RE: [SI-LIST] : Matched Length Traces

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From: abe riazi ([email protected])
Date: Thu Nov 09 2000 - 18:48:31 PST

   Dear Andrew:

    I reviewed the section of the Ultracad article that you referred to and saw the statement:

  "Avoid traces that extend straight and then turn 180 degrees back on itself "

This behavior is often seen in serpentines. There are certain guidelines for correct routing of serpentines (for example it is often recommended to keep the separation between adjacent parallel segments at least 3x to 5x the height above GND plane), but they are used regularly in clock lines to add delay.

Therefore, my conclusion is that traces seen in the attachment of your post (and Figure 1 of my post) are acceptable.

Best Regards,

Abe Riazi

-----Original Message-----
From: Andrew W. Riley III [SMTP:[email protected]]
Sent: Thursday, November 09, 2000 3:55 PM
Subject: [SI-LIST] : Matched Length Traces

 << File: ATT00007.txt; charset = Windows-1252 >> << File: Layer3.jpeg >>

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 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.

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