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Forum: PCB Design & Technology What is 3W Rule of PCB design?


von Lernend B. (Company: KAI Electro Semiconductor Inc.) (lernend05)


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Hey there
Recently, I'm study the pcb design rules, one of them is pcb wire. And I 
read a pcb note which said "To ensure that the distance between the 
lines is large enough, when the distance between the line and the center 
of the line is not less than 3 times the line width. If the line center 
distance is not less than 3 times the line width, 70% of the line 
electric fields will not interfere with each other, which is called 3W 
rule." It is about line distance on the pcb board.
I 'm confused it and have searched it but get a few useful info, is 
there anyone know it?
(The figure is attached, and enclose the ordinary words source 
https://www.apogeeweb.net/electron/pcb-design-basics-wiring.html)

von Christophz (Guest)


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This rule applies to the calculation of line width and distance of 
coupled microstrip or coupled striplines. This are used for differential 
highfrequency signals (for ex. HDMI, SATA, PCIe).

To ensure a good coupling between the positive and negative line, you 
shall follow this 3W rule.

von HildeK (Guest)


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Christophz wrote:
> To ensure a good coupling between the positive and negative line, you
> shall follow this 3W rule.

No.
To ensure a minimal coupling between two separate single ended lines, 
you should have at least 3w distance between these lines to minimze 
cross talk. It depends on the signals on these lines, especially the 
rise/fall time.
If you have enough PCB space and you can spend more than 3w, then more 
than 70% of the line electric fields will not interfere with each other.

On differential lines a coupling between both lines is desired. In this 
case, the distance normally is less than d=3w to get a sufficient 
coupling. The correct value is a result of the impedance calculation. 
The result depends beside the distance d of the PCB thickness (distance 
to GND layer), the desired Z-value, the PCB material and a little bit of 
the line thickness.

If you have several differential line pairs near by near, the distance 
between these pairs should be also at least 3w to avoid unwanted 
coupling.

von P. S. (namnyef)


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This is a rule of thumb (!) to minimize near-end-crosstalk between two 
50 Ohm transmission lines.

It has nothing to do with differential pairs, except maybe for this: If 
you want two 50 Ohm single ended transmission lines to form a 
differential pair with approx. 100 Ohm differential impedance, you 
should keep the two transmission lines at least "3w" apart.

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