Another tip: Probing high-frequency signals, especially when there are 
nearby sources of electromagnetic energy, is tricky.
There are several good articles online, depending upon the type of probe 
you use. If you're lucky (I'm not...) you'll have a differential probe. 
Otherwise, a 10X probe is a must.

BEWARE of ground leads on probes! They will pickup noise. The best solution 
I found is to remove the ground clip and the plastic probe shield, which 
should expose the metal ground connection on the probe. Tightly wrap 
bare-wire around the ground shroud, then solder to the nearest 
circuit-ground. I was able to reduce the ground connection length from 6 
inches to less than 1/4 inch, and it made a big difference. For the signal 
connection, I used bare wire-wrap wire, wrapped a few turns on the 
probe-tip, then soldered to the component being probed. The length was 
around 1/2 inch.

Yes, it's a pain, but there's nothing worse than spending (wasting!) weeks 
chasing-down a suspected noise problem only to find out you created it with 
improper measurement techniques.

Also be careful of non-obvious ground loops. The signal ground of your 
scope is likely to be connected to the AC power-line ground. If your 
circuit ground is connected to the AC line ground (either directly or 
through your power supply), you will likely have aground loop, and that 
will add noise.

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