[SI-LIST] Re: why do I need mixed mode S parameters?
- From: Ken Vaz <kenvaz@xxxxxxxxxxxxxxx>
- To: vince_cavanna@xxxxxxxxxxx
- Date: Wed, 15 Sep 2004 19:54:28 -0400 (EDT)
Vince,
Regarding software which simulate mixed-mode s-parameters, Ansoft's HFSS
can do this. One thing I have noticed about these simulations is that the
single-ended as well as the differential s-parameter output files I can
export from HFSS seem to be almost exactly the same, but when I view the
plots in HFSS, the single-ended and mixed-mode s-parameters are,
obviously, different. But using the Bockelman conversions on these
s-parameters, in say MATLAB, I do end up getting the mixed-mode
s-parameters that I see in the plots in HFSS. So I'm not sure how HFSS
computes the mixed-mode s-parameters. I don't see why it should, but it
seems like HFSS also uses the Bockelman conversions from single-ended to
mixed-mode. If anyone knows if HFSS actually computes the mixed-mode
s-parameters, please let me know. Also, Agilent's ADS computes mixed-mode
s-parameters, but it does it too quick. I'm not sure of the accuracy of
these simulations since it most likely doesn't solve for the fields
first, as HFSS does.
About the VNAs, the four-port VNAs available today use the same
architecture as two-port VNAs, with with a single synthesized sweeper. For
true four-port analysis we would need a VNA with dual synthesized
sweepers, especially if we need to synchronize the two input signals as in
the case of differential signaling.
One assumption that Bockelman has used in deriving the mixed-mode
conversion matrix is that the transmission lines (/DUT) is lossless. If
the transmission lines (/DUT) are lossy, the derivation breaks down and
the single-ended power waves used to stimulate the network cannot be
combined to give the differential and common-mode power waves at each
physical port. This in turn doesnt allow the conversion of the
single-ended s-parameters to mixed-mode s-parameters. This really is a
theoretical limitation more than a practical limitation, but I thought it
would be worth mentioning.
Ken
On Wed, 15 Sep 2004 vince_cavanna@xxxxxxxxxxx wrote:
> 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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