In the PCB design ,in order to reduce crosstalk between lines, we should ensure line spacing is large enough.When the distance between the line center is not less than three times than line width,you can keep most of the electric field noninterference,which is 3W principles.
As shown below: meeting 3W principle enables crosstalk between signals reduced by 70%, and meeting the 10W enables crosstalk between signals reduced by nearly 98%.
3W principle is although easy to remember, but to emphasize that this principle with previous condition is established.Considering physical significance of the causes of cross talk,in order to effectively prevent cross talk,the spacing is related with stack height and line width. For the four-layer board,the distance between traces and the reference plane height (5 ~ 10mils), 3W is enough; but for two-layer board,the distance between traces and the reference plane height (45 ~ 55mils),3W may not be enough for high-speed signal traces .3W principle is generally established at he conditions of 50 ohm characteristic impedance transmission line.
3W principle is that when a plurality of high-speed signal has long-distance traces,its spacing should follow he principle of 3W, such as a clock line, differential line, video, audio signal line, a reset signal line ,and other critical systems which need to follow the circuit 3W principles,not all the wiring on pcb board must enforce compliance with the principle of 3W.
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