Thank you for your feedback. There may be couple of things I am not clear on
based on below explanation. While by definition s-parameter does not
explicitly internalize any conductive connections between ports, it is my
understanding that it does describe all and any conductive paths that exist in
my physical problem. If this is correct, then in my scenario all ports,
including B, not isolated of each other but rather do have non-ambigous
predictable path (it would be ambiguous if no predictable path exists between A
and B planes at DC). Here I am simply breaking up a loop A-B, into let's call
it "forward" and "return" sub-paths. By that reasoning I would think then
choose A side reference as reference for entire s-parameter still should still
be captured correctly?Â
Lenny
On Wednesday, May 26, 2021, 10:41:21 PM PDT, Havermann Gert
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
Hi Lenny,
Vladimir is correct. I think the main problem in your case is how you defined
the reference. S-Parameters only allow one global reference. If you are
interested in seeing differences between planes, then you should define all
planes as nodes and add a separate reference (a plane far away from other ports
and metals) for the S-parameters.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> Im Auftrag von
Dmitriev-Zdorov, Vladimir
Gesendet: Donnerstag, 27. Mai 2021 04:50
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] S-parameter reference shift
Hi Lenny,
Please be aware that in general, replacing a part of your design with
S-parameters may not be an equivalent substitution. What S-parameters (and also
Y, Z) can capture is only relations between port variables (port waves,
voltages, currents), but not the voltages between the terminals of different
ports. Think of the S-parameter block as a model which only defines all
transfer functions between variables of ports (m, n), where m, n are all port
index pairs. Numerical implementation of such transfer functions inside any
simulator is equivalent to ideal transformers, with no conductive connection
between the coils. What it means is that if in the original design you had e.g.
1V DC shift between nodes that now are (+) terminals of the two different
S-parameter ports, you may get a very different "voltage" if it is not a port
voltage.
The two things to remember about S-parameters:
1. Port constraint: the current that enters positive terminal of the port and
the current that leaves negative terminal of this port are forced to be the
same. If such restriction didn't exist in the original design, you may get a
different solution.
2. No conductive connection exists between different ports inside S-parameter
model (it should be defined outside). If such connection existed in your
design, the result could be different.
If two above restrictions make sense for your design, you should be getting
correct port variables, but don't try to compare voltages between "A" and "B"
ports.
If I remember correctly, spice instance of the S-parameter model has two
formats. In one, you only need to define "positive" terminals of each port, by
associating them with node names. In this case, Hspice will automatically
connect all reference terminals of each port to a "global reference". After
connecting each port's reference terminal to the global reference, you can
measure voltages between distant terminals, however the result might be
different from your expectation. Indeed, did you have the voltage at nodes
which now become port reference terminals be all zero? If not, that's an
explanation of why you have different results. There is a more general syntax
for S-parameter ports' terminals assignments, in which you need to explicitly
define the nodes for both positive and negative (reference) terminal of each
port. In this case, you have more flexibility, but still no guarantee that you
will get the same voltage across different ports.
Vladimir
From: "L R" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "lrayzman"
for
Subject: [SI-LIST] S-parameter reference shift
