On Mon, 31 Dec 2012 10:56:51 -0500, Chuck Harris <cfharris@...>
wrote:
J. Forster wrote:
You can play with sampling specs, but 40MS/sec gives a Nyquist limit of 20
MHz.
IMO, you simply cannot trust what you see on the screen of a sampling scope.
-John
If by "trust" you mean see things faster than the Nyquist
limit, I fully agree.
However, any competently designed DSO won't allow signals
faster than the Nyquist limit into the sampling stages...
They typically have a brickwall lowpass filter set at some
point significantly below the Nyquist limit to prevent such
aliasing.
Could you point out any examples of DSOs that do this?
All of the modern ones I am familiar with rely on high real time
sample rates and post processing to prevent aliasing if the record
length is insufficient. Designing a clean analog filter that tracks
the stored sample rate seems like an exercise in futility that would
just result in poor impulse response and even worse, a varying impulse
response at different decimated sample rates.
Older DSO's that had very shallow storage areas frequently
allowed a very sparse representation of the signal to show
on screen. This, when combined with simple vector connect
display methods allowed a variety of interesting Moire patterns
to show on the screen... thus confusing things greatly.
However, modern DSO's don't seem to suffer from that problem.
All of my old DSOs (they are all old enough to drink) will operate in
equivalent time sampling mode and support peak detection. The lack of
the later is the major reason I never picked up the discussed 7D20.
The 70MHz bandwidth spec isn't specmanship. It tells you that
the amplifiers will pass a 70MHz signal with 3dB attenuation.
This spec gives you some assurance that the analog stages won't
be distorting the signals being sampled significantly.
The 7D20 is a little weird but in equivalent time sampling mode, it
can indeed acquire a 70 MHz or faster signal without aliasing using
its 40 MS/sec digitizer. Its maximum equivalent time sample rate,
limited by its record length, is 2 GS/sec.
David Hess