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| Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding) |
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| Mechatrommer:
--- Quote from: rf-loop on October 28, 2023, 08:34:27 am --- --- Quote from: Mechatrommer on October 27, 2023, 05:28:08 pm --- as long as "fundamentals" is not exceeding nyquist limit, there will be no aliasing. --- End quote --- In my opinion, this statement is a bit false... --- End quote --- sorry if the "s" confuses you. i meant "fundamental" the 1st harmonic. i know your other arguments about 2nd, 3rd etc harmonics can alias, yes i agree, esp if signal has excessive ringing and overshoots. but few points to note so you can understand how i think: 1) try drawing a square wave on paper, (a realistic signal you can think) with a little bit distortion in 2nd, 3rd etc harmonics... and then think yourself as sampling machine creating sampling dots a little higher rate than fundamental frequency * 2.5 and connecting lines among dots. fundamental will not alias, i think we both agree, you'll see trapeziods or triangles that still can (barely) capture data (if its like digital serial data transmission). now increase a little bit sampling rate just below 2nd harmonics / 2... yes 2nd harmonics will alias, but when you connect the dots, you can still see square waves closer to original signal and if its serial transmission, you can still recover its data content. 2) now on to 2nd, 3rd etc harmonics. if they alias so badly up to a point rendering fundamental (1st harmonics) unrecognizable, then we have problem with signal integrity, reflection, mismatch and all... or analog signal distortion or oscillations so badly, yes we need higher priced DSO. if we dont have money, we can low down our cheap DSO to single channel only at max sampling rate to tackle problematic signal one by one. but since discussion is on a "fairly good" square waves, then imho aliasing will not affect much our judgement (given the bad Sinc interpolation is turned off) your points are valid on generally all type of analog signals (digital also analog), but we focus on digital signal like mcu clock presented use case earlier, and verified Leo Bodnar pulse with fairly good square wave and no problem with ringing and mismatch. and btw as you also agreed, Sinc can get thing much worse by fabricating what never happened there, amplifying alias that should not be, masking actual signal integrity and all as the article stated... ymmv. |
| Fungus:
--- Quote from: Mechatrommer on October 28, 2023, 09:47:30 am ---sinc can get thing much worse by fabricating what never happened there, amplifying alias that should not be, masking actual signal integrity and all as the article stated... ymmv. --- End quote --- But that's not sincs fault. The problem is undersampling. Turning off sinc is NOT a good idea. Learning to interpret what you see IS. |
| nctnico:
--- Quote from: Mechatrommer on October 28, 2023, 07:03:03 am --- --- Quote from: Fungus on October 28, 2023, 02:27:15 am ---To me that looks exactly like a bandwidth limited square wave: --- End quote --- DIGITALLY bandwidth limited.. it never happened on the actual circuit. but if you like "made up representation", then thats your personal preference i cant do anything about. https://resources.altium.com/p/how-gibbs-phenomenon-produces-measurement-artifacts --- End quote --- IMHO that article is wrong about what the Gibbs effect is in DSOs. This Wikipedia article is more accurate and shows it is a math problem; when approximation a square wave using fourier series, you get a signal which doesn't exist: https://en.wikipedia.org/wiki/Gibbs_phenomenon |
| Mechatrommer:
--- Quote from: nctnico on October 28, 2023, 10:22:06 am --- --- Quote from: Mechatrommer on October 28, 2023, 07:03:03 am --- --- Quote from: Fungus on October 28, 2023, 02:27:15 am ---To me that looks exactly like a bandwidth limited square wave: --- End quote --- DIGITALLY bandwidth limited.. it never happened on the actual circuit. but if you like "made up representation", then thats your personal preference i cant do anything about. https://resources.altium.com/p/how-gibbs-phenomenon-produces-measurement-artifacts --- End quote --- IMHO that article is wrong about what the Gibbs effect is in DSOs. This Wikipedia article is more accurate and shows it is a math problem; when approximation a square wave using fourier series, you get a signal which doesn't exist: https://en.wikipedia.org/wiki/Gibbs_phenomenon --- End quote --- and thats what a Sinc (or whatever digital filter there is) in scope does! Try to approximate (interpolate) what points in between actual sampled points. Turn off sinc, turn on dot mode, calculate how many points and sample rate there is. If you are lucky, and the scope is not transparent enough, you'll be surprised dho800 or low entry siglent can do 10GSps on screen on single channel smallest time div. Where do all those extra points come from? Sampled from actual event? Or simply.. approximated? If your eyes sharp enough, Lecroy sda6000 snapshot i linked earlier did 100GSps.. wow? No! But if you are unlucky, the dots are raw based on sample rate used by dso, you wont see any gibbs anyway. Try it and prove me wrong |
| Mechatrommer:
--- Quote from: Fungus on October 28, 2023, 10:17:36 am --- --- Quote from: Mechatrommer on October 28, 2023, 09:47:30 am ---sinc can get thing much worse by fabricating what never happened there, amplifying alias that should not be, masking actual signal integrity and all as the article stated... ymmv. --- End quote --- But that's not sincs fault. The problem is undersampling. Turning off sinc is NOT a good idea. Learning to interpret what you see IS. --- End quote --- i give up! You win! do whatever you like! ;D |
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