Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding) - Page 1
Forum section: electronics / testgear

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 07:15:38 pm
content: On to perusing this threadhttps://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 byKleinstein (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 analysisFourier ApproximationandGibbs Phenomenoni 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/#msg5076118https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5077231/#msg5077231https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5078086/#msg5078086https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5078230/#msg5078230https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5121846/#msg5121846

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2023, 07:34:12 pm
content: Quote from: Mechatrommer on October 26, 2023, 07:15:38 pmThese 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.

author: ebastler
author_rank: Super Contributor
timestamp: October 26, 2023, 07:44:02 pm
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 07:52:44 pm
content: Quote from: ebastler on October 26, 2023, 07:44:02 pmi 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.

author: iMo
author_rank: Super Contributor
timestamp: October 26, 2023, 07:55:11 pm
content: OT comment - Bodnar's <40ps edges require special cabling, connectors and matching, imho, the BNC connectors are not the best technology here..

author: ebastler
author_rank: Super Contributor
timestamp: October 26, 2023, 07:58:59 pm
content: Quote from: Mechatrommer on October 26, 2023, 07:52:44 pmItisthe real signal, as represented by all its harmonics that fall within the scope's Nyquist limit. Which, one could argue, is what you expect a DSO to show you.Having said that -- personally I do prefer the smoother roll-off used by Rigol and R&S, which dampens or avoids those apparent overshoots. Otherwise it is too tempting to mis-interpret the overshoots as being actually present (and a potential problem) in the input signal.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 08:03:56 pm
content: Quote from: iMo on October 26, 2023, 07:55:11 pmi specifically bought the BNC version to test low end DSO which always have BNC inputs. for high end reputable brand name DSO, there is not much point in testing with Leo Bodnar pulse. so there more is one more adapter seen in my attachment that should not be there, hence expect a little bit more ringing... its ok if i have to add adapter on one test rather than have to do many times of it (less reliable mismatch) on low end DSOs tests. there is no LPA (BMA connector of Lecroy's input) version of Leo's Pulser anyway. ymmv.Quote from: ebastler on October 26, 2023, 07:58:59 pmi believe not, unless you can provide proof that "physical" filter can give this "non-causal" effect. are you telling us that... R&S, HP, Lecroy, Rigol are telling lies by hiding this reality of "Gibbs Ear" on actual "physical" cirrcuit? it is only a crippled version of "mathematical" or "theoritical" representation of the real signal, minus the higher terms that the dsp/dso designer decided to omit due to limited resources. not the actual one on circuit.

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 08:06:11 pm
content: Quote from: iMo on October 26, 2023, 07:55:11 pmYou can connect BNC version directly to scope input without cables...

author: ebastler
author_rank: Super Contributor
timestamp: October 26, 2023, 08:14:12 pm
content: Quote from: Mechatrommer on October 26, 2023, 08:03:56 pmYes, a steep (higher order) analog, "physical" filter will give the same effect.Edit:Hang on -- specifically referring to the "pre-ringing", analog filters willnotexhibit that. They will show the Gibbs phenomenon (apparent overshoot and ringing)afterthe edge though, which is also an effect of the limited frequency components used to reconstruct the signal. The DHO800 does not show that either, due to its smoother filter roll-off.I don't know which design of bandwidth limiting filter Siglent or each of the other scope manufacturers you mention are using. In order to implement a digital cutoff filter you need to oversample the input signal, and especially near the scope's bandwidth there is often not that much headroom in the sampling rate -- so it may well be analog-only filters in most cases?Also, I don't know whether your implication is true that none of the other brands shows a similar Gibbs effect. Do you have access to pulse response data for all of them? For example, Lecroy -- with their focus on "analytical" scopes -- might take a similar approach as Siglent? And it would certainly be plausbile that Siglent takes some guidance from LeCroy as part of their collaboration.

