[SI-LIST] Re: Diff Z for asymmetric coupled striplines
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: otter30@xxxxxxxxxxxxx
- Date: Mon, 20 Oct 2008 09:58:00 -0700
The only public-domain 2D field solver (as opposed to demo versions of
commercial tools) I know of is TNT/MMTL:
http://mmtl.sourceforge.net/
It's not the most comprehensive tool (e.g. can't calculate AC loss, and
assumes frequency independent parameters), but it's pretty easy to use and
can do you asymmetric stripline case and much more. If you have only 2
conductors (other than the ground planes) it will give you differential
impedance directly. It has a graphical frontend so design entry is
straightforward. I found it to be quite accurate, usually within about 1%
compared to other tools as far as impedance is regarded. There are a few
minor bugs that are easy to work around:
- it does not know that far-end crosstalk (FEXT) can't go beyond -6dB (=
50%). I.e. if you increase the line length, the reported FEXT grows
linearly without bounds. Easy to solve, simply truncate any result larger
than 50% to just 50%.
- ground wires are defined by assigning them a name starting with "gr"
(not "g" as the manual indicates).
- never use negative y-offsets for conductors. It confuses the solver
completely even though the graphical display will show the conductors
where they are supposed to be. Instead, drag and drop them with the mouse
to the correct layer in the graphical display.
- never ever use a name with a space in it. It will make the file
uneditable and the solver won't run the file.
Not bugs, but easy to get wrong:
- don't forget to choose the right unit. Default is meters (upper right
corner of the screen). Not important for impedance, but very important for
crosstalk.
- You always need to define at least one ground plane. Thickness is not
important since the solver assumes surface current anyway.
- Don't use too many segments. It will make calculation slow and at some
limit I found it can lead to numerical instability (i.e. results become
inaccurate). Typically between 10 and 50 is a good choice. Always try to
change the number (e.g. double it), re-run the simulation, and make sure
the change in the calculated result is negligible. (That's a good approach
for ANY field solver!)
Hope that helps
Wolfgang
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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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