[SI-LIST] Re: Seperate Sparameter for gnd & supply plane !!
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Thu, 02 Jun 2011 09:22:48 -0400
Steve,
Agree that the shift in return-path potential becomes a problem mostly
when the design is
not done properly, but this does not invalidate the desire to have a
simulation model for
the return path as well. Otherwise, if we always assumed good designs
as a starting point,
we would not need simulation models for the signal path or power rail
either.
It is also true that just for the power-health of a device, we do not
need to model the Vss side.
For signaling through a package-board interface, or for EMI modeling,
some kind of model
for the Vss side becomes necessary. Grid representations or meshed
field-solver models of
the structure will give the proper main terms in the matrices (whether
it is Z or S). The tricky
part is to answer the question repeatedly posed: fluctuation on the Vss
rail with respect to
what. This refers to the mutual terms of the matrices, and though one
could create hypothetical
remote reference planes, the correct way is to know the environment
geometry and use it.
This sadly means we can not create this kind of model for a package, for
instance, without
knowing the surrounding geometry, and if we knew, the model created will
be locked to that
geometry.
Regards,
Istvan Novak
Oracle
On 6/2/2011 6:44 AM, steve weir wrote:
> The problem as: Brad, Larry, and I have all tried to point-out is what
> one means by noise voltage on the gnd rail. What is the noise in
> relation to? Between different points in the interconnect there will
> occur potential differences. What you need to get straight is when and
> where those potentials matter. They usually matter when someone has
> done something very unfortunate in the way that they handle their signal
> returns and/or references.
>
> Take your example of looking at something in the lab. Assume you can
> probe without any radiated noise pick-up. If you probe differentially
> across some chunk of Vss interconnect, you will indeed see the effects
> of DC and AC current drop across that interconnect. Does that matter?
> Well it would if both points you are measuring constitute the power
> connections for some circuit. That's what happens when you do a poor
> job of layout and insert all kinds of series interconnect impedance
> between Vss and Vdd. The noise that you see will be across the Vss and
> Vdd port. It is a single difference potential. As far as the load is
> concerned, if Vss is its reference, then the noise potential all belongs
> to Vdd. If you introduce a third signal that comes from somewhere else,
> and you don't reference that signal to Vss, then you have swapped
> references, and as Larry explained, by doing so inserted a corresponding
> noise source.
>
> You will do yourself well to extricate the word ground from your
> vocabulary. SPICE node 0 is literally where you define it. If you
> take up the practice of using differential voltage probes in SPICE life
> might get a lot less confusing.
>
> Steve.
>
>
> On 6/2/2011 3:15 AM, Rajesh K wrote:
>> lets say, one is interested in simulating some x interface and want to
>> find noise voltage on power and gnd rail separately.
>> s-parameter model (of power/gnd loop) will give only relative noise
>> difference between power/gnd. gnd will always be assumed to be quiet.
>> all noise will appear on power rail. But in actual system, when you
>> take it in lab. one can measure noise voltage independently on power
>> and gnd rail. noise on gnd rail is called ground bounce and noise on
>> power rail is called power noise.
>> How to mimic such scenario in simulation. There are possibilities
>> that gnd noise can be higher than power noise. i need to know which is
>> worse, power plane routing or gnd plane routing? how to find it..
>>
>>
>
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