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

--- Quote from: tautech on October 26, 2023, 08:56:03 pm ---X and SinC interpolation.... captured running and Stop.

--- End quote ---
i just noticed and look carefully that you also proved the non-causal ringing appeared when using Sin(x) ;D and does not exist when doing straight line (X) graph plotting.



2N3055:

--- Quote from: Mechatrommer on October 26, 2023, 08:33:55 pm ---
--- Quote from: ebastler on October 26, 2023, 08:14:12 pm ---Yes, a steep (higher order) analog, "physical" filter will give the same effect.

--- End quote ---
i noticed gibbs effect is worst in siglent at much lower sampling rate, but not at its max smapling rate. from here https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-

lowering sample rate should lower the cut off frequency, not creating more high frequency elements. even sampling below nyquist and straight plot point to point (no sinc interpolation) will not "fabricate" this artifact, if its a real physical filter that caused it, why implement a physical filter that causes artifact that is not seen in high end dso?


--- Quote from: 2N3055 on October 26, 2023, 08:31:21 pm ---Most modern scopes, especially higher BW ones will have that overshoot as a consequence of brickwall AA filter used.

--- End quote ---
see? i think my point will be side tracked again, i was talking about "non-causal" or "precursor" ringing... not overshoot not undershoot that we usually saw :palm: do you care to read my long post above? or at least look carefully at 1st picture attachment, that whats i'm talking about. here again...


--- End quote ---

Please don't get angry again. I read your post. But your post is very complicated and you mixed few different things together.
And you did not understand my images I posted..

So to start from end: Overshoots you see on that Siglent are result of analog Brickwall filter when you feed it with very fast signal, much faster than it's transition band is.  I have exactly same artefacts on my Keysight MSOX3104T. That same effect is very common in higher ends DSOs, that happen to have very sharp Brickwall response filters and sample at 2.5x the BW.
Only way to avoid them is to have more oversampling. DHO800 on the image from Fungus had 100MHz BW and sampling at 1.25GS/s. Factor of 12.5x more.
Images I posted were not answer to that question but to one about insufficient sampling

Lovering sample rate does not change cut off frequency whatsoever. Where did you get that information?
Cutoff frequency is solely function of input channel's BW.
Lowering sample rate simply means you take sample of same signal (with same BW) but with samples more apart in time.

What I have shown you is EXACTLY what happens when you sample signal with 200MHz + frequency content with insufficient sampling rate.
Full bandwidth of signal reaches ADC that cannot reconstruct signal correctly anymore because sample points start to skip parts of signal curve. Since input signal is not synchronous (it has no correlation) with sampling clock of the scope, on every consecutive trigger you get another slightly randomized wrongly reconstructed shape. With those happening thousands of times per second, screen shows the cloud of funny shapes.
And apparent higher frequency content is reconstruction algorithm tryin to curve fit curve through nonsensical data.

It is not artefact of Siglent scopes, but quite the opposite: since I have full manual control of sampling rate and BW on those scopes I could produce such experiment. Normally with scope in Auto memory mode scope you won't be able to make this effect.

And experiment shows you EXACTLY what happens if you have scope with 200MHz BW, insufficient sampling rate and quite common 16MHz square wave signal with 1ns edges. I didn't even shoot Bodnar's pulser in it. I made signal quite simple and common so anybody can reproduce it.

It will happen on any scope that you can achieve those conditions. On DHO800 with 100MHz BW this should not happen that easy or at all.
But if you hack it to 200MHz, enable all 4ch (sample rate drops to 312.5 MS/s) you should see same or similar thing...

That is why I kepp repeating that DHO800 samples properly and DHO900 does not unles in very limited usage scenario.
DHO800 with 100 MHz BW should be largely immune to this. Unless you manually reduce sample rate or memory length at long timebases..

I hope I explained this well. If you don't understand what I mean please ask and I will try to explain it further/better..

Since DHO800 does not seem to have manual sample rate control you can try seeing same effect by reducing memory depth to 1kpts or 10kpts  to force smaller sample rate. Play with it a bit to understand where limits are...

tautech:

--- Quote from: Mechatrommer on October 26, 2023, 09:18:23 pm ---@tautech: refering to picture below. a little bit of "precursor" ringing could exist in real world i think to some specific semiconductors or logic fabrics? maybe something triggered inside the logic gate much earlier that caused ringing before actual logic state switch (rising and falling), but excessive precursor ringing is a bit unnatural, unnaturally "symmetrical" ;D. i wonder if such extreme such as image provided by 2N3055 just above could exist to physical/analog filter at dso front end. anyway, proofs that Loe's pulse doesnt have this excessive non-causality are already presented by other brands TE. fwiw.



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Yet if we look at the real values they are small vs the full signal. From previous readings I always understood these to be Gibbs related.
From Step response adjustments I have done, the step speed (RT) is best to match the capabilities of the front end and if using anything too fast for the scope the response can be impacted so other waveforms are affected.
IMO a tradeoff is required.
YMMV
2N3055:

--- Quote from: Mechatrommer on October 26, 2023, 09:30:55 pm ---
--- Quote from: tautech on October 26, 2023, 08:56:03 pm ---X and SinC interpolation.... captured running and Stop.

--- End quote ---
i just noticed and look carefully that you also proved the non-causal ringing appeared when using Sin(x) ;D and does not exist when doing straight line (X) graph plotting.

--- End quote ---

Because overshoot is artefact of SinC interpolation..

Again, my images has nothing to do with this overshoot behaviour.
It is simulation of insufficient sampling frequency by setting my scope pathologically wrong, manually, on purpose.
tautech:

--- Quote from: Mechatrommer on October 26, 2023, 09:30:55 pm ---
--- Quote from: tautech on October 26, 2023, 08:56:03 pm ---X and SinC interpolation.... captured running and Stop.

--- End quote ---
i just noticed and look carefully that you also proved the non-causal ringing appeared when using Sin(x) ;D and does not exist when doing straight line (X) graph plotting.

--- End quote ---
Stop and SinX was ugly as one expects. Didn't even bother to capture it.
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