Hi, I think the voltage between point A and B (the reference node of port 1 and port 2) is not real and undefined at least for high frequency signal. Any attempt to measure V_AB looks like to be dictated by what is actually tied to node A and B. It could be arbitrary and correlated the geometry of external cable arrangement. _________ +_______| |___________+ | | Port 1 -_______| |___________- Port 2 |_______| A B In my opinion, what is real is voltage between + and - node of each port. JuYoung Ray Anderson wrote: > 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 > ------------------------------------------------------------------ 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