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

Priya

I would recommend loading down the bad signal with a
resistor to ground.  Pick the best resistor value.. This will
help you to temporarily get your circuit working. If the effected
signal is high impedance then it lends itself to more interference from
adjacent signals/power plane noise or crosstalk in your circuit board or noise 
from the florescent light above circuit on the bench.  I have seen total 
disasters from lights in the lab injecting low frequency noise right into the 
high impedance circuit on the bench. Turn the light off and the circuit starts 
to work just fine. This is a very common problem too. 

Load it down to ground with as
much resistance or impedance to ground or +/- VCC RAIL and this will help you 
at least get it going.  Then later you can go into the layout
and fix that problem which causes the problem in the first place. 

I have done this in the past just to get one going. Sometimes high impedance 
signals are highly Susceptible to all kinds of EMI. It happens all the time.  

Just advice based on my past electronic troubleshooting. 

Richard Jungert
http://www.rjungert.com

> From: jeff.loyer@xxxxxxxxx
> To: priyagkr@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Date: Thu, 18 Sep 2008 08:26:19 -0600
> 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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