[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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