[SI-LIST] Re: FW: TDR S-parameter and correlation
- From: steve weir <weirsi@xxxxxxxxxx>
- To: zhenggang.cheng@xxxxxxxxx
- Date: Thu, 30 Aug 2007 17:04:20 -0700
Zhenggang, no a TDR/TDT does not specifically reveal topology, it
reveals behavior as seen by a signal traversing a channel. This isn't a
problem per-se as we are interesting in designing channels with specific
behaviors. A TDR/TDT tells us how we are doing against our goals and
where along the path we have any issues.
Steve.
ZHENGGANG CHENG wrote:
>> 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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