.

　　2. In single panel or double panel, if the power cable is long, add the coupling capacitor to the ground every 3000mil, and the value of the capacitor is 10uF+1000pF to filter out high-frequency noise on the power cable.

　　3, in the single panel and double panel, the filtering capacitor wiring should be filtered by the filtering capacitor filtering, and then to the device pin, so that the power supply voltage is filtered and then the IC power supply, and the IC feedback to the power supply noise will be filtered out by the capacitor first. The position of the decoupling capacitor should be determined according to the actual situation. It is not absolutely placed at the positive pole of the power supply, but may also be placed at the negative pole of the power supply. In principle, the grounding impedance should be minimized.

　　4, clock line, bus, radio frequency line and other strong radiation signal line away from the interface of the outgoing signal line at least 1000mil, to avoid the interference on the strong radiation signal line coupling to the outgoing signal line outward radiation, crystal, crystal oscillator, relay, switching power supply and other strong radiation device layout should be considered.

　　5. The input and output signal lines of the filter (filter circuit) cannot be parallel to each other and crossed to avoid direct noise coupling before and after filtering.

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