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 08:31:21 pm
content: Quote from: ebastler on October 26, 2023, 08:14:12 pmMost modern scopes, especially higher BW ones will have that overshoot as a consequence of brickwall AA filter used.DHO800 does not show it because it's 100MHz filter has gentle slope and combined with 1.25GS/s on one channel will give good response.Enable more than one channel and check then again.Even more will be visible on DHO900 at 200MHz BW...If I go to both my 500MHz and 1GHz scope and enable 200 MHz BW limit, it shows perfect pulse response.Keysight explanation, in attachment.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 08:33:55 pm
content: Quote from: ebastler on October 26, 2023, 08:14:12 pmi noticed gibbs effect is worst in siglent at much lower sampling rate, but not at its max smapling rate. from herehttps://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5121846/#msg51218461GSps:100MSps:lowering sample rate should lower the cut off frequency, not creating more high frequency elements. even sampling below nyquist and straight plot point to point (no sinc interpolation) will not "fabricate" this artifact, if its a real physical filter that caused it, why implement a physical filter that causes artifact that is not seen in high end dso?Quote from: 2N3055 on October 26, 2023, 08:31:21 pmsee? i think my point will be side tracked again, i was talking about "non-causal" or "precursor" ringing... not overshoot not undershoot that we usually sawdo you care to read my long post above? or at least look carefully at 1st picture attachment, that whats i'm talking about. here again...

author: ebastler
author_rank: Super Contributor
timestamp: October 26, 2023, 08:40:39 pm
content: Quote from: Mechatrommer on October 26, 2023, 08:33:55 pmGot your point now -- please see the "Edit" in my most recent post.Maybe your original post was a bit too long for me, with too much text. Talk about a "brickwall"...Good night!Edit: I Still stand by my earlier assessment,QuoteBut getting that "ideal" set of harmonics, without relative phase shifts, can't be done with an analog filter. Hence there must be digital filtering happening in the Siglent.

author: Martin72
author_rank: Super Contributor
timestamp: October 26, 2023, 08:45:39 pm
content: Quote from: Martin72 on October 25, 2023, 09:25:52 pmI have now ordered USB-C breakout boards, once to look at the signal quality of the power supply and as a nice side effect, to check the accuracy of my USB-C powermeter.If someone should have opened the liteon power supply, how many wires come from the connection line ?

author: TimFox
author_rank: Super Contributor
timestamp: October 26, 2023, 08:51:11 pm
content: A lot of posts on this thread, include mention of Lenovo (supplied with my DHO914S) and LiteOn supplies.Has anyone made a direct comparison between DHO800 or DHO900 performance with different power supplies, or, even better, between a supplied unit and a very clean non-switchmode lab supply?

author: Martin72
author_rank: Super Contributor
timestamp: October 26, 2023, 08:53:25 pm
content: No, but it´s a nice idea, will do it in the next days.QuoteWhat performance? Noise ?

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2023, 08:56:03 pm
content: Especially for Mech some screenshots better comparing apples.....at similar timebase settings....max on 1000X HD.Bodnar powered from rear USB port.X and SinC interpolation.... captured running and Stop.

author: Fungus
author_rank: Super Contributor
timestamp: October 26, 2023, 08:59:22 pm
content: Quote from: Mechatrommer on October 26, 2023, 07:15:38 pmI think it's this:https://en.wikipedia.org/wiki/Gibbs_phenomenonIt's counterintuitive that you see somethingbeforethe pulse but it's mathematically correct. That's what you get when you apply a low pass filter to a discontinuity (step) in a signal.

author: ebastler
author_rank: Super Contributor
timestamp: October 26, 2023, 09:05:27 pm
content: Quote from: Fungus on October 26, 2023, 08:59:22 pmIt's sometimes advisable to read the next couple of posts before one fires off a reply.

author: Fungus
author_rank: Super Contributor
timestamp: October 26, 2023, 09:07:59 pm
content: Quote from: ebastler on October 26, 2023, 09:05:27 pmYeah, I just noticed.Still, nobody else posted a link to Wikipedia or said "if you apply a low pass filter to a step..."

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 09:18:23 pm
content: Quote from: Fungus on October 26, 2023, 08:59:22 pmyes i provided the link in my post. sin(x)/x is a highly theoritical and mathematical. iirc i read long ago that you cannot reconstruct back the sampled real signal back on monitor the way its appeared on electrical circuit if you dont do infinite terms reconstruction, at least thats what i understood. with limited terms, what you interpolated on screen will not match the real signal. even lesser terms, the displayed signal will be deviated even more from the actual one.@tautech: refering to picture below. a little bit of "precursor" ringing could exist in real world i think to some specific semiconductors or logic fabrics? maybe something triggered inside the logic gate much earlier that caused ringing before actual logic state switch (rising and falling), but excessive precursor ringing is a bit unnatural, unnaturally "symmetrical". i wonder if such extreme such as image provided by 2N3055 just above could exist to physical/analog filter at dso front end. anyway, proofs that Loe's pulse doesnt have this excessive non-causality are already presented by other brands TE. fwiw.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 09:30:55 pm
content: Quote from: tautech on October 26, 2023, 08:56:03 pmi just noticed and look carefully that you also proved the non-causal ringing appeared when using Sin(x)and does not exist when doing straight line (X) graph plotting.

