[SI-LIST] Re: why do I need mixed mode S parameters?
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- Date: Fri, 24 Sep 2004 00:06:49 -0700
Dear Jeff,
I *did* invest a few ergs before asking about the reference. There is
at least one other Pozar book from which you could have been quoting;
I looked at both of them a few weeks ago in a library and was still
debating which one to purchase. My real question was whether you meant
the one suggested by Jeff from Agilent or "Microwave and Wireless Design
of Wireless Systems" published a few years later. (I'm willing to believe
that it wasn't Pozar's book on antennas.)
The formula quoted immediately below is the one I recognize. My apologies
for misremembering what other two factors entered into the equation. Now
if your two assertions of equality are true, then that equation would
work out the simplified result that you presented earlier. It has been my
experience, however, that as soon as one throws even a connector into
to system, things become so unbalanced (yes, this is non-ideal) that one
must use all four terms. I'm sorry if I didn't recognize the potential
simplification under ideal conditions.
Regards,
Paul
___________________
Loyer, Jeff wrote:
>The formula that I know of is:
>SDD21 = 1/2 (S21 - S41 - S23 + S43)
>
>For the reciprocal, symmetric system, S21 = S43, and S41 = S23. My
>algebra says that, for this system, SDD21 = S21 - S41. I didn't check
>your Maxim reference; please double-check it and let me know if it still
>disagrees with the formula.
>
>Also, a quick Google search of "Pozar book" would've found the
>information about that book - my thanks to Jeff of Agilent for providing
>that info.
>
>Not to beat you up, but please invest a few ergs before responding
>(especially to the entire list) - I invest a bit of time in my responses
>(to keep from asking the obvious, mis-stating information, and/or
>embarrassing my employer), and I appreciate others doing the same.
>
>Aside from that, I welcome folks letting me know if I've goofed and/or
>asking for clarification.
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: Paul Levin [mailto:levinpa@xxxxxxxxxxxxx]
>Sent: Thursday, September 23, 2004 1:03 PM
>To: Loyer, Jeff
>Cc: vince_cavanna@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: why do I need mixed mode S parameters?
>
>Dear Jeff,
>
>Even after correcting S41 to S43, your formula, highlighted below,
>for SDD11 isn't quite correct. If you look at Maxim Application Note
>HFAN-5.1.0 and follow their equations, I think you will find that
>S11 and S22 also figure into the mix.
>
>Also, what book by Pozar are you referring to? Thanks.
>
>Regards,
>
>Paul
>____________________
>
>Loyer, Jeff wrote:
>
>
>
>>My take, some of which was articulated in others' responses.
>>
>>I have been forced to become familiar with mixed-mode S-parameters
>>because they indicate what I need to know about the behavior of a
>>differential bus, which is what many high-speed busses are now. The
>>single-ended S-parameters for a PCI Express Transmission Line (a
>>
>>
>coupled
>
>
>>differential pair), for instance, won't give you much indication of the
>>losses vs. frequency for that T-line. In fact, they may give you a
>>
>>
>very
>
>
>>erroneous representation. You need mixed-mode S-parameters to make
>>sense of your return and insertion losses for differential busses. =20
>>
>>For example, for the following circuit:
>>p1 ------- p2
>>p3 ------- p4
>>
>>p1 & p3 represent the 2 input halves of a differential pair (coupled),
>>routed as microstrip, p2 & p4 the output.
>>Take single-ended measurements of 1/2 of this coupled, microstrip
>>differential pair (S11 & S21, with ports 3 & 4 terminated to 50ohms).
>>If the trace is long enough and your VNA goes to a high enough
>>frequency, you'll find dramatic resonances (S21 drops significantly at
>>certain frequencies). Much of your energy appears to be "lost" at key
>>frequencies. What you've inadvertently created is a "coupled line
>>coupler", described in detail in Pozar's book. If you're unfamiliar
>>with mixed-mode parameters, you might conclude that a terrible thing is
>>happening at those resonant frequencies (and you will be in excellent
>>company, in my opinion). However, if you then measure S41, and from
>>that measurement calculate SDD21 (for reciprocal, symmetric systems,
>>
>>
>mag
>
>
>>
>>
>>
>>
>
>
>
>><>SDD21 is mag(S21 - S41)), you'll find that, for the differential
>>
>>
>case,
>
>
>>there is no resonance. P.S. - nothing too exotic here, this can be
>>
>>
>
>
>
>
>><>duplicated in Hspice. Single-ended S-parameters gave an erroneous
>>indication of a resonance that doesn't occur when the system is
>>
>>
>excited
>
>
>>differentially; mixed-mode S-parameters were needed to judge the
>>
>>
>actual
>
>
>>quality of the system.
