Re: Low-cost oscilloscope recommendations

Noway2 wrote: 
There are some on-line sellers of used test equipment, google for that
phrase or for "used oscilloscopes" and you should come up with plenty.
Ebay, however very well may be your best bet, though I do understand
your concerns.  Honestly, I don't feel that giving someone a "bad
review" to be sufficient response to taking my money on a dud.  While
it would be possible to sue them, it would likely cost just as much.
On the other hand, I know people who buy used test equip on ebay and
they haven't had a bit of trouble.

A word of caution against a 100 Mhz scope:   You are on the right track
about  bandwidth limitations.  The scope bandwidth will significantly
influence your ability to see higher frequency signals.  Modern digital
electronics have very fast rise and fall times, which correspond to
significant engergy in the higher harmonics.  In order to acurately
view these signals you need a scope with significant bandwidth.  The
high speed / high frequency nature of these signals is independant of
the clock frequency of your circuit, meaning that a simple and - or
circuit implemented with LSI chips coiuld exhibit this behavior and
FPGAs, which you mention using above, tend to have 'very fast'
transition times.  A digital scope would also be beneficial with
regards to catching  and non-periodic and non repeating signals which
are quite common in digital circuits.


I can't argue with "faster is better", but it's easy to delude yourself into thinking a faster scope will make it easier to debug a digital thing. Analog scopes are great for periodic signals. Problem
is that your digital thing is far from periodic. Unless you can
trigger the scope on the parameter of interest, it don't matter how fast it is. And the problems hardest to solve are the most intermittent.
I have a GAL20V8 on a breadboard with flying leads as a trigger recognizer. I write a custom program for the GAL to optimize the
sequential event triggering to see what I'm looking for. Tedious, but
powerful.

The sweet spot of price/performance for analog scopes is 50-100MHz.
Much faster and they've got too much custom stuff inside and can't be
repaired by mere mortals. I'd vote for the TEK 465 family. 7704 mainframe
scopes can be had dirt cheap, but get 'em with plugins. Ebay nickel-dime you into the poorhouse filling up the holes. Snag a full
system with manuals and probes.

mike



One thing to keep in mind is that I understand a lot of companies sell
their outdated equipment on ebay when they are upgrading.  What may be
outdated for them could be a treasure for you.




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