[SI-LIST] Re: why do I need mixed mode S parameters?
- From: "Bill Beale" <beale@xxxxxxxxxxxx>
- To: <vince_cavanna@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 15 Sep 2004 22:10:07 -0700
Hi Vince,
The short answer to your question:
'Why do I need mixed mode S parameters?' is =20
YOU DON'T.
Mixed mode S-parameters are nothing more than a linear transformation of =
the
single ended S-parameters. Therefore it is the transformation or the
inverse transformation will let you bounce back and forth between mixed =
mode
and single ended.
If you have N-ports and N is an even number, you can convert your single
ended S-parameters to mixed mode, grouping your ports however you wish. =
You
are correct that it is just as easy to simulate whatever network with =
single
ended (natural) S-parameters, and use MatLab (or some other math =
program) to
do the transformation to mixed mode.
Personally I use just the Sdd quadrant of a 4-port usually and typically
ignore the rest. Well, maybe take a quick glance, but unless I see
something horrific I usually do not care. I make a leap of faith that =
my
common mode rejection is good, and the mode conversion is small.
What I work on is differential outputs and inputs, so the differential
response is what I am most interested in. Which leads in to why one =
would
get mixed mode S-parameters from a device vendor. If you know you are =
using
a differential device, then from a datasheet/S-parameter quick glance =
you
can understand the differential response. In addition, if you make the =
same
assumption I use about ignoring the rest of the mixed mode quadrants =
(Sdc
Scd and Scc) then you can simulate only a two port instead of a 4 port.
(Not that simulating extra ports is a big deal, but two ports simpler in
your own MatLab script or Excel spreadsheet)
Thank you,
Bill
++++++++++++++++++++++++++++++++++++++++++++++++++++
Bill Beale CAE, HiPHY Group
Synopsys phone: 503-547-6523
2025 NW Cornelius Pass Rd. fax: 503-547-6017
Hillsboro OR, 97124 email: beale@xxxxxxxxxxxx
++++++++++++++++++++++++++++++++++++++++++++++++++++
>=20
> I have some philosophical questions about mixed mode S=20
> 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=20
> insight you can =3D
> offer.
>=20
> I understand mixed mode S parameters and can compute them=20
> from standard =3D
> (single-ended) S parameters or from a model - or the other=20
> way around. =3D
> I can appreciate their usefulness in understanding how an=20
> n-port, that =3D
> may have been designed to operate mainly under differential=20
> stimulus, =3D
> responds to (reflects and scatters the incident power)=20
> differential and =3D
> common-mode stimulus.
>=20
> 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=20
> simulation, and =3D
> how to use them. I am also interested to learn what=20
> simulators support =3D
> mixed mode S parameters directly, as using them in a=20
> simulator such as =3D
> Hspice seems cumbersome. My approach today is to simply use=20
> standard S =3D
> parameters directly.
>=20
> The "why" I really don't understand at all. With regards to=20
> the "how", I =3D
> know of one approach but it is cumbersome and does not seem=20
> 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.
>=20
> One way to use mixed mode S parameters, that has been=20
> suggested on this =3D
> mailing list, is to use the S element in Hspice, but=20
> represented with =3D
> the mixed mode S parameters instead of the standard mode S=20
> parameters, =3D
> and recognizing that the ports are conceptual (differential=20
> 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=20
> to wrap the =3D
> device with a circuit that converts the actual port waves of=20
> my circuit =3D
> into the differential and common mode waves that need to be=20
> applied to =3D
> the conceptual n-port. This approach should work but seems=20
> cumbersome =3D
> and, more important to me, I don't understand what I gain from =
it.=3D20
>=20
> The approach I described seems like a round-about way to=20
> attempt to use =3D
> the mixed mode S parameters directly when they can easily be=20
> converted, =3D
> with no loss of information, into standard mode S parameters=20
> and used =3D
> directly with the S element of Hspice. Even better I would=20
> 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=20
> 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=20
> 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.
>=20
> I also don't understand why I would need mixed mode S=20
> 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=20
> mixed mode S =3D
> parameters that have been extracted using a true mixed-mode=20
> (pure mode?) =3D
> VNA, but my understanding is that most VNAs available today actually =
=3D
> apply single-ended stimulus and measure the standard S=20
> parameters, and =3D
> then *compute* the mixed mode S parameters. That means I=20
> derive no real =3D
> benefit from the mixed mode s parameters other than the=20
> convenience of =3D
> not having to do any computations. I don't consider this benefit =3D
> significant since the calculations are quite straightforward=20
> and do not =3D
> suffer from numerical instabilities.
>=20
> I may be missing some fundamental aspect about the mixed mode S =3D
> parameters that would explain their popularity and if so I=20
> would love to =3D
> understand that.=3D20
>=20
> Vince
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