Hi Lenny,
If everything is defined as you described, I think that your expectation are
correct. If S-parameters are properly extracted and include at least
asymptotes to DC, you should see some correlation between voltages at A and
B, but only in the flat parts of a step response (that is also corresponds
to the DC solution). If not, something is wrong. In such cases I would
create a very simplified problem and investigate.
Cross-probing of transients or AC for a distributed electrically large
system may produce some strange useless results, that can be simply
explained by the delays within the system. For instance, AC response of
ideal t-line segment terminated with characteristic impedance and 1 V
travelling wave on one end will give 1 V at the other end, but if we
cross-probe (virtually) the opposite ends we will observe values from 0 to 2
V, depending on the electrical length.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
www.simberian.com
Simbeor THz - Accurate, Productive and Cost-Effective Electromagnetic Signal
Integrity Software to Design Predictable Interconnects!
Simbeor SDK - The Industry-First Signal Integrity Analysis Automation Kit!
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of L R (Redacted sender "lrayzman" for DMARC)
Sent: Thursday, May 27, 2021 10:58 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: AW: S-parameter reference shift
Thank you for your feedback.б= There may be couple of things I am not clear
on based on below explanation.б=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 upб= a loop A-B,
into let's call it "forward" and "return"б= sub-paths.б= By that reasoning I
would think then choose A side reference as reference for entire s-parameter
still should still be captured correctly?б=
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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-----Ursprц+ngliche 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
