[SI-LIST] Re: why do I need mixed mode S parameters?
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: "'vince_cavanna@xxxxxxxxxxx'" <vince_cavanna@xxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 16 Sep 2004 09:02:54 +0100
This example of a practical case might help.
http://pdfserv.maxim-ic.com/en/an/4hfan510.pdf
> -----Original Message-----
> From: vince_cavanna@xxxxxxxxxxx [mailto:vince_cavanna@xxxxxxxxxxx]
> Sent: 15 September 2004 21:59
> 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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