[SI-LIST] Re: Odd mode impedance


Well, you could get a reasonable estimate with the information you've
provided.  For higher levels of accuracy, you'd need a little more info.
Ray assumed a delectric constant 4.6 in his solution, but if you're dealing
with 4.2-4.3...that could create a couple ohm difference in differential
impedance.  It really depends on how accurate you want to be.  Ray's
solution is (probably easily) within 5%.  You could include signal layer
thickness, ground plane thickness, effect of soldermask, etc if you REALLY
wanted to understand what those effects do to your impedance.  It depends on
if you're looking for an approximation or a cause-and-effect type of
analysis.

As a final note, Ray reports Apsim RLGC's numbers for differential and
common-mode impedance.  If what you're really interested in is odd mode
impedance, it's simply half the differential impedance since you meet the
assumption requirements for that case (two identical conductors symmetric
wrt ground have an identical effect one another).

The tool I use most commonly is Apsim RLGC, but there are others.

Hope you find this useful,

robert


Robert M Washburn
Signal Integrity Group
Dell Computer Corporation
512.723.6025
robert_washburn@xxxxxxxx

-----Original Message-----
From: Mir Faiz [mailto:mfaiz@xxxxxxxxxxxxxxxx]
Sent: Tuesday, June 25, 2002 12:14 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Odd mode impedance



Hi,
I have a pair of 11mils wide microstrip lines with 8.7 mils separation and 5

mils FR4 substrte thickness. Can anybody help me calculate or suggest me an 
appropriate tool to calculate the odd mode impedance?
Regards,
Mir


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