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 09:31:30 pm
content: Quote from: Mechatrommer on October 26, 2023, 08:33:55 pmPlease don't get angry again. I read your post. But your post is very complicated and you mixed few different things together.And you did not understand my images I posted..So to start from end: Overshoots you see on that Siglent are result of analog Brickwall filter when you feed it with very fast signal, much faster than it's transition band is. I have exactly same artefacts on my Keysight MSOX3104T. That same effect is very common in higher ends DSOs, that happen to have very sharp Brickwall response filters and sample at 2.5x the BW.Only way to avoid them is to have more oversampling. DHO800 on the image from Fungus had 100MHz BW and sampling at 1.25GS/s. Factor of 12.5x more.Images I posted were not answer to that question but to one about insufficient samplingLovering sample rate does not change cut off frequency whatsoever. Where did you get that information?Cutoff frequency is solely function of input channel's BW.Lowering sample rate simply means you take sample of same signal (with same BW) but with samples more apart in time.What I have shown you is EXACTLY what happens when you sample signal with 200MHz + frequency content with insufficient sampling rate.Full bandwidth of signal reaches ADC that cannot reconstruct signal correctly anymore because sample points start to skip parts of signal curve. Since input signal is not synchronous (it has no correlation) with sampling clock of the scope, on every consecutive trigger you get another slightly randomized wrongly reconstructed shape. With those happening thousands of times per second, screen shows the cloud of funny shapes.And apparent higher frequency content is reconstruction algorithm tryin to curve fit curve through nonsensical data.It is not artefact of Siglent scopes, but quite the opposite: since I have full manual control of sampling rate and BW on those scopes I could produce such experiment. Normally with scope in Auto memory mode scope you won't be able to make this effect.And experiment shows you EXACTLY what happens if you have scope with 200MHz BW, insufficient sampling rate and quite common 16MHz square wave signal with 1ns edges. I didn't even shoot Bodnar's pulser in it. I made signal quite simple and common so anybody can reproduce it.It will happen on any scope that you can achieve those conditions. On DHO800 with 100MHz BW this should not happen that easy or at all.But if you hack it to 200MHz, enable all 4ch (sample rate drops to 312.5 MS/s) you should see same or similar thing...That is why I kepp repeating that DHO800 samples properly and DHO900 does not unles in very limited usage scenario.DHO800 with 100 MHz BW should be largely immune to this. Unless you manually reduce sample rate or memory length at long timebases..I hope I explained this well. If you don't understand what I mean please ask and I will try to explain it further/better..Since DHO800 does not seem to have manual sample rate control you can try seeing same effect by reducing memory depth to 1kpts or 10kpts to force smaller sample rate. Play with it a bit to understand where limits are...

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2023, 09:32:45 pm
content: Quote from: Mechatrommer on October 26, 2023, 09:18:23 pmYet if we look at the real values they are small vs the full signal. From previous readings I always understood these to be Gibbs related.From Step response adjustments I have done, the step speed (RT) is best to match the capabilities of the front end and if using anything too fast for the scope the response can be impacted so other waveforms are affected.IMO a tradeoff is required.YMMV

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 09:36:06 pm
content: Quote from: Mechatrommer on October 26, 2023, 09:30:55 pmBecause overshoot is artefact of SinC interpolation..Again, my images has nothing to do with this overshoot behaviour.It is simulation of insufficient sampling frequency by setting my scope pathologically wrong, manually, on purpose.

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2023, 09:36:19 pm
content: Quote from: Mechatrommer on October 26, 2023, 09:30:55 pmStop and SinX was ugly as one expects. Didn't even bother to capture it.

author: Martin72
author_rank: Super Contributor
timestamp: October 26, 2023, 09:37:22 pm
content: QuoteLike this.

author: Fungus
author_rank: Super Contributor
timestamp: October 26, 2023, 09:37:43 pm
content: Quote from: Mechatrommer on October 26, 2023, 09:18:23 pmUsing only the central part of the sinc function doesn't help but the main problem is lost harmonics due to limited bandwidth.Those harmonics are what keeps the line flat before/after the pulse.

