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Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)

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nctnico:

--- Quote from: gf on November 04, 2023, 02:05:23 pm ---
--- Quote from: Martin72 on November 04, 2023, 12:57:37 pm ---https://cdn.teledynelecroy.com/files/whitepapers/wp_interpolation_102203.pdf

--- End quote ---

The Simple Test described in this paper was also my reconstruction validity criterion when I wrote

--- End quote ---
But you have to keep in mind that sin x / x is not always implemented correctly. I have come across a few oscilloscopes which had broken sin x / x implementations. An easy check is to feed a sine wave into an oscilloscope with a frequency of 2.5 times less than the samplerate (fsample / 2.5). You should be able to get the sine wave on screen properly. If you get the wrong amplitude or a distorted sine wave, then the sin x / x implementation is incorrect.

Mechatrommer:

--- Quote from: gf on November 04, 2023, 02:05:23 pm ---Well, let's assume it comes from the interpolation filter. Why would you design an interpolation filter that cuts off 50% of the recoverable bandwidth? Sure, in practice you have to sacrifice a little, but why so much? Maybe I am missing some information to understand the intention :-// As a result, the interpolation filter does not only reconstruct, but additionally it acts as a bandwidth limiter. And the latter, of course, can still introduce ringing, even if the reconstruction alone at (almost) full bandwidth would possibly not do that.

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i'm not sure how internal Lanczos/Sinc filter is designed/coded, possibly cut off more bw to cut cpu processing speed, there's been try and error by the people who know it, for example change Lanczos kernel width or what not, we will have more or lesser gibbs effect. and secondly, we have to take into account the imperfection of sampling machine in real world.. non consistent sampling interval... a little bit of ADC noises etc. if we read theory, they seems to be interesting, given the limited BW and enough parameters, "you should be able to reconstruct waves PERFECTLY!", but i guess in practice there will be catches here and there to save resources. ymmv.

mr ed:
Anything inbetween the samples is made up data. It's - art. Pick your interpolation. Or rather, it's done for you by the manufacturers. Any digital scope when the sampling gets thin will show some artifacts of some sort. Then the BW limiter adds confusion. Even with it off, the pcb wiring will still have some minor parasitics.

I hate SW options. Hack away!

Fungus:

--- Quote from: mr ed on November 04, 2023, 08:27:00 pm ---Anything inbetween the samples is made up data. It's - art. Pick your interpolation.

--- End quote ---

Nope. It's correct data, providing the bandwidth conditions are met.

nctnico:

--- Quote from: Fungus on November 04, 2023, 11:04:09 pm ---
--- Quote from: mr ed on November 04, 2023, 08:27:00 pm ---Anything inbetween the samples is made up data. It's - art. Pick your interpolation.

--- End quote ---

Nope. It's correct data, providing the bandwidth conditions are met.

--- End quote ---
Indeed. It is signal reconstruction, not interpolation. Signal reconstruction is only there to make a signal visible for humans (as a visual aid) in case the sample points are too far apart on the screen to see the shape. However, when dealing with steep edges, the sampled data represents a step function (infinite risetime) which can not exist in the real world. So there sin x /x will produce pre and post ringing (Gibbs ears).

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