[SI-LIST] Re: Measured waveform is not stable

Priya...are you sure this is not a spread-spectrum clock? -jim

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Loyer, Jeff
Sent: Thursday, September 18, 2008 10:26 AM
To: priya R; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Measured waveform is not stable

I would check the routing carefully to ensure you didn't route the
signals adjacent to a 12V (or other relatively high voltage) plane, even
as the secondary reference plane.  I have seen noise injected on clocks
and other signals from this, especially if the 12V is feeding a
switching regulator.  If you suspect this is the problem, you can find a
probing location for the 12V plane (a cap, for instance) and trigger on
the highest "peaks" of your clock waveforms while monitoring your 12V
plane.  You should see a corresponding high amount of noise on the plane
when your clock signal has its highest noise occurring.  You might find
a particular source that is noisiest at the same time the noise occurs
on your clock - that will be your culprit.

If you're probing the clocks differentially, you might miss this
completely.  It turns out to be a good idea to probe clocks
single-endedly, to ensure they meet all obscure single-ended specs
(Vcross delta, for instance).

Disclaimer:
The content of this message is my personal opinion only and although I
am an employee of Intel, the statements I make here in no way represent
Intel's position on the issue, nor am I authorized to speak on behalf of
Intel on this matter.

Jeff Loyer

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of priya R
Sent: Wednesday, September 17, 2008 7:01 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Measured waveform is not stable

I am seeing a wierd problem when I measure a 100MHz differential system
clock on my board. The waveform is just not stable and it keeps jumping.
I
have ensured that the trigger is fine and also the single ended
waveforms of
p and n are symmetrical although they are also not stable. Does power
supply
noise or any other source of noise can have such an impact on the
waveforms?


Any help on this will be appreciated.

Thanks,
Priya


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