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2023, 09:39:21 pm
content: Quote from: Martin72 on October 26, 2023, 09:37:22 pmStock or hacked ?

author: iMo
author_rank: Super Contributor
timestamp: October 26, 2023, 09:47:59 pm
content: There are 4 things not to be mixed here:1. the "aliasing" creating the fake artifacts/spurious (caused by hw&sw because of "BW vs. sampling rate")2. the "sinc" function used for "creating the nice pictures" via the "sinc interpolation" as seen on the LCD screen (each discrete point sampled is multiplied by sinc function and all that results are added, thus you get "additional points in between", you draw them all on the screen thus you see a solid line instead of just several sampled points, like 5 points for example)3. the process of a "reconstruction/filtering of a square wave" with help of FFT, like sqrwave->fft->filter->ifft->sqrwave4. the "ringing" of the hw circuitry (caused by steep filters, unstable circuits, impedance mismatch)Edit: typos..

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 09:49:46 pm
content: Quote from: Fungus on October 26, 2023, 09:37:43 pmCorrect! And keep the top flat too..And somebody mentioned infinite series to properly reconstruct. That is not needed as soon as you reach resolution of screen, noise floor or step response specifications of the scope.

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2023, 09:50:41 pm
content: Quote from: Martin72 on October 26, 2023, 09:37:22 pmTHANK YOU!!

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 10:21:01 pm
content: Quote from: Martin72 on October 26, 2023, 09:37:22 pmso both siglent and rigol can show false signal at lower sampling ratei was interested that much earlier in this thread, the very false (sinc) signal was used to argue that rigol displayed signal is incorrect, ie hiding "accurate" signal representation, it turned out, that "accurate" representation is just artifact from sinc interpolationthis was my intention, to clear things up and lets argue fairly apple to apple... and not messing up with other aspects as iMo mentioned just above. another factor already answered by Kleinstein about differing input impedance and respond inside the dso front end filter...https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5076118/#msg5076118ps: i need to find where that sinc interpolation setting in rigol and turn it off, its no use anyway at trying to represent "accurate" signal by interpolation.

author: Martin72
author_rank: Super Contributor
timestamp: October 26, 2023, 10:22:30 pm
content: Quote from: tautech on October 26, 2023, 09:39:21 pmRight out of the box (70Mhz).

author: Fungus
author_rank: Super Contributor
timestamp: October 26, 2023, 10:39:12 pm
content: Quote from: Mechatrommer on October 26, 2023, 10:21:01 pmThe sinc function is only averyminor player here.Quote from: Mechatrommer on October 26, 2023, 10:21:01 pmsincisn'tthe problem, lack of bandwidth is.PS: There won't be a way to turn it off. It would be a disaster to just draw lines between the dots.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 26, 2023, 10:40:01 pm
content: Quote from: Mechatrommer on October 26, 2023, 07:15:38 pmYes it cetainly can. Ask anyone* who is competent to design high quality low pass filters for video signals. Passive circuits with capacitors and inductors.( * I am not such a designer but I worked with those who were.)

author: TimFox
author_rank: Super Contributor
timestamp: October 26, 2023, 10:53:42 pm
content: "Non causal" usually means that the output can chang before (in time) the input does: not realizable in the time domain.https://en.wikipedia.org/wiki/Causal_filterA causal filter's output depends only on past and present inputs, not future values of the input.Digital filters operating on a data set can be non-causal.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 11:15:25 pm
content: Quote from: Fungus on October 26, 2023, 10:39:12 pmso then my 230MHz "DHO924" is in better hand than your 100MHz "DHO814"Quote from: Fungus on October 26, 2023, 10:39:12 pmthats too bad. DS1000Z can be switched off. this is useful to visualize and verify (qualitative) what we are seeing is true or possibly a false interpolation. what kind of disaster if we turn it off? at worst we see aliasing. or apparent low BW signal.. interpolation is meant to attempt reconstruct accurate representation, but its proven its not. you are on the side that think the "non-causal"/precursor/gibbs ear ringing is for real do you?i'm on the side with R&S and Lecroyare you saying directly download raw sampled data and plot point to point in PC is a disaster? i've had fair amount of time doing that, apart from sharp edges/triangulated plot instead of nice curvy interpolated plot, there is nothing to be afraid of, brain can certainly make better judgement

