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. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu