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A High-Performance Open Source Oscilloscope: development log & future ideas
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nctnico:
I'm working on an AFE filter. Right now I've arrived at a 5th order Bessel with -3dB at 200MHz. Assuming a samplerate of 500Ms/s it could be a bit steeper (higher order) but then the parts get to unrealistic values. But there will be a 1st order roll-off as well so the -3dB point might need some further tweaking. I think other oscilloscopes use steeper filters at the cost of introducing more phase shift.

I've also recalculated the attenuator part of the schematic I posted earlier. It seems quite usefull and ticks all the boxes (including having a constant capacitance towards the probe); better than I remember.
rhb:

--- Quote from: nctnico on December 11, 2020, 08:59:30 am ---I'm working on an AFE filter. Right now I've arrived at a 5th order Bessel with -3dB at 200MHz. Assuming a samplerate of 500Ms/s it could be a bit steeper (higher order) but then the parts get to unrealistic values. But there will be a 1st order roll-off as well so the -3dB point might need some further tweaking. I think other oscilloscopes use steeper filters at the cost of introducing more phase shift.

I've also recalculated the attenuator part of the schematic I posted earlier. It seems quite usefull and ticks all the boxes (including having a constant capacitance towards the probe); better than I remember.

--- End quote ---

The -3 dB point needs to be around 125 MHz to produce a good step response.  At 80% of Nyquist the edge rings badly.  Also there is no way for a 5th order Bessel to prevent significant aliasing.  With a 50% of Nyquist corner, a 5th order filter will only be about -30 dB at Nyquist whereas you need -42 dB for an 8 bit ADC.

An 80% corner,  5th order filter will be about -7.5 dB at Nyquist with the consequence that FFT displays will be hopelessly borked in certain cases.

Reg
nctnico:
First see how it behaves and go from there. As already stated: the Bessel filter won't be the only part limiting the frequency response. Analog filters also wrap around in the digital domain so you don't need to get to -48dB at Nyquist.
rhb:

--- Quote from: nctnico on December 13, 2020, 08:45:37 pm ---[snip]
 Analog filters also wrap around in the digital domain so you don't need to get to -48dB at Nyquist.

--- End quote ---

WTF?  This is so basic I'm speechless!

Edit: To make clear, an 8 bit ADC can digitize a <7 bit signal range.  Hence the -42 dB stated previously.  This is 80 year old mathematics.  If you want to argue with that, I'll just wander off.
nctnico:

--- Quote from: rhb on December 13, 2020, 09:01:45 pm ---
--- Quote from: nctnico on December 13, 2020, 08:45:37 pm ---[snip]
 Analog filters also wrap around in the digital domain so you don't need to get to -48dB at Nyquist.

--- End quote ---

WTF?  This is so basic I'm speechless!

--- End quote ---
Just think about it and look at it from a practical point of view. Frequency continues to roll off, signals consist of harmonics and at 200MHz you are already over the limit of what can be measured with a standard hi-impedance probe.  The probe itself will already cause a significant high frequency attenuation.

There is a ton of information available on this forum about anti-aliasing filters and DSOs. But since this thread is about an open source design you are free to fit whatever filter you like. I will go for what is the standard approach (which is to have a bandwidth of fs/2.5) for now.

In a nutshell:
From an error perspective: 1% is more than 2 bits (2 bits = 12dB). So if the attenuation is 3dB at 0.4fs, 48 - 12 = 36dB at Nyquist (0.5 fs) and 48dB at 0.6 fs then the amplitude error is less than 1% due to aliasing. Another issue to factor in is that in order to show the shape of a waveform you will at the very least want to see the first 2 (base and 1st) and preferably at least 3 of the harmonic frequencies. For an aliasing error to occur a harmonic frequency would need to be between 0.5 fs and 0.6 fs (and be closer to .5 fs to have the biggest impact). Remember that an oscilloscope isn't a precision instrument nor a data acquisition device and at the -3dB point the amplitude error is already near 30% !

In the end it is all about compromises; getting the highest bandwidth with the least horrible step response. And there is always the option to include two filters; one with the best step response and one with the highest bandwidth.
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