author: EEVblog
author_rank: Unknown
timestamp: October 26, 2023, 11:16:20 pm
content: Quote from: ebastler on October 26, 2023, 07:44:02 pmI showed in my review video two R&S scope responses, MXO4 and RTB2000 and both of them matched the DHO800. The Siglent was indeed the outlier.The MXO4 is a 1.5GHz bandwith scope, so it should show everything on the Leo Bodnar generator warts and all, yet there is nothing there.The Siglent clearly has the problem.I can do more scopes if needed, but if I'm going to trust any scope in my lab it's the MXO44.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 11:29:44 pm
content: Quote from: EEVblog on October 26, 2023, 11:16:20 pmits clear to me now so far, the problem is imperfect sinc reconstruction. on both siglent and rigol if condition is met. so the argument if this scope is more accurate representation than the others is moot.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2023, 11:47:39 pm
content: Quote from: Fungus on October 26, 2023, 11:39:50 pmcan we upgrade to DHO824 meh? i read its impossible, there is no BT7TO20 option, or did i get outdated? btw i saw your leo pulse when hacking to DHO924, its the same with me now (extra overshoot) but last time i read you downgraded to 100MHz DHO814 right?Quote from: Fungus on October 26, 2023, 11:39:50 pmaliasing on fast risetime square 10MHz pulse? i suspect you are confused right now. or am i misinterpreting?

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 12:04:07 am
content: Quote from: EEVblog on October 26, 2023, 11:16:20 pmMr Bodnar sees otherwise as does my 10 GSa/s 2 GHz SDS6204A.From Leo's website a capture from a fast Tek clearly displaying the Gibbs ears other scopes choose to suppress.Better add the falling edge screenshot for the sake of completeness.

author: Fungus
author_rank: Super Contributor
timestamp: October 27, 2023, 12:04:31 am
content: Quote from: Mechatrommer on October 26, 2023, 11:47:39 pmYep.Measured bandwidth is 200Mhz with the BW7T10 option. I'm not 100% sure what's going on inside the 'scope but I like it...I don't need 250Mhz so this keeps it a little bit further away from Nyquist. I also enjoy the lack of junk on screen and the perfect calibration.Quote from: Mechatrommer on October 26, 2023, 11:47:39 pmWith 250Mhz+ analog bandwidth and 312.5MSa/sec. sample rate? Yes, you'll get bad aliasing on a pulse.There's a simple way to know if you're seeing aliasing: Turn some channels off and see if the displayed waveform changes.(assuming you're not STOPped...)

author: wasedadoc
author_rank: Super Contributor
timestamp: October 27, 2023, 12:07:53 am
content: Quote from: TimFox on October 26, 2023, 10:53:42 pmIt is necessary to compare the relative time of filter input and output. Agreed the filter output cannot begin to ring before the time of the input transition. But looking at the output of the Ls and Cs filter there can be preshoot or ring before the main transition. Perhaps that does not meet the strict definition of "non-causal" but has the appearance of non-causal. The screenshots under discussion are such. Only output waveform is shown.

author: boggis the cat
author_rank: Regular Contributor
timestamp: October 27, 2023, 12:22:06 am
content: Quote from: EEVblog on October 26, 2023, 11:16:20 pmHave you tried exchanging the probes between scopes?The probes are part of the measurement system.

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 12:32:21 am
content: Quote from: boggis the cat on October 27, 2023, 12:22:06 amBodnar pulser connects directly to the BNC channel input.

author: Fungus
author_rank: Super Contributor
timestamp: October 27, 2023, 12:33:01 am
content: Quote from: tautech on October 27, 2023, 12:04:07 amWeird that it doesn't have the "pre" ears like the Siglents do...It actually looks a lot closer to the little Rigol DHO800's response than the Siglent's:

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 12:41:55 am
content: Yes well at 100ps/div like Mech showed earlier much more becomes visible than any slow scope displays.

author: Fungus
author_rank: Super Contributor
timestamp: October 27, 2023, 12:44:04 am
content: Quote from: tautech on October 27, 2023, 12:41:55 amSimilar to a fancy Lecroy as well? That's cool.Edit: I'm not picking sides and saying which philosophy is "best" but I think it's cool that my little Rigol behaves like the big expensive scopes.

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 01:00:09 am
content: Quote from: Fungus on October 27, 2023, 12:44:04 amAt 5ns/div what do you see 400ps before the rising edge ? Zip, zero, nada !

author: asmi
author_rank: Super Contributor
timestamp: October 27, 2023, 01:00:28 am
content: Quote from: tautech on October 27, 2023, 12:04:07 amYou would do really well to get some reading comprehension. The discussion is about ringing BEFORE pulse, not AFTER it, and your screens shows none of it. Which is the way it's supposed to be.

