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

Jim,
You failed to mention a couple of other critical parameters.
Dielectric material (and weave) of upper and lower dielectrics Copper weight
Spacing of pairs

With a generic starting point of 4 mils, and 7/4 heights, one can achieve
100 ohms but at a specific spacing and dielectric.

You might try the Polar calculator as well.  MMTL gives good results but
you'll need to know the above info to get a practical result with any tool.

Regards,
Bill

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Kirby Goulet
Sent: Monday, October 20, 2008 1:12 PM
To: otter30@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff Z for asymmetric coupled striplines

Another open source free solver is mdtlc:  http://mdtlc.sf.net

It has a gui and also includes factors such as etch taper of different Er
values for different layers.  These factors may account for the differences
you are seeing.  Its based on the 'atlc' solver which has been available for
some time.



 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Grasso, Charles
Sent: October-20-08 12:36 PM
To: otter30@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff Z for asymmetric coupled striplines

Try the Differential calculator from UltraCad

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of otter30@xxxxxxxxxxxxx
Sent: Monday, October 20, 2008 10:16 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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