Another problem with random sampling as applied to a sampling
oscilloscope and not a storage oscilloscope is that the sampling
strobe kickout can corrupt the trigger pickoff causing the display
near the trigger point to be distorted. The Tektronix 7T11A manual
briefly mentions this issue on page 3-17 but I do not remember if they
discussed it further elsewhere:
"Random sampling permits display of the leading edge, or any other
part, of the input signal as shown in Figure 3-13. This is possible
even when observing fast-rise, short duration pulses, and when using
either internal or external trigger sources (EXT modes of triggering
give better results in Random Sampling)."
That issue threw me off for a while when I was first learning to use
random sampling. Before I started, I suspected there should be some
sort of interaction between the strobe and internal trigger but when I
saw distortion exactly where I expected, I thought that was too
perfect and something else must be wrong.
The distortion is actually useful in some cases where it can identify
the exact trigger edge among many on the display.
On Tue, 01 Jan 2013 20:37:00 -0000, "Ed Breya" <edbreya@...>
wrote:
Actually, sequential sampling presents a faster and fuller display, but needs a vertical delay line to be able to see the front edge of fast steps that are widely spaced (low rep-rate). Random sampling provides this without the delay line, but it is more complicated - there are always tradeoffs.
In reality, all oscilloscopes - including analog - are actually "sampling" the signal intermittently, and only for a certain amount of time. They provide short glimpses of the time domain signal, but are blind to it much of the time. When we have a properly triggered analog waveform display on screen, that is made by repeatedly scanning (sampling, of a sort) multiple waves that we trust are virtually identical from one to the next. The trigger system tells us when to look, the sweep system determines for how long, and the screen phosphor saves the information. It's an equivalent-time system that converts the signal frequency down to where our vision can see it.
Ed
--- In hp_agilent_equipment@..., "J. Forster" <jfor@...> wrote:
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
================
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
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