[SI-LIST] Re: Correlation to lab measurement
- From: "tom" <tom@xxxxxxxxxxxxx>
- To: "'Santangelo, Steven'" <SSantangelo@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 08 Jun 2010 08:53:54 -0700
It could very well be the simulations are correct and your measurements are
tainted by poor measurement technique. Probing boards that were never
designed to be probed can be very problematic. Probing is a SI problem and
the key to good SI is keeping an eye on the return currents. The ground
lead in your probing system looks like an inductor which can cause a
resonant circuit with the input capacitance of the scope probe. Whenever a
resonant circuit is stimulated with an edge ringing will occur. If your
probe point and local ground return point are not very close to each other,
say less than 0.100" and your "ground lead" is almost nonexistent in length
then you will have ringing in your scope's display. When you probe a board
you need to think in terms of how closely your scope probing system
approximates a coaxial connection to the board. How well are the fields
contained in the probing system?
Hope this helps.
Tom Dagostino
Teraspeed Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
www.teraspeed.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Santangelo, Steven
Sent: Tuesday, June 08, 2010 7:23 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Correlation to lab measurement
Hi All,
I'm trying to correlate some lab measurements to my simulations in
Cadence PCB SI (SpectraQuest). I have a 400MHz unidirectional LVDS
interface between two Xilinx devices that is terminated in 100ohms
inside one of the devices. My lab measurements show a pretty good
reflection occurring on the rising and falling edges of the waveform.
I'm trying to determine if this is being caused by the ball to I/O pad
trace length in the package (plus my BGA breakout). My simulation only
shows a slight bend in the waveform rather than the non monotonic ring
that I see in the lab. The bend does correlate well time wise (location
on the edge) with the lab measurement. I don't really know much about
the limitations of IBIS models so I'm wondering if some of the
discrepancy could be due to that.
FYI: I'm using the models that Xilinx "ibiswriter" puts out. The
rise/fall times of the waveforms are in the neighborhood of 300pS.
Thanks
Steve
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