>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