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| Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding) |
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| Mechatrommer:
btw i was stumbled last night with my own code as to why my data introduces Gibbs Effect in "Color Grade" mode where in "Vector Mode" there is none! just now i debug my code just to find out i actually implemented Lanczos Resampler Library which is a derivative or implementing Sinc function in it (a library that i copied from internet that i have no clue about the internal math implementation in it) https://en.wikipedia.org/wiki/Lanczos_resampling i've just forgotten my own code 7 years ago :palm: Lanczos Resampler (interpolator) will be activated whenever data (sample points) count on display is less than screen pixel width size. when there are enough or more points, Lanczos will not be activated. so here i think also a good example of the resampling algorithm that produces Gibbs Effect, where in reality there is none. fwiw... ironically pretty similar result as in here https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5140329/#msg5140329 where i thought "the "imaginary" part of Gibbs disappear, maybe got folded back (aliased) during interpolation process?" i wonder if those scopes are actually implementing Lanczos Resampler? who knows? you can get it online free. |
| Martin72:
I had just discovered pictures of my Bodnarpulser on the Rigol DHO4204 which I had on loan, I put them here. One channel active, 4GSa/s maximum sample rate, the signal looks "good". Two channels active, 2GSa/s sample rate, the signal deforms a bit - but is "faster"(rise time)... 4 channels active, but still 1GSa/s samplerate, the signal looks a bit "blurred" in two places and the rise time becomes slower again. Why is the bandwidth highest at 2GSa/s.... |
| bdunham7:
--- Quote from: 2N3055 on October 26, 2023, 09:31:30 pm ---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. --- End quote --- Which models of Siglent have an analog 'brick wall'? Mine seem to trail off for quite some distance. I've never attempted to accurately characterize them, but I'm shocked to hear them described as 'brick wall'. I was a bit surprised at this discussion because unless I missed something, the various concepts seemed to be mixed together--perhaps like they are in real life. Here's a few thoughts. Gibbs is one thing, and it can be eliminated from the picture by simply turning off interpolation and looking at the dots. The trigger is still interpolated, of course, but that's not too important for this issue. If you consider undulations after the transition to be 'ringing' (whatever the cause) and before to be 'pre-ringing', then the question arises as to the causality issue, which I think was central to Mechatrommer's question. The simple answer is that at a very basic theoretical level, causality is a complete non-issue simply because the display does not appear on the screen until a long time after the actual event, long meaning compared to the timescale of the signal. You can demonstrate pre-ringing in an entirely analog oscilloscope! The obvious culprit in a CRO is the delay line along with the delay line compensation circuit, but group delay is a feature of many analog filters and can show up as pre-ringing. So after you've eliminated Gibbs if you still see a bit of pre-ringing, there you are. Finally, an RLC filter may be underdamped without really making that much of a difference in the ultimate BW and rolloff. That will cause actual ringing due to oscillation, but not pre-ringing. Practically I have no idea whether you can separate this out from group delay for a real-life example. |
| Mechatrommer:
--- Quote from: Martin72 on November 01, 2023, 10:43:10 pm ---Why is the bandwidth highest at 2GSa/s.... --- End quote --- i guess because it produces cleaner less jittery Gibbs (Sinc interpolation) at 2GSa/s (200pts/screen 500ps/pts)? what surprises us is why the properly bw limited scope can still produce gibbs artifact at 10x sampling rate ::) iirc DHO800/DS1000Z dont produce this artifact at max sampling rate 1GSa/s. btw real machine got jittery here and there, internal clock not locked what not resulting sampling at not perfectly consistent interval. making practical application deviates from theory, last night i read about yaigol problem. |
| Fungus:
"Yaigol" was fixed by a firmware update about six months after launch. |
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