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

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Mechatrommer:
On to perusing this thread https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/ to get as much useful info on DHO800/900 hack/mod/upgrade. putting bookmarks on most important and useful posts/infos... now all pages perused, not much to add so far... except few commentaries that got me interested since they are not well, or partially answered. so lets me add something to clear things up. what interest me is peculiar SIGLENT's DSO lines "non-causal ringing" plot just before signal "rise" and signal "down" also known as and let us termed it as "event"... ringing should be caused by the "event", ie after the "event", not before... siglent's plot show as if the "event" is caused by the ringing, not vice versa which should be the natural phenomena in nature in general, and in circuit debugging in specific...

the arguments was that RIGOL's plot are inaccurate due to "no ringing", this "artifact" has been explained by experienced members, for example by Kleinstein (inside DSO's front end artifact), but got sidetracked maybe due to nitpicking agenda? so let me add a bit about this matter. we know ringing can be caused by reflection of mismatched impedance between source and destination, and also due to resonance inside LC networks inside the scope. so different scope will have different internal respond to external signal feed to it. even with my newly acquired DHO800, at its standard setting (70MHz BW) pulses from Leo Bodnar's and UTG962 shows smooth transition with no significant ringing, but when changed to DHO924 FW (230MHz BW), "causal ringing" becomes so significant, due to change in BW limiting mechanism inside the scope. i can provide proves if required, but you can check in my posts if you are interested. anyway, i can explain more, but that is not my main point to tackle here.

the main suspicion is on the "non-causal ringing" on Siglent's DSOs plots, lets stick to the plan remember? just to avoid unnecessary confusion and nitpicking the wrong direction... attached is the area i'm talking about (circled), taken from one of the samples provided here. my take on this is that... the plots are post-processing "fabrication" inside the siglent scope, and it can trick newbies into thinking that is a real signal, but in fact actually it was never there in reality, not even in the dso's front end stage! imho. bad thing about this is it can result in wrong decision making, argumentations and debates later on among newbies vs expert alike. so if we talk about giving false data (signal) i must question again who's giving the false data? this need to be taken into consideration whoever newbies want to buy a new scope, esp the pricey one, iirc this "non-causal" effect also demonstrated by high end $4-5K range siglent scope somewhere outside this thread. when can we see this "non-causal" effect? is when we try to approximate square wave with limited parameters in numerical or fourier analysis Fourier Approximation and Gibbs Phenomenon i believe this is also has to do with "finite" amount of parameters during sinc(x) reconstruction of a signal, so i believe this is happening inside the dsp IC or fpga/asics, but that i cannot be sure,  since i have no proof. its just highly likely, where else it can be? LC circuit cannot produce "non-causal" effect, can it?

now why i dare to take this conclusion? is due to sheer amount of proofs screenshots by other instruments, including the most high end R&S DSO, posted by members and land owners Dave Jones, and HP? or Tek? sampling scope test made by Leo Bodnar himself, that this non-causal (false or lie) ringing does not exist! to add value to the existing one, also attached are plots and setup from Lecroy SDA6000 scope. among the most prominent posts i was refering to and bookmarked are listed below for reference. please dont get it wrong, this is not intended for a shame game, nor a fanboyism dick waving contest. this just to add infos and possibly further discussions, so we can gain more knowledge as to why is this, and newbies can do more informed decision when buying scopes. i stand corrected if you have more academic and technical explanations, i'm also in need of learning. that is for the night, cheers and best regards. ;)

https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5076118/#msg5076118
https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5077231/#msg5077231
https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5078086/#msg5078086
https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5078230/#msg5078230
https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5121846/#msg5121846

nctnico:

--- Quote from: Mechatrommer on October 26, 2023, 07:15:38 pm ---the main suspicion is on the "non-causal ringing" on Siglent's DSOs plots,

--- End quote ---
These are typical 'Gibb's ears' caused by sin x / x signal reconstruction where the signal frequency versus sampling rate approaches the minimum samplerate / nyquist limit where sin x / x works. It is for the user to decide whether the Gibb's ears are allowable or whether the bandwidth of the oscilloscope needs to be reduced.

ebastler:
Two things are important here, in my view:

(a) It's not the case that the ringing is actually present on the Bodnar pulser's output and the DHO800 somehow "glosses it over" while Siglent shows the truth. Tautech had claimed that early in the discussion a few weeks ago, but dropped that claim later.

(b) It is also not the case that the Siglent scopes are "wrong" and produce unacceptable artefacts. They do apparently use lowpass filters with a relatively sharp cutoff, causing the Gibbs reconstruction phenomenon as described. Rigol (and R&S, which Dave showed for comparison too) seem to use filters with a smoother roll-off, resulting in less overshoot in the reconstruction. A matter of philosophy or personal preference, I'd say.

Mechatrommer:

--- Quote from: ebastler on October 26, 2023, 07:44:02 pm ---They do apparently use lowpass filters with a relatively sharp cutoff, causing the Gibbs reconstruction phenomenon as described.

--- End quote ---
i believe the more accurate term is "digital sharp cutoff". i had a play with FFT and did brickwall filter, do inverse FFT and got this Gibbs effect, in the end... its NOT the real signal anywhere on the pcb... thats my main point.

iMo:
OT comment - Bodnar's <40ps edges require special cabling, connectors and matching, imho, the BNC connectors are not the best technology here..

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