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

--- Quote from: tom66 on December 14, 2020, 10:35:16 pm ---
--- Quote from: nctnico on December 14, 2020, 09:47:53 pm ---I have been thinking about this a bit. I think a good approach would be to have several filter banks and use a mux to switch between them to select a different roll-off for a different bit-width and maximum samplerate. A higher order filter using passive components is not very difficult and not very expensive to implement.

--- End quote ---
It is quite expensive to do for 100MHz+ and across 4 channels, though.  Remember, these things also need to be tested during production with a sweep generator, and possibly need manual adjustment.  There are a few manual adjustment varicap points on even cheap oscilloscopes, which appears to be for matching input capacitance.  I'd really like to avoid doing that with the filters, and I don't think there's the grunt to do real time DSP on the ADC samples and correct AFE response there (unless it was very limited in response, and ~all the DSP blocks were used.)

I have tested my Rigol DS1000Z, and it aliases in 4 channel mode.  I think it's just something that's rather difficult to avoid, and the operator just needs to be careful not to exceed the parameters of their instrument.  If the measurement is really crucial, then an external analog filter could be used.

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I think the production can be greatly automated (and the user is also able to do a full recalibration from/by the instrument itself). I have found and ordered some neat parts for self calibration of the attenuators but I need to try them first to see if they deliver before wetting too many appetites.

The filters don't need adjustment; 5% L / C parts will do just fine and 2% parts aren't extremely expensive. Typically you'll see some variation in bandwidth from oscilloscopes so no worries there.
tom66:
I'm more concerned about inter-channel variation.  If you apply a signal near rolloff point on all 4 channels of an oscilloscope, how much would you expect amplitude to vary?  What about the effects on rise time?  I would expect matching to be better than +/-1dB, and rise times to be within +/-10% of each other,  but I admit this is not something I have tested. 

It would be useful to do some Monte-Carlo simulations on any filters you consider to see how influential certain parts would be. 
nctnico:

--- Quote from: tom66 on December 14, 2020, 10:45:16 pm ---I'm more concerned about inter-channel variation.  If you apply a signal near rolloff point on all 4 channels of an oscilloscope, how much would you expect amplitude to vary?  What about the effects on rise time?  I would expect matching to be better than +/-1dB, and rise times to be within +/-10% of each other,  but I admit this is not something I have tested. 

It would be useful to do some Monte-Carlo simulations on any filters you consider to see how influential certain parts would be.

--- End quote ---
Certainly  :)
But since the filter doesn't need different component placement, the first step I want to take is to have a board design to do some testing on. Due to the parasitic capacitances the board itself is as much a component as the rest. I expect it will take a few board spins before getting the board right. If each step also includes converging towards a final design then this approach will catch two flies with one stone.
gf:

--- Quote from: tom66 on December 14, 2020, 10:45:16 pm ---I'm more concerned about inter-channel variation.  If you apply a signal near rolloff point on all 4 channels of an oscilloscope, how much would you expect amplitude to vary?  What about the effects on rise time?  I would expect matching to be better than +/-1dB, and rise times to be within +/-10% of each other,  but I admit this is not something I have tested. 

It would be useful to do some Monte-Carlo simulations on any filters you consider to see how influential certain parts would be.

--- End quote ---

Were it prohibitively costly to sweep each AFE during production with a VNA, and to store the measured response as calibration data, in oder that corrections can calculated and applied in the digital domain?
Marco:
If you do Chebyshev for the capture ... how well can you correct the group delay with a high order digital filter?
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