[SI-LIST] Re: IBIS simulation LVPECL
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'Bob Tarasewicz'" <Bob.Tarasewicz@xxxxxxxxxx>, "'Istvan Novak - Board Design Technology'" <Istvan.Novak@xxxxxxx>, "'Cotton, Chris'" <Chris.Cotton@xxxxxxxxxxx>
- Date: Fri, 03 Oct 2008 11:21:55 -0700
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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