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| Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding) |
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| wasedadoc:
--- 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 --- It is only correct data if the bandwidth conditions are met and the interpolation is "sufficiently perfect". In theory, perfect interpolation requires an infinite length filter but in practice it is pointless going beyond a length and arithmetic precision where the ADC quantising error dominates the combined quantising plus interpolation error. |
| Fungus:
I didn't say it was perfect, I said it was correct. :) To be "perfect" you need an infinite amount of samples. |
| Mechatrommer:
--- Quote from: bdunham7 on November 05, 2023, 12:30:24 am --- --- Quote from: Mechatrommer on November 05, 2023, 12:16:05 am --- too bad dho cannot show dots... --- End quote --- Is that true--it does not have a dot mode? --- End quote --- yep. maybe someone need to burst tantrum all over rigol sites on this. or else, we just need to get along with this limitation. make sure only work with risetime within nyquist range, or at least need 3 or 4 points sampling for risetime alone. need mental calculation everytime. btw: a "step" in sampling reality got dt is not zero, that means, not perfect vertical, more like diagonal slope, so its real, not only exist in theory, but when doing Sinc interpolation, and ADC noise included af f = sampling rate / 2 and aliasing and all, things can become funny as already presented. btw, recently probing 150MHz digital lines with DS1054Z, gibbs and ringing everywhere, probably energy content in GHz range? change to 500MHz tektronix 1:10X probe reduced those artifacts and digital signal can be seen clearly, enough for my need, probe with one channel (2 channels 500MSa/s possible) and then save long memory to PC one by one channel, done all 14 channels lane and do post-processing analysis later on, job done. just dont expect doing signal integrity check on 1-2 points per risetetime, that request is unrealistic with $400 scope.. ymmv. |
| mawyatt:
We've used 2 samples per rise/fall edge for sample rate minimum, or Sample Rate => (1/tr)*2. So for something like 100ps rise/fall time this equates to having a sample rate => 20GSPS. Best, |
| gf:
--- Quote from: Martin72 on November 04, 2023, 11:29:59 pm ---I had read through the whitepaper I linked to and connected the bodnar pulser to the scope again, in this case the DHO804(100Mhz). --- End quote --- Is it really the 100MHz model? Not hacked? There are no measurements enabled, but the risetime seems to be in the order of 1.6ns. This risetime rather suggests that the actual bandwidth is twice as much. --- Quote ---The signal from the bodnar is 10Mhz, so it should be interpolated cleanly, regardless of whether linear interpolation or sinx/x is used. --- End quote --- No, that's the wrong conclusion. It is not the fundamental frequency of the square wave which matters here, but the "smoothness" of the edges and corners. --- Quote ---But it doesn't, even at 1.25GSa/s you can already "discover" something --- End quote --- The superimposed persistent traces from subsequent acquisitions line up quite well and are not "blurred" (ilke in 312MSa.png), which suggests that there is no interpolation/reconstruction issue here. Most likely, the small overshoot is either already present in the original signal or introduced by the analog frontent. --- Quote ---and in the worst case, the 312.5MSa/s, it already looks pretty blurred. --- End quote --- Which is indeed an indication for a reconstruction problem at this bandwidth / sample rate combination. --- Quote ---So I fed in a square wave signal from the SDG2122X with the same frequency and amplitude. The only difference is that the signal is very "slow" in terms of rise time - 9ns, which shouldn't be a problem for the interpolation, and lo and behold, it is. --- End quote --- IMO It is not. Again, the superimposed persistent traces are not "blurred" (ilke in 312MSa.png), which suggests that there is no interpolation/reconstruction issue. --- Quote ---(Why the "roof" looks so "wobbly", I'll have another look tomorrow). --- End quote --- Check the waveform also on a different scope. I could well imagine that the top is not perfectly flat. A square wave is not an easy task for an AWG either, which is based on Nyquist-Shannon reconstruction from samples. --- Quote ---Now I also understand the behavior of the DHO4204, thanks for the food for thought@rf-loop. --- End quote --- I still do not understand it. Your doubt was well justified when you asked why the rise time of the 2Gsa/s trace was shorter than the rise time in the 4Gsa/s trace. This suggests that the front-end was running at a higher bandwidth when capturing the 2GSa/s trace than when capturing the 4GSa/s trace. But this would not make sense. |
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