Products > Test Equipment
Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)
gf:
--- Quote from: Fungus on October 29, 2023, 11:10:57 am ---I love how everybody is busy trying to recreate a technique that was abandoned years ago in favor of sinc:
https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-sampling
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Not recreate, but emulate -- without the expenses for a hardware trigger circuit and TDC that used to be required for ETS and its variants in the past. Particularly for low-end scopes it was certainy a cost factor. Digital trigger is cheaper, as it is just software on the FPGA, and offers more oportunities. But in the past when real-time sample rates were only in the order of 100MSa/s, one could hardly renounce ETS.
Mechatrommer:
--- Quote from: tautech on October 29, 2023, 08:26:15 am ---Please stop this BS about dot interpolation.
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your tone is a bit too high for a representative ;D didnt you still get it that Lecroy scope that i attached earlier shows interpolated dots? do you call them BS? i wish there is option to only shows sampled data, instead of many interpolated data that are too good to be true. but SDA6000 cannot be easily turned on right now and i havent played much time with it. still i would love to pull data from it raw which i have not yet.
on the topic of interpolation, i had play with some techniques in photo 2D and 3D object rendering (graphics) such as bezier, spline, bicubic curve etc... bezier is unsuitable for DSO purpose as interpolated data will not cross true sampled data, but interpolator like spline is good candidate as sampled data is included in the interpolated data. since Sinc in its limited resources form apparently doesnt make a good job anyway, maybe investigation into other method from graphics arena can be done, i think i will have no time for it. although spline may not recreate high order frequency such as in Gibbs Phenomenon, but it could be a safer bet on limited BW and sampling rate system, although Sinc is theoretically sound in data sampling arena, but in its limited form i would say it just possible for it to do a "wild guess" on what the signal should looks like, enough for now. people can google themselves on several data interpolation methods if they are interested cheers.
what most relevant to me are snapshots from tautech below, because it proved the scope is honest enough to reveals its sampled data without interpolation. and from there we can mentally figure that there are indeed ringing on the rise time. the rest of pics are just overlapped dots thats confusing to analyze. and the third pic below in vector also can help brain visualize to suspect if there is ringing or not. if from dots we dont see up and down much, there is no high freq near nyquist exist, so try to imagine there is ringing is just a wild guess. Sinc theoretically will be able to reconstruct the signal acccurately given 2 conditions (1) bandwidth of input data is limited with brickwall filter at nyquist freq, even a slight magnitude can screw it up and renders theory unapplicable (2) infinite parameters (or whatever practical the screen can show) is used. i suspect the visible gibbs effect is due to "not enough parameters" in Sinc interpolation OR BW filter of front end (analog) is not match brickwall at nyquist, any excess BW MUST be zeroed before it reach ADC! imho. i cant be sure about Sinc parms being not enough, i'm not a Sinc interpolater programmer, but what i'm certain, its not perfect implementation in some DSO condition. this is why also if sampled at much lower rate, by theory, "a properly designed" DSO should also switch BW limiter to match nyquist limit, such as if sampling at 100MSps, BW limiter must be brickwalled as well at 40-50MHz, at 10MSps, brickwall at 5MHz etc... do you think its easy for DSO designer? ::) fwiw...
Mechatrommer:
--- Quote from: Fungus on October 29, 2023, 09:10:43 am ---
--- Quote from: Mechatrommer on October 28, 2023, 10:05:44 pm ---then that probably causal... a real signal. who knows whats inside semiconductors IC?
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Substrates that can predict the future? :)
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looking at tautech pics (1st and 3rd picture in my post just above)...there is rippling before event, many other things can happen such as mismatch and reflection, but need to look further what happened before the snapshot. same thing on the rise area there are infinite rippling to the end of pic. waves and interactions with parasitics (pcb and components) is a mystery things.
2N3055:
--- Quote from: Mechatrommer on October 29, 2023, 12:26:22 pm ---
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My biggest problem with your posts is that I cannot, for the love of God, figure out what the heck you are trying to say and prove.
You post a flurry or unconnected statements, insults and misrepresented data and then when people reply to you, you follow up with more of the same.
You keep talking about interpolated dots in LeCroy without explaining to people that LeCroy scopes have Enhanced sample rate mode (ESR), that is upsampling by factor of 2, from 5GS/s to 10GS/s. That being a controversial point is irrelevant. If you don't like it, just disable it and stop bitching about it. I also prefer not to use it, for various reasons.
That being said, SDS6000A also supports ESR. SDS2000X HD does not. My images also show SDS6000 data WITHOUT ESR.
That is one confusion out of the way.
As for Sin(x/x) reconstruction , that is ONLY proper way to reconstruct signal if you cannot afford to have oversampling that is pretty much factor of screen resolution. That means that you should at least have one sample point per pixel on screen, and then you don't need interpolation.
For a screen 1000 pixels wide, you would need 100 samples per division. On 1ns/div that means 100GS/s. Good luck with that.
So sampling theory to the rescue.
Sin(x/x) reconstruction filter is our friend. Math works. Those who don't understand that, back to school.
wasedadoc:
Here is some evidence that Gibbs ears are real and can be seen on the output of a passive analogue filter.
The filter is a low pass video filter. Yes this one has many Ls and Cs. I don't say it is necessary to have such a complex filter to show the effect. Just that this is a filter that I still had up in the loft from prior to 1980 when I was designing some video equipment and I knew it would certainly be suitable for this demonstration.
The input is a 100kHz square wave. Shown on CH2 and and the scope triggered on it. Generator is a Uni-T UT962E whose rise and fall time is not less than 15ns. The output impedance is 50 Ohm whereas the filter is designed for 75 Ohms in and out and that is quite critical. Hence the parallel 56 Ohm and 47 Ohm in series on the input and the 75 Ohm resistor across the output with the CH1 probe across it.
Scope is Rigol DS1054Z enhanced to 100MHz. The 20MHz BW limit is not enabled on any channel. CH3 is turned on but not used. This is to enable the button to turn off sin(x)/x on the scope. Sampling rate is 250Msa/s. Vector mode or dot mode does not change the ears.
The delay through the filter is about 1.2 us. The "left ear" around each transition begins at the same time as the input transition.
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