[SI-LIST] Re: N-port model limitations in simulators

• From: "Joel R. Phillips" <jrp@xxxxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx, Larry.Smith@xxxxxxx
• Date: Fri, 25 Apr 2003 19:24:16 -0700 (PDT)

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>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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