[Open-graphics] Re: Digital 'scope & spectrum analyzer

[Dieter]

> >>> 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?  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?  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?

> 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.

> 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.