[SI-LIST] Re: FW: TDR S-parameter and correlation
- 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
>
>
>
>
>
>
>
>
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