[SI-LIST] hapice simulation of Z_tdr
- From: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx>
- To: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 31 Aug 2007 15:50:40 -0700
Hi all,
Thanks for your clear explanations!
I have another interesting question about my hspice simulation deck for
simulating the TDR Z (differential) profile (vs time):
The Z profile in the sim deck is defined as (V(t)_p - V(t)_n)/I(t). V(t)_p
is probed at the point right after source impedance (50ohms) for positive
lane, and V(t)_n is probed at the point right after source impedance
(50ohms) for negative lane. The source is a step voltage source connecting p
and n lanes.
It seems, sometimes my simulated Z profile can be very close to
measurements, but sometimes it is not, especially for DUT with many large
discontinuities inside.
I am wondering if (V(t)_p - V(t)_n)/I(t) is sufficient and correct to
represent the real TDR Z profile?
Look forward to your explanations. Thanks.
ZG
2007/8/31, Loyer, Jeff <jeff.loyer@xxxxxxxxx>:
>
> 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.
>
> 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
> differentcircuits 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
> *<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<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<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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