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Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)

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Serg65536:

--- Quote from: Mechatrommer on October 27, 2023, 05:32:10 am ---gibbs ears are before the "EVENT".

--- End quote ---
That's complete nonsense.
Linear phase filters (as should be used in oscilloscopes) produce ripple BEFORE AND AFTER fast transients in the signal.
I have been using the Fourier transform analysis almost daily for many years.

Please, read the article carefully: https://en.wikipedia.org/wiki/Gibbs_phenomenon, https://wiki.seg.org/wiki/Dictionary:Gibbs%E2%80%99_phenomenon

Mechatrommer:

--- Quote from: Fungus on October 27, 2023, 06:04:03 am ---
--- Quote from: Mechatrommer on October 27, 2023, 05:32:10 am ---4) gibbs ear are fabricated in mathematical post processing in DSP IC due to imperfect (not infinite) terms sinc interpolation.

--- End quote ---

Nope.

A pulse is the sum of a load of sine waves (infinite). Gibbs ears are caused by some of those sine waves being missing.

(because 'scopes have finite bandwidth)

--- End quote ---
i know, and your explanation is exactly what i tried to explain about missing hi order math components. the irony is you are contradicting your own explanation by denying it ;) this is the effect when people too obsessed with math while losing grasp of what reality is imho.. you are mixing up analog filter with digital filter. Aliasing and under damped analog filter respond..

This is what analog filters do when bw limited


This is what digital filters do when bw (parameters/terms) limited


They have opposite respond/effect/representation.

And this is 'under-damped' analog filter respond, and possibly less aggressive digital filter processing.. NOT aliasing!



Please note all differing screenshots above came from the very same pulser device, same dso, except the later has been hacked upgraded... can you tell me how the actual pulse in reality supposed to look like?

Until you can read up some preliminary literatures on these subjects, we have not much to add. Its amazing to see how people are less and less interested in reading and understanding technical literatures, more and more interested in giving personal thought/belief or hearsay... it fits nicely with the prophecy's prediction ;)

I remember when i was very young going back home on plane. next to me is a factory operator, we were discussing how a plane fly and gravity. . he told me gravity is what makes a plane floats. But I told him gravity is what makes us fall.. he insisted on his point, so i stop the discussion and quiet, let him win.. cheers


 

Mechatrommer:

--- Quote from: Serg65536 on October 27, 2023, 06:36:34 am ---
--- Quote from: Mechatrommer on October 27, 2023, 05:32:10 am ---gibbs ears are before the "EVENT".

--- End quote ---
That's complete nonsense.
Linear phase filters (as should be used in oscilloscopes) produce ripple BEFORE AND AFTER fast transients in the signal.
I have been using the Fourier transform analysis almost daily for many years.

Please, read the article carefully: https://en.wikipedia.org/wiki/Gibbs_phenomenon, https://wiki.seg.org/wiki/Dictionary:Gibbs%E2%80%99_phenomenon

--- End quote ---
there is no mentioning anywhere in the article about actual physical filter that produces giibs effect, the article is talking about mathematical property, did you read?

--- Quote ---The Gibbs phenomenon is a behavior of the Fourier series
--- End quote ---
do you know what is fourier series? and your linear phase filter is also the digital one https://en.wikipedia.org/wiki/Linear_phase https://en.wikipedia.org/wiki/Finite_impulse_response you have many thank you there, so congratulation if played well enough with FFT. if you do brickwall at f (cutoff freq), setting FFT elements past that f to zeros, and do inverse FFT, you'll have gibbs effect. unfortunately i tried to relate it (digital filter imperfection whether FIR or FFT or whatever implemented) to commentaries and confusion that its fabrication on screen as if its the actual signal and most accurate representation as it happened on circuit, which is not and a total misinformation to readers. you (Fungus and Tautech and possibly bunches of people thanking you) really missed the fact that gibbs ear (the pre-ringing one) effect is not in existence when plotted with R&S, Lecroy and Tek sampling scope used by Leo Bodnar. are you saying those 10GSps scopes not using proper "linear phase" filter?

iMo:
You cannot get those ears before the rising edge starts to rise in a real hw system.
You see it on the LCD display because the sinc interpolation has been used to interpolate between couple of sampled points as I wrote above. Thus it is a graphical tool only. No FFT or any miracles are involved.
If you looked on raw data (off the memory after the ADC sampling) you would not see the ears before the start of the rising edge of the signal (for example).

PS: a low pass or any other filter (hw or sw) starts to ring "after" something happened at its input (the "causality principle" Mechatrommer mentioned above). The real physical system cannot react (knowing what to do) before comes to an input disturbance/change (like the rising/falling edge).

For example the Thermodynamic/Causal Arrow of Time..
https://en.wikipedia.org/wiki/Arrow_of_time

Mechatrommer:

--- Quote from: iMo on October 27, 2023, 02:03:10 pm ---You cannot get those ears before the rising edge starts to rise in a real hw system.
You see it on the LCD display because the sinc interpolation has been used to interpolate between couple of sampled points as I wrote above. No FFT or any miracles are involved.
If you looked on raw data (off the memory after the ADC sampling) you would not see the ears before the start of the rising edge of the signal (for example).

--- End quote ---
yes this is exactly my point. i think when we present educational materials in public we should be careful by removing other factors that may mess up the subject we are discussing. the issue was "accurate representation" of the actual signal... imho we should turn off the questionable digital filter (sinc) feature in scope, use vector or dot plot, turn on peak detect etc and then claim it to be the most accurate. like in this capture...



i would say, this is "acceptable", Lecroy also presented a little bit of pre-ringing... but not as extreme as presented by 2N3055 in https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5121846/#msg5121846 on subject of "not properly designed DSO due to aliasing"



this is trying to prove thing at it worse as we can see Sinc is messing the otherwise perfectly square wave.. and OT on that subject, he should feed the siglent while at 100MSps with > 50MHz sine or square signal instead of 16MHz arduino clock, aliasing guaranteed! whether Sinc is turned on or off. but then it seems even "aliasing" can be confusing to some people discussing it. cheers.


https://en.wikipedia.org/wiki/Aliasing

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