author: Fungus
author_rank: Super Contributor
timestamp: October 27, 2023, 01:14:23 am
content: Quote from: tautech on October 27, 2023, 01:00:09 amYep, and that's exactly what I want when I've got a good sample rate to bandwidth ratio (ie. with a single channel enabled).Would I like a sharper, Siglent-like bandwidth rolloff when I turn on more channels and the ratio changess? Yes, but I can't have everything.That's the dilemma... the choice a manufacturer has to make when they design an oscilloscope front end.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 05:32:10 am
content: Quote from: tautech on October 27, 2023, 12:04:07 am1) gibbs ears are not "after the EVENT" (rising or falling edge), gibbs ears are before the "EVENT".2) although overshoot and undershoot after the "EVENT" are similar (symetric) appearance. thats why its easily confused, if not read carefully.2) overshoot and undershoot are real, gibbs ears are not...3) overshoot and undershoot are "causal" phenomena, caused by the rising and falling EVENT, gibbs ears, if it really exist in reality, are "non-causal"4) gibbs ear are fabricated in mathematical post processing in DSP IC due to imperfect (not infinite) terms sinc interpolation.5) you yourself demonstrated the gibbs ear by showing 2c captures, one is using X, another is sinc interpolation, but it turned out you are looking at the wrong side (the actual post-event) of "gibbs ear"you have similar confusion as Fungus who are mistakenly identify Gibbs ear as aliasing, thats why we keep reacting round and round... here again, on the correct side (the unrealistic, the DSP fabricated) of the gibbs ear i was talking about. not the other side...here again (attached) i should have provided more proper picture, i thought i was clear enough (my mistake). and esp to sales representative need to get this right. cheers

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 05:52:10 am
content: Quote from: Fungus on October 27, 2023, 12:04:31 amaliasing is false low freq representation of actual high frequency element above nyquist/2.5 leo bodnar test shows no high frequency ringing, so no, i dont see any aliasing. either you look at the wrong side, or misinterpret DSP math post-processing with aliasing. aliasing will not caused non-causality... the picture you showed, is math post-processing, unless you can give better proof, fwiw..

author: Fungus
author_rank: Super Contributor
timestamp: October 27, 2023, 06:04:03 am
content: Quote from: Mechatrommer on October 27, 2023, 05:32:10 amNope.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)

author: ebastler
author_rank: Super Contributor
timestamp: October 27, 2023, 06:16:57 am
content: Quote from: Fungus on October 27, 2023, 06:04:03 amThanks. And actually it's "nope" to most of the other statements in Mechatrommer's list of claims in that post. But I think we have already beaten this to death here and this discussion is going nowhere.Could we all agree that the DHO800 pulse response is fine, and go back to the scheduled program?(Edit: For clarity, I consider the Siglent pulse response fine as well. But that is the controversy over the past X pages of the thread, and the part where I think the discussion with Mechatrommer is not getting anywhere. And it is off-topic in a "DHO800 unbox and teardown" thread.)

author: Serg65536
author_rank: Regular Contributor
timestamp: October 27, 2023, 06:36:34 am
content: Quote from: Mechatrommer on October 27, 2023, 05:32:10 amThat'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

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 07:11:28 am
content: Quote from: Fungus on October 27, 2023, 06:04:03 ami 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 itthis 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 limitedThis is what digital filters do when bw (parameters/terms) limitedThey 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 predictionI 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

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 01:39:51 pm
content: Quote from: Serg65536 on October 27, 2023, 06:36:34 amthere 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?Quotedo you know what is fourier series? and your linear phase filter is also the digital onehttps://en.wikipedia.org/wiki/Linear_phasehttps://en.wikipedia.org/wiki/Finite_impulse_responseyou 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?

author: iMo
author_rank: Super Contributor
timestamp: October 27, 2023, 02:03:10 pm
content: 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

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 02:25:41 pm
content: Quote from: iMo on October 27, 2023, 02:03:10 pmyes 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 inhttps://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5121846/#msg5121846on 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

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 02:33:50 pm
content: Quote from: Mechatrommer on October 27, 2023, 02:25:41 pmYou have been busy digging out my previously posted screenshots from SDS6204A.

author: iMo
author_rank: Super Contributor
timestamp: October 27, 2023, 02:34:43 pm
content: That is the same as this example:I give you 0 apples on Sunday, 1 apple on Monday, 2 apples on Tuesday, 3 apples on Wednesday.On Thursday I calculate the average (for example), like (0+1+2+3)/4 = 1.5 and I write on your DHO804 LCD "You got from me 1.5 apples per day since Sunday".Where the 1.5 apple on Sunday came from ??

