Chuck,
The random sampling is intentional and is done to reduce the artifacts of
precisely spaced samples.
I think the 3T77A was the first of Tek's attempts. I clearly remember that
one of the 3 Series sampling sweeps was labeled that way.
-John
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David wrote:
There are many ways to get a waveform using sampling. All of those
that sample waveforms that are higher bandwidth than the sampling rate
are storing only small parts of many, many, repetitions of the signal
under test. In the case of the 7D20, and the 7854, you may be looking
at snippets of hundreds of repetitions of that signal, just to get a
look
at a single copy. In the days of old, these were called sampling
oscilloscopes.
My old Tektronix catalogs always refer to them as digitizers or
digital storage oscilloscopes. The term sampling was always
associated with instruments that had actual sampling front ends.
It doesn't matter what they call it, if it can't store the whole
waveform in one shot, it is a sampling scope, just as sure as the
old N, 1SXX, 7SXX, etc. plugins were. The prime difference is the
old type N, 1SXX, and 7Sxx plugins used the screen's phosphor, in
combination with the refresh rate, to "store" the sampled bits long
enough for you to see the full waveform. The 7D20, and 7854 use
digital storage bins, filled in a fairly chaotic way, to store the
sampled bits for view. If you have ever watched a 7854 store a
400MHz sine wave using its 50K sampling rate, you know what I mean.
If you have it set to display the stored bits as they come in, you
will see dots randomly appear on the screen (like snowflakes) as
the waveform is generated in the digital memory.
I never picked up a 7D20 because it lacks peak detection but the slow
waveform regeneration rate of my 2230 has only rarely been a problem.
I use a 7854, or a 7D20, quite a lot... but only in the single shot
mode. I usually only need storage to handle things that are slower
than my visual refresh rate.
-Chuck Harris
