[SI-LIST] Re: N-port model limitations in simulators
- From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 22 Apr 2003 14:41:34 -0700
Ray,
I think it all depends on how you (or your tool) count your beans
(I mean ports).
To me, the two-port you drew with a common GND (second figure) would
be like a single wire with two ends, and your guess is as good as mine
for where the return path is. However, I can imagine let's say, a two
wire twisted pair (or even a one wire coax with shield) to be modeled
with "two ports" on the left and two other ports on the right side,
plus one common GND for all. In this case the common GND does not
prevent you from describing the return path. (Of course this would
be 2*n + 1 nodes, using your equations).
Arpad Muranyi
Intel Corporation
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-----Original Message-----
From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx]
Sent: Tuesday, April 22, 2003 2:07 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] N-port model limitations in simulators
This message deals with what I am perceiving as some
significant limitations in the n-port model implementations
in current day SI simulators. For those who have the stomach=20
to wade through my prose I pose a few questions to base
further discussion on at the end of the message.
I think many SI engineers will agree that the use of
s-parameters to characterize certain circuit elements
has become an important tool in today's high-speed
simulation environment.
=09
S-parameters can fully characterize an arbitrary n-port.
In this message I'll restrict the discussion to 2-ports
to simplify things.
A 2-port representation of a network has separate
reference nodes for each port. Hence a 2-port has
4 nodes or terminals associated with it.
(in general a n-port has 2*n terminals): =09
_________
+_______| |___________+
| |
Port 1 -_______| |___________- Port 2
|_______| =20
Many popular simulation engines now provide native support
for n-ports characterized by s-parameters. However it seems
that many of the models utilized by these simulators only
support n-ports with a common reference node (n+1 nodes):
_________
| |
Port 1 --------| |------- Port 2
| |
|_______|
| =20
|
Common
=20
Having a common reference node limits the utility of these
n-port models for a variety of purposes:
1 Can't have a DC offset between the input and output ports
=09
2 Common nodes that are physically separated can't be modeled
as such (connectors for example).
=09
3 Can't be utilized to accurately model planes which are spatially
large.
=09
4 Can't be utilized for SSN simulations. (seems like return paths
are being ignored)=09
=09
5 and the list goes on and on .......... =09
=09
It seems that the restriction of a common reference node harkens
back to the mythical global ground concept (node 0) of spice.
All voltage measurements are taken in reference to some reference node.
In DC and low-frequency simulations you can get away with the concept
of a global ground in a lot of cases, however for high-speed simulation
one might as well forget the global reference concept as it certainly
isn't useful in cases where delays in the picosecond range can be
significant. =09
=09
In the case where several n-ports are cascaded to model a signal
propagation path (say from a driver, through a package, through a
PCB trace, through a connector, through more PCB trace, through
another package and ultimately to a receiver) the assumption the
the reference node at the driver package is the same as the reference
node at the receiver package is just wishful thinking. It isn't so!
Some simulators have a proper n-port model (in the case of ADS there
is a proper 2-port model [S2pmdim] but all the other n-ports s1p to
s99p have a single reference. There may be other simulators that
handle the problem correctly but I'm not sure which ones.
So the question is: How do people handle this issue?
1) Ignore it and hope it goes away
=09
2) Use a simulator that supports a correct model
(which one)
=20
3) Create 'black-box' models with an external tool that
provides multiple references and use these models
in spice or whatever.
=20
4) Combine n-ports into a 'big' n-port via T or ABCD parameters
off-line and then use the composite n-port in a simulator
that only supports a single reference in the n-port model.
=20
5) Come other solution.
=09
=09
I'm posing these questions not in search of a simple answer, but as
a springboard for discussion. Is the problem real? Why the restriction
to a single reference? Is the restriction based on programming
considerations or actual mathematical restrictions? Workarounds?=09
-Ray Anderson
Sun Microsystems Inc.
=09
=09
=09
=09
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