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A High-Performance Open Source Oscilloscope: development log & future ideas
2N3055:
--- 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. The DSOX2000A likely does some kind of peak-detect because the display part works with decimated data while other DSOs do not.
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Agree!
DSOx/MSOX3000T is also very resilient to aliasing, screen representation keeps outside signal envelope far into sampling rates that should alias badly. I also think they use peak detect internally for display all time, and decimate for waveform buffer in accordance with sample rate.
I tried enabling /disabling peak detect and saw no difference, so I presume that's it.
Marco:
--- Quote from: nctnico on December 16, 2020, 09:25:36 am ---otherwise you'll be distorting the signal due to phase delays introduced by filtering.
--- End quote ---
You get exactly the delay you want with digital filtering. if you really want a 100th order Gaussian response filter you can get it in digital, analogue not so much. The subsampled signal will never be the original signal, all you get to chose is what type of distortion you want ... no distortion is not an option.
Outside of subsampling, I'm told Tek has response correction on by default. The analogue filter introduces group delay based distortion, digital can correct it.
tom66:
Well, throwing away samples is certainly easier than filtering them. But, I don't understand why you'd go to so much trouble to build a good antialias filter for the AFE side if you just risk aliasing when your sample rate drops off? Switching to an automatic 'peak detect' mode is an option, although it would double the memory required as you need to store a min and max for each sample.
I see ERES or equivalent as being comparably easy to achieve. Bin N samples (where N is a power of two) into an accumulator, take the top N bits (probably 16 bits so it functions with 14 bit mode up to 4x ERES) and then save into RAM.
I need to rewrite the acquisition Verilog so it can handle all 4 channel configurations (well, 3 modes as only 1/2/4 are truly supported and 3 channel mode is treated as a subset of 4ch mode.) Then it needs to be able to discard samples on a binary division rate (all but 2nd, all but 4th, all but 8th, etc.) and re-order these into RAM correctly. The 'real trick' here is then getting that data lined up with the digital trigger. And then ideally include the capability for MSO support. Needs a good amount of thought to make that work.
nctnico:
--- Quote from: tom66 on December 16, 2020, 05:52:41 pm ---Well, throwing away samples is certainly easier than filtering them. But, I don't understand why you'd go to so much trouble to build a good antialias filter for the AFE side if you just risk aliasing when your sample rate drops off?
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Aliasing is not a bad thing perse but the user needs to be aware of it. Without filtering you'll still be able to measure the RMS and peak-peak values of a signal which has a fundemental at a higher frequency then Nyquist for the lowered samplerate. IOW: you will still be able to make out the amplitude. In case of narrow pulses you'll miss some pulses. With automatic filtering you will suddenly see no signal or a completely different signal on your screen after changing the time/div which will confuse the user.
tom66:
Makes sense - but, in that case, why not omit the input filter altogether and allow the user to cautiously use their instrument up to Nyquist? All filters risk eliminating signals that you intend to look at - part of operating a scope is understanding approximately what you expect to appear on the screen before you even probe it.
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