[SI-LIST] Re: IBIS simulation LVPECL
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- Date: Fri, 3 Oct 2008 16:10:42 -0600
Good catch, Tom.
For the probe model I'd use a series RLC like so: 1 pF, 10 nH, 100 ohms. If
you want a better model measure the probe on a VNA. The "1 pF" number given
by the manufacturer doesn't tell enough.
Dave Cuthbert
On Fri, Oct 3, 2008 at 12:21 PM, Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
> What you are measuring and what you are simulating are not the same. When
> you put the probe on the net you are adding an additional set of parasitics
> from the probe that are not in the net you are simulating. And from what
> you describe below the half pF you changed the package model by is likely
> about the magnitude of the probe's parasitics. Try adding the probe to the
> simulation and see what you get.
>
> Tom Dagostino
> Teraspeed(R) Labs
> 13610 SW Harness Lane
> Beaverton, OR 97008
> 503-430-1065
> 503-430-1285 FAX
> tom@xxxxxxxxxxxxx
> www.teraspeed.com
>
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> 401-284-1827
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Bob Tarasewicz
> Sent: Friday, October 03, 2008 9:02 AM
> To: Istvan Novak - Board Design Technology; Cotton, Chris
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: IBIS simulation LVPECL
>
> Istvan & Chris
> The step is about 200ps wide. I am measuring at vias right under
> the BGA pads. I don't know how accurate the package model is; the
> processor vendor provided it. Here is the package model info for the 2
> reciever pins AV21 and AU21
> AV21 Len=0 L=0.1n R=1.7m / Len=11391.99 L=0.4p C=0.11f R=0.06m /
> Len=0 R=3m / Len=0 L=0.4n R=2.54m /
> Len=0 R=3m / Len=0 L=0.08n R=0.04m C= 0.03p /
>
> AU21 Len=0 L=0.1n R=1.7m / Len=10354.47 L=0.4p C=0.11f R=0.06m /
> Len=0 R=3m / Len=0 L=0.4n R=2.54m /
> Len=0 R=3m / Len=0 L=0.08n R=0.04m C= 0.03p /
>
> I have to make some pretty significant changes in the receiver
> ibis model 'Define Package Model' in order for simulation to start showing
> a "step". If I change the 'Len' by a factor of 2x and the C by a factor
> of 5x (ie make Len=20000 and C=0.55f for both AV21 and AU21) , then
> simultion will show a "step". However, I'm not sure if making these
> kinds of changes to Len and C is still within a reasonable range or not.
>
> Thanks for your responses.
> Bob
>
>
>
>
>
> Istvan Novak - Board Design Technology <Istvan.Novak@xxxxxxx>
> Sent by: Istvan.Novak@xxxxxxx
> 10/03/2008 09:56 AM
>
> To
> Bob Tarasewicz <Bob.Tarasewicz@xxxxxxxxxx>
> cc
> si-list@xxxxxxxxxxxxx
> Subject
> Re: [SI-LIST] IBIS simulation LVPECL
>
>
>
>
>
>
> Bob,
>
> How wide is the step? Could it come from the round-trip delay from the
> pin connection where
> you measure, to the die, including possible via(s), package traces, bond
> wires -if any - ?
>
> Regards,
>
> Istvan
>
>
> On 10/03/08 10:42, Bob Tarasewicz wrote:
> > My simulations of an LVPECL net (apprx 3" long, 'Y' terminated just
> beyond
> > receiver) do not seem to be correlating at all with in-lab measurements.
>
> > In the past I've had very good luck with simulations agreeing pretty
> well
> > with actual scope measurements; however, this is my first experience
> > simulating LVPECL. Is there a fundamental reason why perhaps IBIS
> models
> > won't model the behavior of LVPECL accurately? Or is it that the faster
>
> > edge rate of LVPECL is highlighting flaws/limitiations in my simulation
> > methodolgy and/or tool?
> > Simulations "at the pin" as well as "at the die" show nice
> rising/falling
> > edges at the receiver. However, scope captures with a good diff probe
> > show a 'step' in the rising/falling edge at the receiver. The 'step'
> > looks like what one would expect halfway down a series-terminated net;
> not
> > sure why I'm seeing a 'step' with an end-terminated net.
> >
> > Any guidance would be apprecitated.
> >
> > Bob Tarasewicz
> >
> >
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