*From*: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx>*To*: vladimir_dmitriev-zdorov@xxxxxxxxxx, si-list@xxxxxxxxxxxxx*Date*: Thu, 30 Aug 2007 16:14:30 -0700

> > Vladimir, Thanks for your detailed explanations. I still have a question, since many topologyies can produce the same S-parameters, the real TDR of these equivalent circuits with different topologies would still be the same? TDR can see the characteristic impedance discontinuities inside the circuits in terms of delay/flight time. I thought the different topologies would give different TDR restults. ZG -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of Dmitriev-Zdorov, Vladimir > Sent: Thursday, August 30, 2007 3:03 PM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] TDR S-parameter and correlation > > > 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 > > > > ----------------------------------------------------------------------------------- > This email message is for the sole use of the intended recipient(s) and > may contain > confidential information. Any unauthorized review, use, disclosure or > distribution > is prohibited. If you are not the intended recipient, please contact the > sender by > reply email and destroy all copies of the original message. > > ----------------------------------------------------------------------------------- > ------------------------------------------------------------------ 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

**Follow-Ups**:**[SI-LIST] Re: FW: TDR S-parameter and correlation***From:*steve weir

**[SI-LIST] Re: FW: TDR S-parameter and correlation***From:*Loyer, Jeff

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