EMI desigm principles in the high-speed PCB circuit

- Aug 11, 2016-

Electronic engineers pay more and more attention, almost 60% of the EMI problems can be solved by the high-speed PCB. The following are nine rules:

Rule number one: the high-speed signal traces blocking rules

In the high-speed PCB design, clock and other key high-speed signal lines, the lines need to be shielded process, if there is no screen or only a portion of the shield, EMI will cause leakage. Shielded cable is recommended every 1000mil, punching the ground.


Rule number two: high-speed signal traces a closed loop rule

Due to increasingly high-density PCB board, PCB LAYOUT engineers during many traces, it is easy emergence of a mistake, that the high-speed clock signal, etc. signaling network, multilayer PCB traces generated when the closed-loop results this result will be a closed loop loop antenna to increase the intensity of the radiation of EMI.


Rule number three: high-speed signal traces ring-opening rules

Rule number two mentioned closed-loop high-speed signal can cause EMI radiation, but the ring-opening will also cause EMI radiation.

Clock signals and other high-speed signal network, multilayer PCB traces when once produced the results of the open-loop will produce a linear antenna, increase the intensity of the radiation of EMI.


Rule Four: Characteristics of continuous high-speed signal impedance rules

High-speed signals, when switching between layers must ensure continuous characteristic impedance, otherwise it will increase the EMI radiation. That is, with the width of the wiring layer must be continuous, different layers of trace impedance must be continuous.


Rule 5: High-speed PCB design rules routing directions

Between two adjacent traces must follow the principle of vertical trace, otherwise it will cause crosstalk between lines, increase EMI radiation.

In short, the adjacent wiring layer wiring follow the direction vertical vertical horizontal flat, vertical wiring can be suppressed crosstalk between lines.


Rule Six: high speed PCB design topology rules

High-speed PCB design, PCB topology of the characteristic impedance of the load control and multi-case design directly determines the success or failure of the product.


Rule Seven: take the length of the resonant rules

Check whether the frequency of the signal line length and constitute a resonance signal, that is, when the wiring length is 1/4 wavelength of the signal when the integer multiple of the wiring will have resonance, and resonant electromagnetic radiation will produce interference.


Rule Eight: return path rule

All high-speed signal must have a good return path. As far as possible to ensure the high-speed clock signal return path is minimized. Otherwise it will greatly increase the radiation, and proportional to the size and signal path and the return path of the radiation being surrounded.


Rule Nine: decoupling capacitor placement rules device

Decoupling capacitor placement is very important. Placed unreasonable hardly be decoupling effect. The principle is: close to the supply pin, the area and the capacitance of the power traces and ground surrounded minimum.


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