[SI-LIST] Re: N-port model limitations in simulators
- From: "Raj Raghuram" <raghu@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 28 Apr 2003 09:50:59 -0700
I think Ege made some very good points. The use of n-port S-parameters (or Z
or Y) with a commong reference is not as restrictive as may seem at first
glance. As Ege pointed out, the original ports should be defined in such a
way that the two terminals of each port are close to each other. This makes
it necessary to have separate negative terminals for the ports as we are
talking of distributed circuits separated in space and often by several
wavelengths. However, when making a circuit model of these S-parameters it
is usually okay to have a common reference.
When using a circuit model of n-port S-parameters with a common reference,
the other elements of the circuit (such as drivers and receivers) must be
connected carefully. If one of the following two rules is followed, all the
original information is preserved.
1. Assume that the n-port model can be called by a subcircuit call such as
X1 port1 port2 port3...........portn ref
We ensure that no circuit elements connected are connected across two ports
such as portx and porty. It there are 3-terminal elements such as
transistors, it is not a problem. For convenience they may be viewed in
terms of their equivalent circuits containing only 2-terminal elements.
We also make sure that we measure all output voltages between portx and ref
and not between two ports such as portx and porty.
2. If rule 1 is too restrictive, we allow connections across two ports portx
and porty only if portx and porty have a common negative terminal in the
original n-port representation.
If we follow these rules, we can have different negative terminals for each
port. One could even have Gnd as a negative terminal for one port and VCC
as negative terminal for another. When a circuit made with one common
reference is hooked up in a SPICE file, even the DC values would come out
right.
Best Regards,
Raj Raghuram
Sigrity, Inc.
"Achieve what others can't"
raghu@xxxxxxxxxxx
http://www.sigrity.com
4675 Stevens Creek Blvd. , Ste 130
Santa Clara, CA-95051
PH: 408-260-9344 x116
CELL: 408-390-7614
FAX: 408-260-9342
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Joel R. Phillips
Sent: Friday, April 25, 2003 7:24 PM
To: si-list@xxxxxxxxxxxxx; Larry.Smith@xxxxxxx
Subject: [SI-LIST] Re: N-port model limitations in simulators
>Date: Fri, 25 Apr 2003 18:18:11 -0700 (PDT)
>From: Larry Smith <Larry.Smith@xxxxxxx>
>However, For the Y and Z parameters, I believe we need a common
>reference point in order for voltage to be defined. If there is no
>common reference point, then what do we measure the voltage on port 1
>with respect to? If there is a reference point for each terminal, we
>don't get unique voltages. If ports 2, 3, ... have a different
>reference levels than port 1, I don't think the equations work. Sure,
>you can do the math and convert S to Z or Y matrices, but how do you
>interpret the voltages if there is not a common reference node. How do
>you hook it up in a circuit simulator?
Larry,
I think you are mixing up two separate processes: the process of trying to
model a circuit component, or sub-circuit, and the process of trying to
solve a set of circuit equations, which are assembled from many separate
sub-circuits, all (presumably) connected together.
When the circuit simulator goes to solve its equations, it must select a
reference node in order to obtain a unique solution. It is not actually
necessary to specify this node in advance -- in principle, the simulator
can pick *any node whatsoever*. Circuit designers usually have some idea
which node they want to be the global voltage reference, and most
simulators by default pick that node for their reference, but it's actually
an arbitrary choice as far as the simulator is concerned (or the real
world, for that matter). But at this point, and only at this point, you
must make your choice.
When constructing models, on the other hand, it is only necessary to know
how N port currents relate to N relative voltage difference on the nodes of
the device. The simulator knows (or should know) how to hook such models
up in a consistent manner; once it has all the models and sources, it can
perform the full circuit solution. At that point the simulator will set
the (hopefully unique) absolute values of the voltages on the port nodes,
relative to the single, global reference node. You cannot expect to get
absolute voltages before that point because you have not fully specified
the problem, the behavior of the subcircuit acting in its environment, you
have only specified the behavior of the subcircuit.
For that matter, until that point, you can't even know that there *are*
unique voltages. If parts of the circuit are completely disconnected,
there is no unique solution. If what is inside the black-box is N
disconnected resistors, and you connect current sources in parallel outside
the box, the solution is highly non-unique. You can pick N arbitrary
voltage offsets, one for each port, and still have valid solutions to the
KCL/KVL equations. Your black-box model must admit this possibility if it
is to faithfully represent the behavior of the original circuit.
Regards,
***********************************************************************
Joel Phillips Cadence Berkeley Laboratories
jrp@xxxxxxxxxxx 2655 Seeley Rd, MS 1A1
Tel: (408) 944-7983 San Jose, CA. 95134
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