Re: [SI-LIST] : Matched Length Traces

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Wed Nov 22 2000 - 09:38:10 PST


Traces that loop back on each other don't turn into antennas any more
than traces that are straight. The antenna effect happens only when the
underlyig plane is removed.

The bigger issue with this serpentine routing is that the segments
interact with each other resulting in an electrical length that is
shorter than anticipated.

Lee

Andrew W. Riley III wrote:

> Hello SI-LIST members,
>
> One issue that I am unclear on is all the turns in the traces shown in the attached file 'Layer3.jpg' and in the picture Mr. Riazi attached for reference to his post on "Plane Splits Inspection".
>
> We have our designs sent to another company for Layout. In a section of the layout instructions I specified net pairs to be of equal length with an extremely generous tolerance (I think) of 0.200" but ONLY if excessive bends were not added to the trace. The board is relatively small; approximately 2"x6", and densely packed.
>
> I also referred to UltraCAD's Design Note www.ultracad.com/t001.pdf under the Radiating Points section, which states;
> "Right angle turns and "T's": A trace that extends in a straight line is relatively clean. One that extends straight and then turns 180 degrees back on itself looks just like an antenna (like those on a tall building!)"
>
> Layer3.jpg is a sample of what was sent back to me for review and is what I thought we were trying to avoid. Also, I was somewhat surprised to see similar routing in "Figure 1" attached to Mr. Riazi's initial post "Plane Splits Inspection". This style of routing will not present any problems?
>
> Am I wrong in thinking the routing shown in Mr. Riazi's and my picture are examples of the 'antennas' mentioned in UltraCAD's Design Note? And to that end, would someone be so kind as to explain why?
> Any references supplied would be most helpful, too.
>
> I will do my best to clarify anything that I neglected if at all possible.
>
> ------------------------------------------------------------------------
>
> Name: Layer3.jpg
> Layer3.jpg Type: JPEG Image (image/jpeg)
> Encoding: base64

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