[SI-LIST] Re: DDR2 Signal Probing on SODIMM
- From: Alex Horvath <alexh1@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 22 May 2007 15:48:58 -0700 (PDT)
That's why I qualified it with a "for my purposes". I was not trying to measure
with <50mv accuracy nor was trying to trigger on once a day events. Also, I
stated that I did a comparison of various ground lengths for the signal I was
measuring and again the results were well within the accuracy required for what
I was doing. It is possible to rig the 1" ground using the scope holders but it
wasn't worth the effort in my case.
I was recently at a company where they asked me to help out in the DVT area
and 2 engineers were struggling to measure some DDR signals (a third engineer
may have made the measurement feasible but it was physically impossible to get
3 people in close proximity to the board). In fact they finally gave up in
frustration. With the probe holders I was alone able to measure 4 signals
simultaneously with my hands free to manipulate the scope.
Another method I have had great success with is the use of board mount SMC
connectors glued onto the board with <0.25" wires to ground and signal. You can
use the Loctite Tac Pac kit to instantly glue the SMA connector and it's
completely reversible to boot. Of course this will load your circuit with 50
Ohms so it's generally not useful for the signals typically discussed here
unless a passive scope probe type divider is implemented.
Doug Smith <doug@xxxxxxxxxx> wrote: Ouch! A 3 inch ground lead will make your
probe very sensitive to
every noise source on your board and in the room. Lots of little and
large spikes will appear over time as people move in the room due to
ESD events. This can make it impossible to trigger on the "once a day"
glitch.
Many Agilent scopes have a clean ~100 ps to 300 ps edge (depending on
model) available at the front panel to check probe response to the
intended signal.
There are many problems with using a ground lead of most any practical
length with probes older than 5 or 6 years. For many of the popular
probes, 1/4 inch is too much connection length. Newer probes have
damping resistors in the design that make it possible to tolerate a
ground lead at the expense of sharply lowered bandwidth and of course
sensitivity to every noise source for 50 feet.
Doug
Alex Horvath wrote:
> I try to avoid probing DDR signals whenever possible but if I have to these
> work wonders even on vertical boards using the vias as signal contact points.
> You have to get the adapters for your scope probe model ? obviously the
> default probe tip they sell is good for DC only!
>
> http://www.agdevices.com/pages/product/qp/qpmain.html
>
> Of course it is extremely difficult to simultaneously ground the 1" scope
> probe ground. However, I have done tests comparing the 1" ground, 2" and 3"
> ground to a nearby soldered post @ 200 Mhz and I did not see a large
> difference in signal quality for my purposes. Of course I highly recommend
> that you perform your own tests if you use this method.
>
> Sharma Rajeev-B01961 wrote: Hi All,
> What is the best way to probe the DDR Signals on SODIMM?Is there any
> readymade solution(in terms of Connectors) available to probe the DDR
> SODIMM signals?
> I am facing issue in probing the signals on SODIMM as the via's are very
> small and moreover it becomes difficult to hold the probes on these
> vias.
> In my attempt I have used some wire straps with Damp Resistors and
> soldered them on the vias.But just want to understand if there is some
> other better way to do this.
> Thanks & Regards,
> Rajeev Sharma
>
>
>
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