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

  • From: Ege Engin <engin@xxxxxxxxxx>
  • To: Raymond.Anderson@xxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Wed, 23 Apr 2003 11:13:19 +0200

Dear Ray,

S parameters characterize a spatially distributed circuit. As you
stated, a global reference node may not be defined for distributed
circuits (electric field is non-conservative, therefore voltage drop
along a conductor, say the ground line, is undefined). But, there may be
several local reference nodes, such as the 2 -(minus) nodes in your
first figure. In this case, the voltage between a node and its local
reference node is defined (e.g., between + and - nodes of port 1), but
the voltage between the two local reference nodes is undefined (e.g.,
between the 2 - nodes). 

If an S parameter matrix is implemented in a circuit simulator, it
actually divides the rest of the circuit (all the other linear and
non-linear elements) into groups, that are only coupled to each other by
means of this S parameter matrix (due to the fact that an S parameter
matrix represents a distributed circuit). Since the voltage drops
between the local reference nodes in various groups are undefined, they
can be connected to each other in an arbitrary manner. Maybe the
simplest way is to short circuit them to form a common reference node,
as it is implemented in many simulators. In this way, you can also
simulate several n-ports cascaded to model a signal propagation path
without loss of generality, but only the voltages between a node and its
associated local reference node would be meaningful.

As an exception, in some cases a network matrix (like the S parameter
matrix or the impedance matrix Z) can be used to characterize a system
whose reference nodes are close to each other, such that the voltage
drops between the reference nodes are defined as well. In this case, the
connections between the reference nodes cannot be done arbitrarily. A
well-known example is an n-port Z matrix representing the partial
inductances and resistances of an n-conductor interconnect. To implement
this n-port Z matrix in a circuit simulator, n reference nodes have to
be given explicitly (if all the reference nodes were connected to a
common node, then one side of the interconnect would be
short-circuited). If such a model does not exist in the circuit
simulator, a possible workaround would be to create an equivalent
2n-port chain matrix with some math, and use this chain matrix in a
model with a common reference node. 

Regards
Ege

Ray Anderson schrieb:
> 
> 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
> 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.
> 
> 
> 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):
>                         _________
>                 +_______|       |___________+
>                         |       |
>         Port 1  -_______|       |___________-   Port 2
>                         |_______|
> 
> 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
>                         |       |
>                         |_______|
>                             |
>                             |
>                           Common
> 
> 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
> 
> 2       Common nodes that are physically separated can't be modeled
>         as such (connectors for example).
> 
> 3       Can't be utilized to accurately model planes which are spatially
>         large.
> 
> 4       Can't be utilized for SSN simulations. (seems like return paths
>         are being ignored)
> 
> 5       and the list goes on and on ..........
> 
> 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.
> 
> 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
> 
>         2) Use a simulator that supports a correct model
>            (which one)
> 
>         3) Create 'black-box' models with an external tool that
>            provides multiple references and use these models
>            in spice or whatever.
> 
>         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.
> 
>         5) Come other solution.
> 
> 
> 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?
> 
> -Ray Anderson
> Sun Microsystems Inc.
> 
> 
> 
> 
> 
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