RE: ibis VT curves

For what it's worth,

From experience: I now assume that SpecctraQuest always uses the dT data. I
had a simulation which gave very strange results. Turns out, the dT data
was bogus (~0.5 pS); everything else was just fine. And the dT data is easy
to check if you have VT data.

Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@Dell.com
(512) 723-3592

> -----Original Message-----
> From: Katja Koller [mailto:Katja.Koller@icn.siemens.de]
> Sent: Friday, November 17, 2000 7:26 AM
> To: ibis
> Subject: Re: ibis VT curves
>
>
> Todd Westerhoff wrote:
> >
> > Simon,
> >
> > The answer to your original question, as I understand it:
> >
> > SPECCTRAQuest (a.k.a SQ) doesn't use the dV/dT data when
> rise/fall curves
> > are present. As Betty said, the V-I curves and V-T data
> are used instead.
> > SQ only uses the dV/dT data when the rise/fall curves are
> NOT present (i.e.
> > the so-called "linear" ramp model).
> >
> > Some experimentation yesterday revealed that when V-I
> curves are present
> > (but NOT V-T), SQ uses the dT information, but NOT the dV
> data. That makes
> > sense (the high and low operating points are determined by
> the output's V-I
> > characteristics and the power supplies). The dT
> information is used to
> > define the length of time it takes the output to change state.
> >
> > There are some interesting implications to this, worthy of note:
> >
> > If you have an output with a 2V swing that takes 1 nS to
> switch, you have a
> > ramp rate of 2V/nS. Theoretically, you could specify that
> as 2V/1nS, or
> > 1V/0.5nS, or 10V/5nS - all of which would give you the same
> ramp rate.
> > BUT - only the 2V/1nS value would give you the switch time
> you expect,
> > because SQ is only using the dT value.
>
> As far as I know, the ibis spec says, that the ramp rate has
> to be from
> 20% to 80%.
> In the case of an output with 2V swing within 2 nS, this has to be:
>
> dV/dt = 1.2V / 0.6 nS
>
> Unfortuately, I often get ibismodels where this is not the case. This
> can be
> checked by looking at the crossing of the VI-curves with the
> "50Ohm-load-line"
> (if the load is 50Ohm).
>
> Perhaps others have similar experiences.
>
> Katja
>
> >
> > Morals of the story:
> >
> > 1) Ramp data is not used by SQ when V-T data is present.
> > 2) When using Ramp Data, tell SQ the actual dV and dT
> values for the whole
> > output transition.
> >
> > That's the story as I understand it. If anyone has seen
> different, please
> > let me know!
> >
> > Todd.
> >
> > > -----Original Message-----
> > > From: Simon Schaeffer [mailto:simon.schaeffer@cern.ch]
> > > Sent: Thursday, November 16, 2000 8:13 AM
> > > To: ibis
> > > Subject: ibis VT curves
> > >
> > >
> > > Hi,
> > >
> > > I'm working on ibis models and I met the same problem as
> > > Betty Luk, it's
> > > to say that in playing around with my IBIS model (changing
> > > the ramp data
> > > manually), I didn't see any changes with Sigxplorer (Specctraquest
> > > signal explorer expert). However, in making the same changes
> > > in the ibis
> > > model used in pspice I saw differences.
> > > I have done this with an ibis model with and without the
> VT curves.
> > > I have used "ibis2spice" from Intusoft but this program
> > > doesn't convert the VT curves, it just uses the [Ramp] parameters.
> > > So, I am trying to build a new ibis2spice program using
> the VT curves.
> > >
> > > My questions are:
> > >
> > > 1) Why doesn't the [Ramp] modifications influence the
> results (Betty
> > > Luk's question)
> > >
> > > 2) Why use the VT curves instead of the [Ramp] parameters?
> > > Is it "only"
> > > to get a more accurate curve?
> > >
> > > 3) I am puzzled by the apparent contradiction that might
> arise if both
> > > the
> > > VT and VI curves are available. For example, a VT curve has been
> > > specified with the following load:
> > >
> > >
> > > out -----/\/\/\/\/-----|
> > > 50 |
> > > + ___|___ 1.25V
> > > ---
> > > |
> > > |
> > > |
> > > GND
> > >
> > > Going from low to high, the VT curve switches from 400mV to 2V.
> > >
> > > How can I use this information for other loads?
> > > For example:
> > >
> > > - 50 Ohm load only
> > > (i.e. the dc offset has been removed. My simulated output
> > > will now want
> > > to be something defined by the VI curve. How do I
> sensibly use the VT
> > > curve in this case?)
> > >
> > > - 100 Ohm load only
> > >
> > > 4) Do I have to take into account the Rfixture and Vfixture
> > > values used
> > > while specifying the VT curves?
> > >
> > >
> > >
> > > Thanks for your feedback
> > > Simon Schaeffer
> > >
>
> --
> Katja Koller ICN M TC 6
> Siemens AG Hofmannstr. 51 81359 Muenchen
> Katja.Koller@icn.siemens.de
> Tel: 0049 89 722 45719 Fax: 0049 89 722 44692
>