>This is what you can expect to see if you cut
>a square wave off at 10 times its frequency:

Umm, maybe, but that graph only has a subset of the harmonics in it. In 
practice, the cut off frequency will normally be at -3db, and so there will 
be some higher harmonics at a lower amplitude than they would otherwise be. 
The result is that on an analogue scope you don't tend to see the ringing 
except at the edges.

However I guess the device in question is using an A/D to digitise the 
signal, so whatever filter is in there will have a much steeper slope with a 
true filter going down to a notch, so that graph may well be closer to what 
one would see. 

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