I think Ege made some very good points. The use of n-port S-parameters (or Z or Y) with a commong reference is not as restrictive as may seem at first glance. As Ege pointed out, the original ports should be defined in such a way that the two terminals of each port are close to each other. This makes it necessary to have separate negative terminals for the ports as we are talking of distributed circuits separated in space and often by several wavelengths. However, when making a circuit model of these S-parameters it is usually okay to have a common reference. When using a circuit model of n-port S-parameters with a common reference, the other elements of the circuit (such as drivers and receivers) must be connected carefully. If one of the following two rules is followed, all the original information is preserved. 1. Assume that the n-port model can be called by a subcircuit call such as X1 port1 port2 port3...........portn ref We ensure that no circuit elements connected are connected across two ports such as portx and porty. It there are 3-terminal elements such as transistors, it is not a problem. For convenience they may be viewed in terms of their equivalent circuits containing only 2-terminal elements. We also make sure that we measure all output voltages between portx and ref and not between two ports such as portx and porty. 2. If rule 1 is too restrictive, we allow connections across two ports portx and porty only if portx and porty have a common negative terminal in the original n-port representation. If we follow these rules, we can have different negative terminals for each port. One could even have Gnd as a negative terminal for one port and VCC as negative terminal for another. When a circuit made with one common reference is hooked up in a SPICE file, even the DC values would come out right. Best Regards, Raj Raghuram Sigrity, Inc. "Achieve what others can't" raghu@xxxxxxxxxxx http://www.sigrity.com 4675 Stevens Creek Blvd. , Ste 130 Santa Clara, CA-95051 PH: 408-260-9344 x116 CELL: 408-390-7614 FAX: 408-260-9342 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Joel R. Phillips Sent: Friday, April 25, 2003 7:24 PM To: si-list@xxxxxxxxxxxxx; Larry.Smith@xxxxxxx Subject: [SI-LIST] Re: N-port model limitations in simulators >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 ------------------------------------------------------------------ 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