Dear Larry I do not agree with you that "If you cannot define a common reference for the N terminals, then you cannot convert the S parameters to Y or Z parameters." S parameter is defined by the voltage, current and impedance. Y and Z are also defined by the voltage, current. From S parameter to Y or Z parameter, matrix transfer is ok. Best Regards Zhangkun 2003.04.26 ----- Original Message ----- From: "Larry Smith" <Larry.Smith@xxxxxxx> To: <si-list@xxxxxxxxxxxxx>; <arpad.muranyi@xxxxxxxxx> Sent: Saturday, April 26, 2003 2:31 AM Subject: [SI-LIST] Re: N-port model limitations in simulators > > Arpad - Yes, we throw around the word "port" a lot but not everybody > understands the meaning of the word in the same way. > > In Pozar's book (page 216 of 1990 edition), he says > > "The term port was introduced by H.A.Wheeler in the 1950's to > replace the less descriptive and more cumbersome phrase > 'two-terminal pair'." > > He then cites some historical references. > > Ray Anderson and I have been kicking this discussion around a lot > lately. For the measurement and definition of S parameters, we believe > that a port has two terminals. V+ and v- (the forward and reverse > traveling waves) are measured between these two terminals. The current > into the port goes into one of the terminals and returns back out the > other terminal. > > The confusion comes when we convert S parameters to Y or Z parameters. > Brian Young has a nice diagram on page 154 of his book. He shows a > black box with N ports, each with a voltage and a current. The black > box has a single wire connected to a ground symbol, so the box has N+1 > terminals. I believe all voltages for this box are measured with > respect to the ground symbol terminal. The current on the N terminals > are defined as going into the box. No doubt the ground terminal > carries what ever current it has to in order for the total current into > the box to sum to zero (Kirchhoff). On page 155, he talks about the Z > matrix being the inverse of the Y matrix and he makes the comment, "If > a datum node has not been defined, then the matrix inverse does not > exist." > > I believe this is the source of the confusion. For S parameters, each > port has two terminals, leading to 2N terminals for N ports. For Y and > Z parameters, I believe there must be N+1 terminals for for N ports. > Pozar lists a bunch of conversions between S, Y, Z and ABCD parameters > on page 235. I believe these conversions require the use of matrix > inversion and Brian's comment about a "datum node" applies to all of > these conversions. If you cannot define a common reference for the N > terminals, then you cannot convert the S parameters to Y or Z > parameters. > > If you follow this line of reasoning far enough, I believe you find > that an electrical circuit with a single reference node has a unique S > parameter representation. But a set of S parameters (matrix of > magnitudes and phases for each port) does not have a unique circuit. > You can convert the S parameters to Y or Z parameters (for a black box > with a single reference node), but there are many circuits that could > be built that will have exactly the same port characteristics. > > There are incredibly important implications for this as we try to > convert S parameter data into a black box that is to be used in a > circuit simulator. > > regards, > Larry Smith > Sun Microsystems > > > From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx> > > To: <si-list@xxxxxxxxxxxxx> > > > > I would like someone to give us a definition of "port". > > > > The reason I am asking for this is because I think there > > is an important detail that makes things confusing in these > > responses. Is a port just one node, *assuming* a universal, > > global reference (as in SPICE node 0)? Or does a port > > consist of two nodes, neither one of which is GND (node 0) > > between which the measurements are done with respect to > > each other? The first one could also be called single=20 > > ended, and the second one differential. There may be far > > reaching implications depending on how we define "port". > > > > Arpad > > -----Original Message----- > > From: Marek Schmidt-Szalowski > > [mailto:marek.schmidt-szalowski@xxxxxxxxxxx] > > Sent: Friday, April 25, 2003 2:02 AM > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: N-port model limitations in simulators > > > > > > > > Dear All, > > > > 1. N-port scattering parameters do not say anything about > > voltages between ground terminals of different ports. In > > particular, they do not say whether you have a common ground > > terminal or not. Thus, in my eyes, S-parameters give only a > > *partial* description of a multiport device. =20 > > > > 2. Although both twoports and fourpoles have four terminals there > > is no general one-to-one relationship between twoports and > > fourpoles. In case of a twoport we define 2 voltages and 2 > > (balanced) currents. In case of a fourpole we define 4 > > voltages and 4 currents (only 3 voltages and 3 currents are > > independent). Thus, the fourpole formalism gives more > > information on the device than twoport S-parameters. In > > particular, it is possible to define a common terminal by > > zeroing one of the voltages. > > > > 3. Generally speaking, there is no global ground node in a > > distributed circuit. A good microwave circuit simulator > > defines a local ground node for each cluster of lumped > > components. Some simulators automatically connect the local > > ground nodes one with each other. Otherwise, you must connect > > them explicitly. You must do it cautiously or simulation > > results will be completely wrong. > > > > 4. There are cases when the partial description of fourpole > > offered by twoport S-parameters is not enough for a circuit > > simulation. Imagine a lumped twoport A (obviously having a > > well-defined ground node) to which you connect a distributed > > twoport B, e.g. long coaxial cable. In this case twoport B > > does not divide the circuit in two parts with local ground > > nodes. The current at both ports of the cable are in general > > no longer balanced. For instance the current though the inner > > connector can be much larger then in the outer one. However, > > S-parameters fail to describe this kind of behavior. > > > > Assume, that you have a lossless cable whose length is 1 > > lambda. If you replace it with a 10 times longer cable you > > will have still the same S-parameters but twoport A will see a > > completely different connection between its ports.=20 > > > > Or remove the outer connector and leave the inner one.=20 > > S-matrix is now {{1,0},{0,1}}, thus no transmission. However, > > if twoport A gives an alternative return-current path it will > > see some transmission.=20 > > > > 5. If you simulator implements only a common-ground-node > > S-parameter component but also a four-terminal transformer you > > may still simulate generic S-parameters (S-parameter with > > local ground nodes). In order to de-couple the ground nodes > > use an S-parameter component you have and connect a 1:1 > > transformer to one of its ports. > > > > 6. The above issues are mere consequences of a circuit theory.=20 > > The type of propagation mode (TEM, waveguide etc.) is here > > irrelevant. > > > > with kind regards, > > Marek > > > > > > M.Schmidt-Szalowski > > Philips Semiconductors > > > > > > ------------------------------------------------------------------ > > 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: =20 > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages=20 > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > =20 > > ------------------------------------------------------------------ > > 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 > > ------------------------------------------------------------------ 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