[SI-LIST] Re: Diff Z for asymmetric coupled striplines

Hello Jim,

By using your values in Polar, I get a differential impedance of about
89 ohms. I think, one important consideration is using the correct value
for your dielectric constant. How did you obtain that value of 4.3? Was
that provided by your pcb fabricator? Or did you take it out from a
material datasheet based on your resin/glass content? Sometimes the
value provided by your PCB fabricator and the original FR4 data may not
match. The pcb fabricators usually back calculate the value of
dielectric constant from a set of samples they manufacture and it might
not even be close to the value you expect and still get a 100 ohm
differential impedance. I never understood how that was possible. Maybe
they used a different material from what they claimed to have used. 

If you already have the board, the best way is to measure the
differential impedance and estimate the dielectric constant using a TDR.
Otherwise, you'll have to work closely with your pcb fabricator to
obtain the correct values. 

Thanks,         
Abhilash Rajagopal


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of otter30@xxxxxxxxxxxxx
Sent: Monday, October 20, 2008 9:16 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Diff Z for asymmetric coupled striplines

Hello,
I need to determine the differential impedance of pair of coupled
stripline traces. They are edge coupled and are asymmetric in the sense
that the distances to the 2 reference planes are different - both traces
are
4 mils from the upper plane and 7mils from the lower.

Our pcb fabricator told us that this configuration would have a diff Z
of 100 ohms, but I have gotten a value from the cross section calculator
in Cadence Allegro of 79 ohms. I'd like to find another method to settle
this, as our board does have SI problems.

The diff Z formula for this configuration isn't in any of the well known
SI or TL texts, although the symmetric case (H1 = H2) is. Can anyone
either:
 1) Point me toward the correct equation
 2) Suggest a free field solver that would let me calculate this value?

I've downloaded the student version of Ansoft Maxwell, and am trying it
out. If it can extract the mutual capacitance and inductance, is there a
general method of getting the diff Z from these?

Any help is greatly appreciated,
Jim


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