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 02:46:08 pm
content: Quote from: tautech on October 27, 2023, 02:33:50 pmOn to perusing this threadhttps://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5136138/#msg5136138then i came across some "hilarious" posts imho thats difficult for me to let gonot particularly you, but whoever did thateven now i have to face one of rigol fanboyismwill that made me change side? no! i'm just here, on my own sidecheers.

author: 2N3055
author_rank: Super Contributor
timestamp: October 27, 2023, 03:29:04 pm
content: Quote from: Mechatrommer on October 27, 2023, 02:25:41 pmCould you please stop twisting what people said and misrepresenting data? Please?You are taking images I made for specific purpose with very specific set of parameters to illustrate something.Then you use that image and say it is something else and babble about evil Sinc reconstruction filter..It has been explained in 1948. Long ago..What I have shown is what happens when you undersample square wave signal, how it looks.I used 16 MHz deliberately to illustrate point of very common signal (Arduino clock), being square wave, will have much larger frequency content of 16 MHz. This is common mistake with beginners, they buy 100 MHz BW scope to look at 50 MHz square wave because it is only 50 MHz, right.With square wave, how much frequency BW is there in signal is visible in rise/fall time of the signal.You can have square wave signal that has repetition frequency of 1 kHz, that will have 4GHz BW of spectrum..16 MHz square wave (with 30ps edges) will have harmonics into gigahertz range. Any of these harmonics above nyquist will alias. And then at reconstruction will be mixed back into waveform as lower frequencies, distorting pulse shape.Undersampling will also result in not enough timing accuracy for digital triggering. That will result in trigger jitter.So combine that with changed pulse shape and you get that shape.Scope cannot violate math and physics. But it is not ideal representation of infinite functions.In attachment:10 MHz square wave on:Keysight 3104T : 1GHz full BW 1ns/div

author: nctnico
author_rank: Super Contributor
timestamp: October 27, 2023, 03:29:21 pm
content: Quote from: Mechatrommer on October 27, 2023, 02:25:41 pmNo. Dot and vector mode will show a signal which is even further from what the signal looks like in reality. As I wrote before: if you want to prevent Gibb's ears then make sure the signal has a limited bandwidth.

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2023, 03:32:35 pm
content: Quote from: 2N3055 on October 27, 2023, 03:29:04 pmFat fuzzy traces.

author: 2N3055
author_rank: Super Contributor
timestamp: October 27, 2023, 03:54:33 pm
content: Quote from: tautech on October 27, 2023, 03:32:35 pmYep. But exactly same response as SDS6104.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 03:59:43 pm
content: Legends Never Die - Rigol DHO924S (ermmm... 804) looking at 100MHz RF signal (EraSynth Micro) with amplitude -70dBm to 10dBm

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 04:09:01 pm
content: We must remember a DSO is NOT a scope in the sense of the older analog scopes like the ones Tek provided, these portray signals in a more "realistic" sense because they are "almost" real time, "almost" in the perspective of just slightly delayed in time to the signals occurring at the probe tip which is shown as a electron beam sweeping across a screen (not considering triggering and retracing). Note the signal of interest is delayed, scaled/amplified and filtered but NOT sampled as in a DSO, and some non-linear effects may be introduced along the way to the screen for viewing. Whereas a DSO is actually a data acquisition system, disguised as a scope which captures data with Analog to Digital Conversion with sampling techniques (both of which are non-linear functions). The sampled waveform data stored (just digital bits now) in memory and displays a rendering of the data (digital bits) captured in memory with manipulations on a computer type screen (pixels) to render what the DSO "thinks" was the original waveform in a from the user prefers. Totally different than the phosphor based screen with sweeping electron beams used with analog scopes. Here, the DSO "thinks" involves all sorts of digital data manipulation, some user requested, some not, and varies highly with DSO brand, settings, and development date.If one thinks for minute an ideal squarewave has infinite harmonics, which the analog scope low pass filters and displays as the pulse step response on each edge, thus some of the harmonics are attenuated, and higher one towards obscurity. We often think of this "response" as the Step response of the filter, but can also be viewed as a consequence of affecting the harmonics in the original waveform step. However because these harmonics are filtered in real time by the analog filters, and displayed by the pseudo-real time analog screen, the resultant display possess no magic non-causal effects (Gibbs ears).A DSO behaves quite differently as mentioned being a sampled data system, and displaying results long after the fact and highly manipulated. Especially true wrt to the actual display, and how it's rendered, which involves "waveform reconstruction" in the sense of the discrete time digital data is arranged/manipulated to display an apparent continuous waveform to the user by means display "filters". One must remember the DSO screen is NOT continuous in time nor amplitude, but yet another discrete X and Y system, and the sampled signal digital data which is discrete in time (X) and amplitude (Y) must be "fit" into the display area for the user to interpret!!Regarding the Gibbs ears, this is a manifestation of the digital rendering of the captured analog signal by the non-linear processes of Analog to Digital Conversion and Discrete Time Sampling, with the effects of both analog and digital filtering involved, and manipulation for displaying on the DSO screen to add further confusion.So it should be no surprise there are significant differences between the two methods of time domain waveform displaying. One is entirely continuous in time and amplitude (analog), the other is highly discontinuous in time and amplitude (DSO).Best,

