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Bode Plot Computational Time for various DSOs

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Fgrir:

--- Quote from: Someone on November 08, 2022, 09:47:28 pm ---Practical real world scopes include anti-aliasing filters ahead of the sampling
--- End quote ---
Do they? My RTB2004 happily aliases whatever I throw at it while my MSOX3024A appears to do some kind of sample rate dithering unless you turn on FFT in which case it happily aliases whatever I throw at it. Not trying to prove a point, just genuinely curious whether other scopes are anti-aliasing by default.

nctnico:

--- Quote from: Fgrir on November 09, 2022, 07:31:56 pm ---
--- Quote from: Someone on November 08, 2022, 09:47:28 pm ---Practical real world scopes include anti-aliasing filters ahead of the sampling
--- End quote ---
Do they? My RTB2004 happily aliases whatever I throw at it while my MSOX3024A appears to do some kind of sample rate dithering unless you turn on FFT in which case it happily aliases whatever I throw at it. Not trying to prove a point, just genuinely curious whether other scopes are anti-aliasing by default.

--- End quote ---
They don't. None of the mainstream oscilloscopes I have seen (and I have had a lot of different models through my hands) adjust the anti-aliasing filter depending on the samplerate. The anti-aliasing filter is part of the analog front-end and consists of components soldered to the board. So what gf wrote is right: any signal & noise components beyond fs/s (nyquist) are folded / aliased back to frequencies between 0 and fs/2.

Now some oscilloscopes are sold with the same hardware for different bandwidth models and use an adjustable filter to set the bandwidth according to what it says on the badge but again, the bandwidth is not adjusted when the samplerate is lower. It is entirely up to the user to set the oscilloscope's bandwidth limit to what is appropriate for the measurement at hand.

Besides that, for maximum effect of getting extra bits from an ADC you'll need enough Gaussian noise to make it work. On the Tektronix TDS700 series high-res will actually works worse when the 20MHz bandwidth limit is turned on.

mawyatt:

--- Quote from: Fgrir on November 09, 2022, 07:31:56 pm ---
--- Quote from: Someone on November 08, 2022, 09:47:28 pm ---Practical real world scopes include anti-aliasing filters ahead of the sampling
--- End quote ---
Do they? My RTB2004 happily aliases whatever I throw at it while my MSOX3024A appears to do some kind of sample rate dithering unless you turn on FFT in which case it happily aliases whatever I throw at it. Not trying to prove a point, just genuinely curious whether other scopes are anti-aliasing by default.

--- End quote ---

Apparently our SDS2104X+ doesn't either!! It's easy to get under-sampled aliased artifacts just viewing a single channel without any additional "Signal Processing" taking place, or other channels active!!

Best,

Someone:

--- Quote from: Fgrir on November 09, 2022, 07:31:56 pm ---
--- Quote from: Someone on November 08, 2022, 09:47:28 pm ---Practical real world scopes include anti-aliasing filters ahead of the sampling
--- End quote ---
Do they? My RTB2004 happily aliases whatever I throw at it while my MSOX3024A appears to do some kind of sample rate dithering unless you turn on FFT in which case it happily aliases whatever I throw at it. Not trying to prove a point, just genuinely curious whether other scopes are anti-aliasing by default.
--- End quote ---
Sure, you can find ways to cause aliasing, how about the ways to not do that? Do the high-resolution modes suppress that out of band noise?

Taking measurements from the display (as the Keysight does) continues to capture the amplitude of the original signal even when undersampled, that's using random decimation to make the display look nice (no visible aliasing). Upsides and downsides. If you're trying to reduce the number of samples offloaded from the scope for measurement, that's really not the mode to use.

RoGeorge:
Haw fast?  Challenge accepted.
A single chirp should be plenty!  ;D

This is the first I want to try:
- sample the DUT output
- get a 90* phase shifted DUT signal (here approximated by a delay looking at the 1/4 of the wavelength behind)
- that would be as if it were a quadrature sin and cos of the measured signal
- square each one and sum them together to get instantaneous
- because sin2x + cos2x = 1 at any x, we get a DC voltage representing the frequency response
- square each ADC sample, then square the ADC sample from 1/4 wavelength behind, then add the two results, and that will give the A2 as a DC along the chirp frequency sweep
- can be computed on-the-fly, sample by sample while reading from the ADC  :D

These are LTspice simulation of an RC and an RLC implementing my method above:





Not the final method, only a simulation I happen to post in https://www.eevblog.com/forum/programming/extract-precise-amplitude-and-phase-from-a-frequency-sweep-(vna-from-dsoawg)/

So far the idea is work in progress, the speed was not addressed yet.  It feels like it should be possible to do all in a single chirp of a continuous frequency sweep.  In terms of speed, this would mean less than a second for the audio range, and for RF chirps it would be faster than the refresh rate of the display.  ;D

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