>>
>>Similar things might occur if you only measure single-ended
>>
>>
>S-parameters
>
>
>>for a differential pair going over a slot in a reference plane -
>>single-ended S-paramaters show horrible return loss (reflection),
>>
>>
>while
>
>
>>mixed-mode S-parameters indicate little reflection.
>>
>>As another example, take the circuit below:
>>
>>SE_p1 ------ SE_p2
>>Diff_p1 Diff_p2
>>SE_p3 ------ SE_p4
>>
>>
>>SE_p5 ------ SE_p6
>>Diff_p3 Diff_p4
>>SE_p7 ------ SE_p8
>>
>>Here, SE_p1 and SE_p3 represent the 2 input halves of one differential
>>pair (Diff_p1), while SE_p5 and SE_p7 represent the inputs of another
>>differential pair. If these are signals going through a connector,
>>you're probably most interested in SDD11 (differential return loss),
>>SDD21 (differential insertion loss), SDD31 (differential NEXT), and
>>SDD41 (differential FEXT). S21, S63, S33, etc. won't tell you much
>>about the behavior of the differential signals going through the
>>connector. Also, you may not be interested in things like SCD31
>>(near-end crosstalk that ends up as common mode).
>>
>>Again, mixed mode S-params are necessary to give you info. about what
>>you care about - single-ended S-params won't.
>>
>>To my knowledge, no VNA equipment exists to measure mixed-mode
>>
>>
>S-params
>
>
>>directly (for the GHz frequencies I care about). You must measure the
>>single-ended S-params, and mathematically derive the mixed mode
>>
>>
>S-params
>
>
>>from those.
>>
>>A very good explanation of mixed-mode S-params is the "RF Balanced
>>Device Characterization" webcast by Greg Amorese and David Ballo of
>>Agilent.
>>
>>I think many future models will have to be 12-port representations (2
>>differential aggressors, 1 differential victim), possibly as 12-port
>>S-params (single-ended). Results from simulations that use those
>>
>>
>models
>
>
>>may then be converted to mixed-mode S-params to understand those
>>results.
>>
>>Hope this helps without muddying the waters...
>>
>>Jeff Loyer
>>
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx
>>
>>
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
>
>
>>On Behalf Of vince_cavanna@xxxxxxxxxxx
>>Sent: Wednesday, September 15, 2004 1:59 PM
>>To: si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] why do I need mixed mode S parameters?
>>
>>I have some philosophical questions about mixed mode S parameters that
>>
>>
>I
>
>
>>=3D
>>have been struggling to understand as I re-enter the field of signal
>>
>>
>=3D
>
>
>>integrity and attempt to catch-up on some of the new =3D
>>measurement/analysis techniques. I would appreciate any insight you
>>
>>
>can
>
>
>>=3D
>>offer.
>>
>>I understand mixed mode S parameters and can compute them from
>>
>>
>standard
>
>
>>=3D
>>(single-ended) S parameters or from a model - or the other way around.
>>=3D
>>I can appreciate their usefulness in understanding how an n-port, that
>>
>>
>=
>
>
>>=3D
>>may have been designed to operate mainly under differential stimulus,
>>
>>
>=
>
>
>>=3D
>>responds to (reflects and scatters the incident power) differential
>>
>>
>and
>
>
>>=3D
>>common-mode stimulus.
>>
>>What I am trying to understand is why I would ever want to use mixed
>>
>>
>=3D
>
>
>>mode S parameters in a time-domain or frequency domain simulation, and
>>
>>
>=
>
>
>>=3D
>>how to use them. I am also interested to learn what simulators support
>>
>>
>=
>
>
>>=3D
>>mixed mode S parameters directly, as using them in a simulator such as
>>
>>
>=
>
>
>>=3D
>>Hspice seems cumbersome. My approach today is to simply use standard S
>>
>>
>=
>
>
>>=3D
>>parameters directly.
