[SI-LIST] S-parameter reference shift
- From: "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 27 May 2021 02:49:31 +0000
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
Hello SI-Listers,
I am describing a physical model with s-parameter, where I have a group of
ports, call them A-group, referencing a local plane. There is then another
group of ports, B-group, referencing another local plane. There are general
paths between A-group and B-group which is what I am interested to describe by
s-parameter. An important detail is that all B-group connections, including
returns, are ports which are referenced to B-plane as opposed to conventional
method where a port represents both forward and return paths. (This is done
intentionally for reasons that are not as relevant here.)Suppose the planes are
interconnected through a conductive path, with conductivity perhaps few orders
of magnitude higher than conductivity of local planes. Logically this is
similar to a split plane scenario except there is a weakly conductive path
connecting the planes, to ensures a predictable conductive path from A-plane
to B-plane at DC. The s-parameter is solved with an explicit conductive s olve
at 0Hz, presumably correctly. I then use this parameter in spice, where I am
careful to properly connect circuits to ports such that there is no improper
cross-plane interaction. What I mean by that is, if A-group ports are
referenced to same spice node as a reference for s-parameter, then connectivity
for B- ports referenced only with respect to each other. In this way my
intention is allow currents to flow through through immediate paths between
A-and B- group with (theoretically) no/negligible DC currents to flow through
path between planes. Now, I energize a set of A- ports with known voltage
source. This will energize the circuit on B- side. What I expect is voltages
on any B-side node to remain within levels of A- side (since B- circuitry is
drawing and not sourcing, and as measured both relative to B-side and A-side).
What I may often see on B- side is the entire group is shifted *outside* of the
range of the A-group voltage, typically below the reference level of A-group.
However, without looking ac ross A- to B- boundaries, each A- or B- side
locally is behaving as expected. Note that this entire discussion is at 0Hz as
computed by spice DC operating point for the complete circuit. I don't think
all this is some illegal use of s-parameters -- this is just simply calling out
a different location as port reference. I am wondering if any one has some
insight into what could be causing this. More importantly is there a method to
detect this condition? It seems satisfying passivity is not sufficient here.
Best regards,Lenny Rayzman
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