[time-nuts] Pitfalls of Digital 'Scopes

[Philip Pemberton]

On 06/06/11 18:40, J. Forster wrote:
> IMO, the lesson is that digital scopes do not always accurately depict
> what a circuit is doing. Even a $50 analog 'scope would never have this
> issue.

Out of idle curiosity, what sampling mode were you using? (ACQUIRE menu 
on my TDS2024B).

In SAMPLE mode, you're guaranteed to see this kind of aliasing. It's 
usually best to run digital scopes (especially the cheaper ones with 
limited acquisition RAM) in PEAK DETECT mode.

What this does is keep the acquisition front-end running at maximum 
speed and stores the minimum and maximum values recorded in each 'sample 
bin'. You still only have 2500 data points out of the (say) 2.5 million 
the scope might have analysed, but the min/max will at least tell you 
"there's a signal here" even if the scope doesn't have the memory / 
resolution to tell you what that signal *is*.

If you decide that the burst is interesting enough to look at (and you 
know what the repetition rate is), you zoom in (TIME/DIV and H POSITION 
controls) as normal.

I seem to recall someone covering this in a Youtube video. Probably Jeri 
Ellsworth (jeriellsworth on Youtube or Twitter) or Dave Jones (EEVBlog).


It's a big, nasty beartrap and one that's all too easy to fall into.


-- 
Phil.
lists at philpem.me.uk
http://www.philpem.me.uk/