[SI-LIST] Re: FW: TDR S-parameter and correlation
- From: "Chris Cheng" <Chris.Cheng@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 31 Aug 2007 13:55:21 -0700
This is a good point on the phase matching issue.
I am always curious about the direct FFT approach like Iconnect vs. GPOF =
approach.
It seems the later may have a better potential in matching the phase =
part.
Besides, my standard approach is TDR->Iconnect->rational fit->SPICE. =
With GPOF, I can potentially save one step if passivity can be enforce.
This is not to say Iconnect is not good. It is the best $8K I've spent =
and I strongly encourage everyone who is interested to try it out.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff
Sent: Friday, August 31, 2007 11:20 AM
To: zhenggang.cheng@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: FW: TDR S-parameter and correlation
If the S-parameters are identical, then the TDR/TDT results will
likewise be identical (assuming proper risetime to match the frequency
limit of the S-parameters).
But you have to remember that the phase of S-parameters are just as
critical as the magnitude (in evaluating the circuit). Often we only
focus on the magnitude, and forget the phase. =20
Another caveat is that we tend to concentrate on S21, which is the
frequency equivalent of TDT. This can be very similar for different
networks, even though S11 (and TDR) are dramatically different.
I believe these 2 factors add up to make it appear that very different
circuits have the same S-parameters: you might only be looking at S21
magnitude, when you should be looking at the magnitude and phase of S11,
and phase of S21 also.
You might look at some work Cherry Wakayama did, comparing S-parameters
derived from VNA vs. TDR/TDT measurements (see link below). There were
several very different structures, and the S-parameters varied
accordingly, especially when you look at phase and S11, not just the
magnitude of S21.
http://www.tek.com/products/oscilloscopes/sampling/interconnect_analysis
/customer_papers/vna_tdr_correlation.pdf
Unfortunately, we didn't include the original TDR waveforms in the
paper, but they contained all kinds of various "bumps" in them before
being converted to S-parameters.
P.S. - I'd also like to thank Vladimir for his great explanation.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of ZHENGGANG CHENG
Sent: Thursday, August 30, 2007 4:15 PM
To: vladimir_dmitriev-zdorov@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: FW: TDR S-parameter and correlation
>
> 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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