*From*: "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>*To*: <si-list@xxxxxxxxxxxxx>*Date*: Thu, 30 Aug 2007 15:03:07 -0700

Hi ZHENGGANG, As I understand, the issue is about "general type" model, not just a segment of T-line. The answer to your first question is YES provided that (a) DUT's S-parameters were measured correctly, (b) the equivalent circuit is correctly built from S-parameters and (c) the way you assign ports in your equivalent circuits is consistent with how the measurements was done. Topology cannot be uniquely derived from S (or Y/Z/G/H... etc.) parameters unless you have it predefined, like in case of T-line. There could be many circuits with quite different topology producing the same S/Y/Z... parameters. Yes, S-parameters only characterize the model from outside ports, but this is exactly what other pieces of your design see from this model: this is sufficient to use the model in many simulation procedures, unless you are interested in voltages/currents inside the model itself. The problems above named (b) and (c) are sometimes not well understood. First of all, any measured S-parameters (think of touchstone file) contain the data in a limited frequency range, while any equivalent circuit is 'defined' from DC to infinite frequency. Hence, they cannot be equivalent if S-parameters do not cover sufficiently wide range of frequencies, presumably from the lowest frequency where they start changing up to the highest frequency where they stop changing and approach to constant level. In all other cases, be prepared that the circuit does not accurately capture the model behavior at very low and high frequencies. Building equivalent circuit requires rational polynomial fitting. This may be done by different tools with different accuracy. In many cases, passivity enforcement is required on the post fit stage to prevent unstable model behavior, especially if the upper frequency in touchstone data is not sufficient. "Passivation" adds some inaccuracy to the fitted model. Typically, representing the poles/residues with circuit elements does not bring much error. These are main sources of discrepancy we may have between given S-parameters and equivalent circuit. Plus, time domain simulation of the equivalent circuit, performed with finite resolution, adds LTE (local truncation error). The problem (c) may sometimes be insidious. Imagine the S-parameters of DUT were measured for two ports, without a 'common' ground. Essentially, these measurements only characterize how the wave may propagate between these two ports. Then, an equivalent circuit was built with four external nodes making two ports. Now, someone use this model not only by apply input to the first port and measure output at the second, but also making arbitrary connections between any of external nodes of this model. The result: behavior inconsistent with the original device. (Same also possible with common ground). That is, we need to make sure the model is used the same way the measurements were done for S-parameters. Vladimir Msg: #15 in digest Date: Thu, 30 Aug 2007 12:30:19 -0700 From: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx> Subject: [SI-LIST] TDR S-parameter and correlation Hi, My question is: If we TDR (assuming the TDR method is 100% correct) the equivalent circuit converted using S-parameter, will the result is exactly the same as real TDR of the same DUT? (assume the converting error and bandwidth are not issues) Assume this DUT has many large discontinuities inside. To me, the real TDR can distinguish all the discontinuities inside a DUT; however, the S-parameter is only the characterization at the ports rather than inside. Will two equivalent circuits give the same S-parameter but have two different topologies? Look froward to your replies. Thanks, ZG ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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

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