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A High-Performance Open Source Oscilloscope: development log & future ideas
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nctnico:

--- Quote from: gf on December 15, 2020, 07:55:31 pm ---If the ADC always runs at full speed, and the currently selected sampling rate is lower, then at least the decimation filtering needs to be done in real time, before storing the samples.

--- End quote ---
No DSO uses a decimation filter in case of a lower sampling rate becaue this conflicts with the specified bandwidth. The only decimation 'allowable' in realtime is peak-detect. Unless ofcourse there is a specific signal filtering feature the user enables.

For post-processing a quick peak detect -ish decimation is used to compress the data quickly into a display record. This can lead to odd behaviour for specific signals like sweeps though which isn't easy to solve. Compressing the data realtime is a more time sensitive operation compared to sampling because any delay will result in sluggish operation.

This is for the GW Instek GDS-2204E but similar intermodulation effects can be made to appear on other (much more expensive) DSOs as well:



--- Quote from: rhb on December 15, 2020, 10:58:59 pm ---@nctnico  200 MHz corner and 500 MHz Nyquist will give very fine results.  A 5th order filter will be ~ -32 dB at Nyquist and if one picks up another -10 dB elsewhere in the AFE the 8 bit case is done.  Though, personally I'd go to a 7th order just to be sure I had the roll off.  Steep slopes cause heavy ringing, so they are not a good idea.

--- End quote ---
I have to see about the exact impedances to be used in the circuit but so far anything over 5th order is resulting in unpractical part values. And as tom66 noted a Monte-Carlo simulation needs to be done to verify component variation sensitivity. Other roll-offs in the AFE will likely be 1st order anyway.
gf:

--- Quote ---The only decimation 'allowable' in realtime is peak-detect
--- End quote ---

Don't forget ERES. This is basically a decimation filter - whatever filter response they may have realized.
[ I guess in most cases just a boxcar averaging of the samples in the decimation interval, i.e. the lower fs/2 still falling on the main lobe of the sinc frequency reponse, not at a zero. ]
tom66:
The ADC must run at full speed to function as a digital trigger source.

But, in some cases, less data must be stored,  although the trigger is always going to work on realtime data.  The CIC filter looks like an interesting, inexpensive way to downsample.  It's certainly better than alternative of just throwing away (N-1)/N samples.

My experience is the Rigol DS1000Z does not downsample correctly - the scope will alias very easily - but the Agilent DSOX2000A does not.
nctnico:

--- Quote from: tom66 on December 16, 2020, 08:30:33 am ---The ADC must run at full speed to function as a digital trigger source.

But, in some cases, less data must be stored,  although the trigger is always going to work on realtime data.  The CIC filter looks like an interesting, inexpensive way to downsample.  It's certainly better than alternative of just throwing away (N-1)/N samples.

My experience is the Rigol DS1000Z does not downsample correctly - the scope will alias very easily - but the Agilent DSOX2000A does not.

--- End quote ---
Do not filter! Throwing away samples is the only correct way (when in sample mode); otherwise you'll be distorting the signal due to phase delays introduced by filtering. The DSOX2000A likely does some kind of peak-detect because the display part works with decimated data while other DSOs do not.
rf-loop:

--- Quote from: nctnico on December 16, 2020, 09:25:36 am ---
--- Quote from: tom66 on December 16, 2020, 08:30:33 am ---The ADC must run at full speed to function as a digital trigger source.

But, in some cases, less data must be stored,  although the trigger is always going to work on realtime data.  The CIC filter looks like an interesting, inexpensive way to downsample.  It's certainly better than alternative of just throwing away (N-1)/N samples.

My experience is the Rigol DS1000Z does not downsample correctly - the scope will alias very easily - but the Agilent DSOX2000A does not.

--- End quote ---
Do not filter! Throwing away samples is the only correct way (when in sample mode); otherwise you'll be distorting the signal due to phase delays introduced by filtering.

--- End quote ---

 :-+
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