[SI-LIST] Re: why do I need mixed mode S parameters?
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Thu, 23 Sep 2004 13:03:09 -0700
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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