RE: [SI-LIST] : Matched Length Traces

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From: Chris Rokusek (crokusek@innoveda.com)
Date: Thu Nov 09 2000 - 18:09:48 PST


Andrew,

Just an opinion...

The comment appears to stem from the effect of NEAR fields increasing when
traces are looped or folded back. I assume this because there is mention of
"probing the board."

I disagree with the recommendation to avoid loops or 180 degree turns
because:

1) One cannot conclude that a strong NEAR field will always map to a strong
FAR field. Especially when the NEAR field is well-controlled in the sense
that the trace's return is a plane.

2) For this particular case your trace separation (XTalk margins) should
have been defined in such a way that the NEAR field cannot be coupled
significantly onto another trace anyway.

In fact, a very weak near field that is carried by a very efficient antenna
is a much bigger problem to concern yourself with (think common-mode
currents on return planes).

Yes there is definitely a correlation between near and far field deltas for
many cases, but for this particular case it sounds like an
over-generalization.

Again, just an opinion,

Chris Rokusek
Innoveda

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Andrew W. Riley III
> Sent: Thursday, November 09, 2000 3:55 PM
> To: SI LIST
> Subject: [SI-LIST] : Matched Length Traces
>
>
> 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.
>
>
>

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