Dieter wrote:
You need to be able to *capture* the data in real time, in
order to do a single sweep mode, for non-periodic signals.
The processing and display of that data don't have to be real
time.
Yes, that is true for some applications, but unless this is a
real time spectrum analyzer ($$$$) -- which is useful for some
applications such as looking for noise and intermittent
distortion -- you are going to be working with a stable
periodic wave form.
That was in reply to:
To be more specific, a digital oscilloscope displays a
periodic signal on the screen and the refresh scans rather
slowly from left to right.
Single sweep mode is a very useful, often essential, feature for
an oscilloscope.
Yes, a one shot scope is a useful device. With analog this was a
scope with a storage CRT. With digital this requires a very fast
flash ADC (perhaps more than one). I think that this is going to
be rather expensive compared to a scanning type D-scope for stable
AC signals.
So this "scanning" type would use a A/D with a fast sample time but a
long time between samples?
Actually not. The time between samples would be just slightly more than
1/f -- (N + 1)/N. At least at the maximum frequency. I presume that
you could take multiple samples per cycle at lower frequencies but I am
not familiar with that type of scope.
It would collect data for a long time, and then calculate a time or
frequency domain graph? So what sort of bad things happen when the
signal isn't periodic?
The same sort of things that happen with an analog scope.
A time domain graph would be impossible, right? Could you sample for
a long time and get a frequency domain graph that would be an
average?
Yes, there are commercial products that do this called digital signal
averagers. They are basically a digital scope with a lot of memory.
The nice feature of one shot D-scopes is that they can record a lot
more data than an analog one -- you can record a lot more data
than will fit on the screen at once.
Very nice if the test is time consuming to set up, or worse, if it is
a destructive test. And you don't need a storage CRT or camera.
Upload the data easily, Remote access. Could be used for automated
tests.
An inherent issue is that you can't take data faster than your ADC and
faster ADCs aren't as accurate. It depends on your application which is
more important. I presumed that you would be using a spectrum analyzer
to accurately analyze the harmonic content of a constant and periodic
signal. An instrument good for that type of application isn't going to
be as good for non-repetitive signals.
This has always been a serious issue with an analog spectrum
analyzer. In theory, it should be a Gaussian distribution. This is
not realizable because it would have to extent to infinity. But
even taking a polynomial distribution, it is still impossible to
exactly realize a band pass filter with that response function.
And, the filter also needs to have linear phase response. So, this
-- the scan filter response shape -- is an important feature and
something that is better in more expensive units.
I suspect that the band pass response shape in these devices is not
anything near a Gaussian response or linear phase.
Does this band pass response shape issue go away with a digital SA?
Digital filters class was a long time ago, but IIRC you can get any
response you want since it is math.
There is no band pass response with a digital spectrum analyzer since it
assumes that the signal is periodic, with a known period, and computes
the harmonics up to the Nyquist limit -- a filter isn't needed to do
this. An analog spectrum analyzer doesn't know the period so it can't
work that way.
--
JRT
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