>>
>>The "why" I really don't understand at all. With regards to the "how",
>>
>>
>I
>
>
>>=3D
>>know of one approach but it is cumbersome and does not seem
>>
>>
>worthwhile.
>
>
>>=3D
>>I would be interested to know if there are circuit simulators that =3D
>>handle mixed mode S parameters directly but most important I need to
>>
>>
>=3D
>
>
>>understand why I need them.
>>
>>One way to use mixed mode S parameters, that has been suggested on
>>
>>
>this
>
>
>>=3D
>>mailing list, is to use the S element in Hspice, but represented with
>>
>>
>=
>
>
>>=3D
>>the mixed mode S parameters instead of the standard mode S parameters,
>>
>>
>=
>
>
>>=3D
>>and recognizing that the ports are conceptual (differential and common
>>
>>
>=
>
>
>>=3D
>>mode) as explained in [ref1]. In order to interface the conceptual =3D
>>n-port to my circuit (which expects real ports) I then have to wrap
>>
>>
>the
>
>
>>=3D
>>device with a circuit that converts the actual port waves of my
>>
>>
>circuit
>
>
>>=3D
>>into the differential and common mode waves that need to be applied to
>>
>>
>=
>
>
>>=3D
>>the conceptual n-port. This approach should work but seems cumbersome
>>
>>
>=
>
>
>>=3D
>>and, more important to me, I don't understand what I gain from
>>
>>
>it.=3D20
>
>
>>The approach I described seems like a round-about way to attempt to
>>
>>
>use
>
>
>>=3D
>>the mixed mode S parameters directly when they can easily be
>>
>>
>converted,
>
>
>>=3D
>>with no loss of information, into standard mode S parameters and used
>>
>>
>=
>
>
>>=3D
>>directly with the S element of Hspice. Even better I would prefer to
>>
>>
>get
>
>
>>=3D
>>standard S parameters for my components so I don't need to do any =3D
>>conversions at all. In my simulations I prefer to see the physical
>>
>>
>ports
>
>
>>=3D
>>rather than the conceptual differential port and common mode port =3D
>>described in [ref1], and so the most appropriate model for me seems to
>>
>>
>=
>
>
>>=3D
>>be the standard s parameters. I can easily compute the various =3D
>>differential or common quantities from the circuit if that is what =3D
>>interests me.
>>
>>I also don't understand why I would need mixed mode S parameters of a
>>
>>
>=
>
>
>>=3D
>>device from a vendor when I can compute them from the single-ended S
>>
>>
>=3D
>
>
>>parameters. I do understand that there may be benefit in mixed mode S
>>
>>
>=
>
>
>>=3D
>>parameters that have been extracted using a true mixed-mode (pure
>>
>>
>mode?)
>
>
>>=3D
>>VNA, but my understanding is that most VNAs available today actually
>>
>>
>=3D
>
>
>>apply single-ended stimulus and measure the standard S parameters, and
>>
>>
>=
>
>
>>=3D
>>then *compute* the mixed mode S parameters. That means I derive no
>>
>>
>real
>
>
>>=3D
>>benefit from the mixed mode s parameters other than the convenience of
>>
>>
>=
>
>
>>=3D
>>not having to do any computations. I don't consider this benefit =3D
>>significant since the calculations are quite straightforward and do
>>
>>
>not
>
>
>>=3D
>>suffer from numerical instabilities.
>>
>>I may be missing some fundamental aspect about the mixed mode S =3D
>>parameters that would explain their popularity and if so I would love
>>
>>
>to
>
>
>>=3D
>>understand that.=3D20
>>
>>Vince
>>
>>[ref1]
>>Combined Differential and Common-Mode Scattering Parameters: Theory
>>
>>
>and
>
>
>>=3D
>>Simulation
>>David Bockelman and William Eisenstadt
>>IEEE Transactions on Microwave Theory and Techniques, vol 43, no. 7,
>>
>>
>=3D
>
>
>>july 1995
>>=3D20
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>
>
>
--
Paul A. Levin
Senior Principal Engineer
Xyratex, Manhattan Beach
(310) 372-7352 - home & office
(310) 291-8199 - cell
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