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TurboPC
Joined: 14 Mar 2002
Posts: 148
Helped: 1
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 17 Feb 2003 5:02 Controlled Impedance Trace with ground fill |
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I have a 6 layer-board with 10-mil spacing layers and FR4 material. My 50-Ohm trace is on component side and the ground layer is the next layer (10-mil distance).
Using the formulas, I found that the trace should be around 17 mils thick to obtain the 50-ohm controlled impedance.
For EMI reasons, I did a ground fill on the component side but I kept a clearance around the controlled impedance trace to avoid affecting the controlled impedance.
What should be this clearance? Some say 3 times the trace width. Some say 3 times the "ground plane to trace" distance. What do you think?
Best regards, T.
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 17 Feb 2003 5:48 use appcad |
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| Appcad which can be downloaded from http://www.hp.woodshot.com/ has a coplanar transmission line with ground plane under it calculator that you can use to make the calculations.
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jzo777n
Joined: 10 Sep 2002
Posts: 113
Helped: 2
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| 18 Feb 2003 11:55 |
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I think that you refer to the rule of thumb for crosstalk, 3 times ground plane to trace distance
This is a formula for crosstalk between parallel lines from by Dr. Howard Johnson, http://www.sigcon.com .
The worst case value of the coefficient of coupling K (i.e., the crosstalk coefficient) is approximately:
K = 1 / (1 + (D/H)^^2)
Where D is the separation between the centerlines of the traces, and where H is the effective height above the nearest reference plane.
But I think that you must use a field solver for a more correct answer
jzo
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