[SI-LIST] Re: why do I need mixed mode S parameters?

  • From: "Charles Hill" <chuck@xxxxxxxxxxx>
  • To: <vince_cavanna@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 15 Sep 2004 15:40:00 -0600

Vince,

The value I see in mixed mode S parameters is that in cases where common and
differential modes are mostly uncoupled, it allows easier conceptualization
of the excitations on the network through analogy with a simple 2-port
network.

For example, with experience one can scan through a 2-port S parameter file
and tell how well matched it is, or the gain, or the reverse isolation.
Same thing with a mixed mode file, if you can separate out (with your eyes),
the appropriate terms.

As far as modeling goes, I agree with you that a mixed mode S parameter file
for input to a simulation is an unnecessary complication.


Regards,
Chuck Hill, consultant


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
vince_cavanna@xxxxxxxxxxx
Sent: Wednesday, September 15, 2004 2:59 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 =
have been struggling to understand as I re-enter the field of signal =
integrity and attempt to catch-up on some of the new =
measurement/analysis techniques. I would appreciate any insight you can =
offer.

I understand mixed mode S parameters and can compute them from standard =
(single-ended) S parameters or from a  model - or the other way around. =
I can appreciate their usefulness in understanding how an n-port, that =
may have been designed to operate mainly under differential stimulus, =
responds to (reflects and scatters the incident power) differential and =
common-mode stimulus.

What I am trying to understand is why I would ever want to use mixed =
mode S parameters in a time-domain or frequency domain simulation, and =
how to use them. I am also interested to learn what simulators support =
mixed mode S parameters directly, as using them in a simulator such as =
Hspice seems cumbersome. My approach today is to simply use standard S =
parameters directly.

The "why" I really don't understand at all. With regards to the "how", I =
know of one approach but it is cumbersome and does not seem worthwhile. =
I would be interested to know if there are circuit simulators that =
handle mixed mode S parameters directly but most important I need to =
understand why I need them.

One way to use mixed mode S parameters, that has been suggested on this =
mailing list, is to use the S element in Hspice, but represented with =
the mixed mode S parameters instead of the standard mode S parameters, =
and recognizing that the ports are conceptual (differential and common =
mode) as explained in [ref1]. In order to interface the conceptual =
n-port to my circuit (which expects real ports) I then have to wrap the =
device with a circuit that converts the actual port waves of my circuit =
into the differential and common mode waves that need to be applied to =
the conceptual n-port. This approach should work but seems cumbersome =
and, more important to me, I don't understand what I gain from it.=20

The approach I described seems like a round-about way to attempt to use =
the mixed mode S parameters directly when they can easily be converted, =
with no loss of information, into standard mode S parameters and used =
directly with the S element of Hspice. Even better I would prefer to get =
standard S parameters for my components so I don't need to do any =
conversions at all. In my simulations I prefer to see the physical ports =
rather than the conceptual differential port and common mode port =
described in [ref1], and so the most appropriate model for me seems to =
be the standard s parameters. I can easily compute the various =
differential or common quantities from the circuit if that is what =
interests me.

I also don't understand why I would need mixed mode S parameters of a =
device from a vendor when I can compute them from the single-ended S =
parameters. I do understand that there may be benefit in mixed mode S =
parameters that have been extracted using a true mixed-mode (pure mode?) =
VNA, but my understanding is that most VNAs available today actually =
apply single-ended stimulus and measure the standard S parameters, and =
then *compute* the mixed mode S parameters. That means I derive no real =
benefit from the mixed mode s parameters other than the convenience of =
not having to do any computations. I don't consider this benefit =
significant since the calculations are quite straightforward and do not =
suffer from numerical instabilities.

I  may be missing some fundamental aspect about the mixed mode S =
parameters that would explain their popularity and if so I would love to =
understand that.=20

Vince

[ref1]
Combined Differential and Common-Mode Scattering Parameters: Theory and =
Simulation
David Bockelman and William Eisenstadt
IEEE Transactions on Microwave Theory and Techniques, vol 43, no. 7, =
july 1995
=20
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