[PCB_FORUM] Re: Impedance controlled line widths

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"Jean Bratton" <jean.bratton@xxxxxxxxxxxxxx> 
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08/29/2007 10:11 AM
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[PCB_FORUM] Impedance controlled line widths






Hi folks,
We have a design with impedance rules built in at the schematic level. The 
stackup appears to be correctly defined as provided to us, but Allegro is 
not calculating the trace widths at the same width as the stackup 
indicates they should be. The impedance value appears to be the prevailing 
rule, not the trace width we have set up in the constraints. If we delete 
the impedance property, we can get the board to route with the trace 
widths we specify, but we?re wondering if there?s a way to change the 
priority?to have it look at what the design rules specify for trace width 
rather than what the impedance rule would automatically define. 
 
Also, does anyone know of a good impedance calculator online? The one we 
used to use isn?t set up for a board as thick as this one is, so we need 
another. We need to know which is accurate, the stackup provided to us, or 
the calculations Allegro is making.
Thanks for any insight,
Jean
 
Jean Bratton
Sr. Printed Circuit Designer
Freedom CAD Services, Inc.
603-864-1349
jean.bratton@xxxxxxxxxxxxxx
Visit us at http://www.freedomcad.com
 
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