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Re: Digital Oscilloscopes



Ludwik Kowalski wrote:

I learned today that the concept of "sweeping time" (period
during which the signal is being displayed in an analog
scope) does not apply to digital scopes. The corresponding
concept is "sampling" which keeps going continuously at
a selected rate.

Well, yes and no.

-- At the extreme high-frequency end of the scope's
performance range, it will use sampling. Obviously
this only works on a repetitive signal.
-- In contrast, in the lower ranges, the scope will
quite happily take data in order and throw it on the
screen. This is useful for nonrepetitive signals.
... You can see the difference between the two modes
if you select "single-shot" triggering mode. It
obviously is limited to taking the data in order.

Also note that not all digital scopes even have a
sampling mode. The cheapest scopes don't have it (the
just take data in order, and limit the high-frequency
end of the performance range) and the most-expensive
scopes don't have it (they throw enough parallel
circuitry at the problem to permit taking data in
order at the highest imaginable rate).

But the concept of two triggering time bases (each with
its own selectable delay) is still a mystery to me.

The delayed sweep mode as previously discussed obviously
requires two timebases. This mode is typically called
"B delayed by A" which means timebase A controls the
delay and timebase B controls the "sweep" (or "data
acquisition window" in sampling mode).

There's also "A intensified by B" mode which is handy
for setting up "B delayed by A" mode. A decent description
of this can be found at

http://archives.e-insite.net/archives/ednmag/reg/1995/042795/09column.htm

which also explains why you want both the A and B timebases
to be triggerable.

==========================================

Also note that the "trigger" need not precede the
useful data. This is one of the great advantages of
the digital scope over the ordinary analog scope.
The way this works is that the scope just takes
data, and throws it away if it doesn't meet the
"trigger" criteria.

That's why some scopes call them "qualifiers" rather
than "triggers" these days.

Having multiple triggers (qualifiers) is useful even
if you aren't using multiple timebases.
-- The various trigger-inputs can be ANDed and ORed
together to form complex qualifier expressions, e.g.
a coincidence detector.
-- You can also exercise control over how much signal-
averaging will be done.

=========================================

I say again that students in a typical introductory
physics course should not be messing with all this
multiple-trigger multiple-timebase stuff. I'm having
a hard time imagining any elementary physics apparatus
that is capable of generating a signal complex enough
to require such features.

Just because the capability is present doesn't mean you
have to exercise it. I have the "capability" of catching
wild animals in my back yard and cooking them for dinner,
but I don't need to, and it's not worth the trouble.