Products > Test Equipment
Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)
TimFox:
An important detail about Gibbs' phenomenon in the Fourier series of a square wave is that, although the height of the Gibbs' ear grows with the number of harmonics included, the area under the ear decreases.
wasedadoc:
For those interested in the filter I used, the photo shows log magnitude and phase of the through response measured on a VNA. Horizontal is linear from 100kHz to 5MHz. Vertical is 10 dB per div and 90 degrees per div. The VNA is 50 Ohms in and out but I did not take the trouble to make up complete matching pads. Just added 25 Ohms in series with filter in and out.
Filter was designed by a specialist colleague. I wanted to use it as a presampling filter for sampling the luminance of 625/50 video at 5MHz. Nyquist frequency is 2.5MHz. Beginning frequency of roll-off and steepness are a tradeoff between blur, ringing and aliasing. We never selected those parameters just from a theoretical or measured frequency response plot. Always eyeballed the result on both test signals and real picture material.
Only about 6dB down at half sample rate. Good phase linearity in the passband is evident.
AIUI the design technique is a combination of a low-pass to give the shape plus an all-pass which has frequency dependent delay to balance that of the low pass.
Apologies for the photo quality but it plus the text above give the salient details.
tautech:
--- Quote from: gf on October 29, 2023, 10:16:05 am ---
--- Quote from: tautech on October 29, 2023, 08:26:15 am ---A selection, mostly dots but some vectors when we need compare.
Toggled captures between Dot and Vector mode.
Most in Stop mode but a few running, all easy to identify.
Any in Stop mode are also toggled between Dot and Vector mode.
Max at 200ps/div. Dot count in the Timebase tab.
All in Auto memory management mode.
--- End quote ---
Does run mode superimpose multiple traces on the screen, even though Persistence is off?
Apparently it does. How many traces?
--- End quote ---
Several.
Related to refresh rate I believe which will also be related to timebase settings.
WFPS and display refresh rate obviously has been optimized for Run mode Print not to just show a single sweep of dots. eg, useless for a Single trigger capture at fast timebase settings.
But there is always interpolated Vector display mode.....
--- Quote from: gf on October 29, 2023, 10:16:05 am ---In run mode (e.g. ..._9.jpg) there still seems to be some interpolation going on. Not interpolation in the traditional sense, but trigger point interpolation, aligning the traces horizontally by fractional sample time offsets. Since this signal is not undersampled, the trigger point interpolation is expected to work quite well here, and apparently it does. But I wonder how well it works if the edges were undersampled?
--- End quote ---
Dot mode = true sample points before any interpolation.
In the acquisition mode I use these DSO's* everything that is reported in the info boxes is just one screens width so when pts are listed it's for the 10 horizontal divisions.
Here we have complete control of what we see and how we might capture it.
Run and Print shows some overwrites in dot mode whereas Stop and Print shows a single sweep of data points.
Count them, what's reported and the physical count match.
* We could instead chose Fixed Mem mode and zoom out capability of several orders of magnitude if that is your thing. ;)
Mechatrommer:
--- Quote from: 2N3055 on October 29, 2023, 04:23:23 pm ---I give up...
--- End quote ---
sorry i keep missing your post, because whenever you post, i only see "show the post" for some reason... and some posters like to post pedantic stuffs creating distractions. why give up? the challenge its not so hard, but it will reveals what i want to explain that some of you people very hard to comprehend. this is only basic stuff i dont claim i have phd... why give up? because what you'll find is a fact that i already explained here? https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5140329/#msg5140329
now if you still dont get what i want to prove? 1st i already explained in 1st post and various places here (i have to keep repeating again and again) this is for awareness... if you admit the facts i was presenting, simply admit it or simply do not reply (admission by silence) otherwise, present proofs to prove my points are wrong, in maturely and wisely and most relevant manner. forum is for healthy debate to avoid misinformations in scientific, technical community alike.
now since you (and others) dont want to take the task, may i turn to tautech, to do it for me? i need to ask favor again. because so far, he is the one the most transparent on this... and also since he stated earlier this interpolated data and stuffs are BS... here mr tautech... https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5140950/#msg5140950
here what i suggested for you to do earlier.... check my challenge posts earlier. dial back to where you got the gibbs effect like in this picture you showed here...
[img]
screen capture it again to show the gibbs effect as reference... and then.... this is the most important part you need to get right.... change your scope to Sinc OFF, dot mode ON, and single trigger it, until it triggered and stopped. so we can see few points on screen paused, not overlapping around. and do the 2nd capture, and post here again for comparison and analysis... cheers.
here an example of the 2nd capture i need, like tautech have made... (i would love to see same time scale as the one you got the gibbs effect above 200ns/div)
[img]
i was asking it here earlier but maybe not clear enough... https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5139081/#msg5139081
anyone? not just tautech and 2N3055..
btw: when we try to make a claim or swear about this device not being good enough, not accurate enough, make sure we have good background knowledge to back it up, otherwise people of knowledge can easily see what is wrong with your claim and continuos attempt at spreading misinformations to people who just got into the arena. swearing about others without good knowledge is an INSULT, to the designer and brand, and to the users too... and not trying to defend your point in maturely and calmly manner is RUDE at least thats the culture around here (my place, not this forum)... imho.
gf:
--- Quote from: TimFox on October 29, 2023, 07:02:16 pm ---An important detail about Gibbs' phenomenon in the Fourier series of a square wave is that, although the height of the Gibbs' ear grows with the number of harmonics included, the area under the ear decreases.
--- End quote ---
That's IMO the important point. Strictly speaking, the "Gibbs Phenomenon" is less about the ringing itself, which happens near discontinuities when Fourier series are truncated, but it is more about the convergence when more and more harmonics are included. As you said, the strange thing making it a "phenomenon" is that the mean square error of the approximation does converge to zero, but the overshoot (or maximum deviation between true function and approximation) remains constant and does not converge to zero.
There seems to be some disagreement in the literature as to whether the ringing itself (due to Fourier series truncation) should also be called "Gibbs". Some say yes, while some purists just call it overshoot/ringing and associate "Gibbs" only with the convergence phenomenon.
EDIT: In this thread, "Gibbs" seems to be mostly associated with the ringing itself, and not with the convergence phenomenon.
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