RE: [SI-LIST] : How to model effect of vias on nearby traces?

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Thu Mar 22 2001 - 14:50:06 PST


Let's say for argument that a trace runs directly over the center of a
circular antipad area on the reference plane, and the antipad diameter is
larger than the trace width.

Yes, there is no reference under the trace, but wouldn't the return paths of
least inductance go around the rims of the circular antipad? That is not a
much longer path length unless the antipad is huge, so maybe that is why
there does not seem to be much effect.

What seems to really affect things is where you have a gap such that the
return currents have to seek a much longer path length.

Hmmm?

Larry Miller

-----Original Message-----
From: Greim, Michael [mailto:mgreim@sonusnet.com]
Sent: Thursday, March 22, 2001 2:40 PM
To: 'Larry Miller'; 'Patel, Bhavesh'; SI_LIST (E-mail)
Subject: RE: [SI-LIST] : How to model effect of vias on nearby traces?

Wouldn't part of the effect (perhaps a more significant
part) be how big the antipad is on the reference plane and
whether the signal in questions is routed over it or not?
I have seen some antipad requirements so large that a trace
could be routed over a wafer thin piece of plane to the
point that the trace is essentially routed over no reference
at all.

Comments?

Best Regards,

Michael C. Greim Sonus Networks
mgreim@sonusnet.com 978-589-8336

Making the world safe for digital signals everywhere

And all this science I don't understand
It's just my job six days a week

The time is gone. The email's over
Thought I'd something more to say......

-----Original Message-----
From: Larry Miller [mailto:ldmiller@rhapsodynetworks.com]
Sent: Thursday, March 22, 2001 4:42 PM
To: 'Patel, Bhavesh'; SI_LIST (E-mail)
Subject: RE: [SI-LIST] : How to model effect of vias on nearby traces?

I have seen some high speed connector TDR plots where traces are routed
through pin fields on back/midplanes. This is similar to passing near a via,
even worse I would think. I have also done TDR's on our own boards.

The effect seems to be very small (a few % of 50 ohms) and of short duration
in time, which of course corresponds to very high frequencies (10's of GHz).

One view.

Larry Miller

-----Original Message-----
From: Patel, Bhavesh [mailto:bpatel@cyras.com]
Sent: Wednesday, March 21, 2001 8:06 PM
To: SI_LIST (E-mail)
Subject: [SI-LIST] : How to model effect of vias on nearby traces?

Hi! I wanted to know how do I simulate the effect on the impedance..
reflection on a trace/signal when it is very close to a via. It does not go
thru the via but passes very close to it. Can I model this in HSPICE? And if
yes how/
Thanks in advance
Bhavesh

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