[sdiy] Pocket oscilloscope

[Richie Burnett]

>> Like all digital 'scopes it cannot display analogue signals.  Good luck 
>> making any sense of the aliased pixelly mess.
>
> Like all digital scopes, it displays analogue signals within the limits of 
> its sampling rate and resolution. If you've got an aliased pixelly mess on 
> the screen, you're using it wrong.

Modern professional digital storage oscilloscopes from the likes of Agilent 
(HP/Keysight), Tektronix and LeCroy usually capture the analogue input 
signal at the maximum sample rate the ADC will run at.  They then decimate 
the data for display purposes using filtering algorithms depending on what 
timebase setting you have selected.  This greatly reduces the aliasing 
effects that you used to see on early digital storage scopes where the 
sample rate was varied to suit different timebase settings but the analogue 
front-end bandwidth was always left wide open!  Such practice is always 
prone to aliasing, missing brief pulses that occur in between the samples, 
etc.

The "always sampling flat out" approach is very good at picking up the 
briefest of transients even if you have the timebase dialed down to a fairly 
slow sweep.  It also lets the manufacturers do their "Digital Phosphor 
Emulation" stuff because you've got a lot of additional waveform information 
recorded at that high speed that can be used to make histograms and 
represented as brightness variations when it is dressed up for displaying on 
the screen.  The downsides of running the ADC's flat out at maximum rate are 
that you need a lot of fast memory, and it can be power hungry.

You get what you pay for though.  It's likely that cheap Chinese scopes like 
Digimess, Rigol, Tenma, etc, still might not use these tricks, or might cut 
corners to keep down the memory requirements / cost.

-Richie,