Re: Linux, PC-based, PCI or USB oscilloscopes?
- From: Wolfgang Draxinger <wdraxinger@xxxxxxxxxxxxxxxx>
- Date: Mon, 05 Nov 2007 11:21:06 +0100
lbrtchx@xxxxxxxxx wrote:
~
My son is quite a bit enthusiastic about computer electronics
and he wants to use an oscilloscope for his learning/tinkering
around
Then buy him one. For his first experiences it does not need to
be a state of the art one. You can get good refurbished analog
oscilloscopes in the 60MHz bandwidth range for about 400$.
Especially for a beginner I recommend a analog oscilloscope,
since digital ones can set traps you don't fall into with analog
ones (one of those traps is undersampling, which you also
mentioned).
~
Oscilloscopes are not really mass-market toys, but very
expensive pieces of equipment, so, I was telling him to check
his options and he would now settle for a PC-based
oscilloscope
Hehe, that can mean anything. For example the LeCroy Waverunner
series (really state of the art and VERY expensive) are in fact
very fast multichannel A/D samplers, connected to a PC on which
runs: Windows... All in one housing. You can even connect
keyboard, mouse, USB storage and stuff like that.
Now I think you might have thought about one of those USB
connected A/D samplers, with some oscilloscope frontend
software. We're using them here in university for educational
purposes on _slow_ signals. We used to use analog servo
XY-plotters for that, but those things caused so much trouble,
that we were happy to replace them.
For those problems where you really need an oscilloscope, those
cheapo softscope USB boxes are IMHO not worth the bucks. For the
same money you can get some 10MHz 8 channel A/D converter board
you plug into the PCI slot, which easyly outperforms them, plus
for most of those there are open source drivers in the Comedi
package.
I am more of a software guy, so, I don't know much about it,
but I was trying to make him understand that he could
certainly not use a PC with a chip pulsing at a given
frequency to gauge another one with a (n equal? or) higher
frequency
Yes, you're right: To sample a signal of frequency n you need a
sampling rate of at least 2*n, which is described by the Nyquist
theorem.
That's why CD digial audio haa a sampling frequency of 44,1kHz,
as this results in a cutoff at 22,05kHz which is at the upper
limits of human hearing capabilities. Since some individuals can
hear beyond 22,05kHz (I'm one of those - so far, this changes
with age), nowadays 48kHz sampling rate are used in recent audio
applications (audio track on DVD AC3, DTS etc.). In production
you also want to have some bandwidth reserve, so in the whole
mastering process usually 48kHz*2 = 96kHz are used.
However once you've sampled the signal there are no longer
constraints. You can do any signal processing with any turing
complete machine, no matter how slow it is. Of course for
reproduction you've to go with the sample rate again.
Am I basically right? What other aspects should be consider
while getting a PC-based oscilloscope? Which other
functionality are you throwing out the window when you use a
PCO?
If you can get a very fast A/D converter (we're talking about at
least 300MHz single shot sampling rate at 12 bits resolution) a
softscope gives you a lot more possiblilties, than a analog
scope, except: Analog scopes natually expose a far better
visualisation of the signal. E.g. if you've got some signal with
harmonics on it a digital sampling scope will show you some
jaggy wave - you can use the averaging function to cancel that
out, but at the loss of the harmonics, the envelope will mostly
show you the harmonics at loss of the main signal. A analog
scope will however show you the main signal, but the shape gets
blurred out by the harmonics giving you a much better idea of
what's really going on.
There are BTW so called comboscopes which you can switch between
analog and digital sampling mode. I own such a thing, a Hameg
1507-3, which you can even connect to a computer to download
digitally sampled waveforms, or upload reference waveforms you
can overlay into the display. It IMHO a very cool scope but it
also costs something. The official price for a refurbished one
is 1900€, I got one for about 1000€ a few weeks ago.
Wolfgang Draxinger
--
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