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 04:39:04 pm
content: Quote from: 2N3055 on October 27, 2023, 03:29:04 pmAgree, very common for folks to "think" a squarewave is just a fundamental waveform with a few "Harmonics" at 3X and 5X, whereas "seasoned" folks know the fast edge speed can yield results well into the Multi-GHz regions!!A good question to ask, "what's the frequency content of a Step function, or an Impulse function!!!"Best,

author: nctnico
author_rank: Super Contributor
timestamp: October 27, 2023, 04:43:31 pm
content: Quote from: 2N3055 on October 27, 2023, 03:29:04 pmIn theory yes, but given the spacing between harmonics and their levels, the chance of harmonics aliasing back as a visual artifact are not very high as you have to consider the anti-aliasing filter in a DSO. Especially when dealing with a square wave which has mostly odd harmonics which are spaced at twice the fundamental frequency.

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2023, 04:53:09 pm
content: Also, the odd harmonics in a symmetric square wave fall off as 1/nin amplitude (voltage), so they eventually fall below the noise in the system.

author: 2N3055
author_rank: Super Contributor
timestamp: October 27, 2023, 05:11:46 pm
content: Quote from: nctnico on October 27, 2023, 04:43:31 pmProblem is that they will all get compressed in same energy "blob" and will add up.As they get folded down in frequency domain. Equivalent in time domain is, if you can imagine a small differential triangle formed by fast edge and edge of risetime of scope. Area of that triangle gets folded on top of the rising edge with a overshoot pulse that has period of 1/risetime....If energy enters the ADC it will show somewhere..

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 05:22:39 pm
content: One might be temped to create the "Gibbs ears" by means of a fast edge squarewave, then low pass filtering the waveform to reduce/eliminate higher order harmonics to create the "ears" as shown in earlier computer generated visualizations of adding harmonics to create a squarewave approximation.If we can agree the analog scope does not "self-inflict" the Gibbs effect, then view on the analog scope and DSO at various settings.This exercise is left to the reader!!!Best,

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2023, 05:27:37 pm
content: Of course, the output waveform of a "fast" square wave (e.g., the output of a good binary flip-flop) passed through an elementary low-pass filter (e.g., a simple one-pole R-C circuit) should not exhibit the Gibbs phenomenon on a high-bandwidth analog oscilloscope or a fast-enough sampling digital oscilloscope.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 05:28:08 pm
content: Quote from: nctnico on October 27, 2023, 04:43:31 pma perfect square wave is just that... from 0 suddenly 1 (or whatever Vmax it is, dont be pedantic), sampling rate is just that... sampling at certain dt interval. too slow sampling rate, the machine will miss the details during the step. plotting straight line point to point will just looks like trapezoid, or slowly risetime square, or at worst a bunch of triangles. add signal reconstruction/interpolation to it, the imperfect one, thats where the devil is where people misunderstand it "aliasing", in fact it is just a digital post processing artifacts. maybe people got deluded due to they think in frequency domain, not in time domain, where the samples capturing happens.as long as "fundamentals" is not exceeding nyquist limit, there will be no aliasing. granted the displayed squares will be distorted here and there, but not aliased, maybe due to jittery square wave on circuit, or just that.... artifacts from imperfect interpolation. even if it is indeed an aliasing, if harmonics amplitudes are not too much, it could be unimportant in digital circuit, otherwise the dut square wave got problems, too much oscillations and all, in this case, to verify we must go up the DSO ladder to 10-100GSps class. on the subject of adding LP filter for the sake of antialiasing, the true signal's shape will get hidden in the process, this is what low end DSO is about, we wont know if the signal is good or oscillates in real life.fwiw.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 27, 2023, 05:33:58 pm
content: Quote from: TimFox on October 27, 2023, 05:27:37 pmTrue, but a more complex LC filter with linear phase response can be physically constructed which will show the phenomenon.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 05:40:43 pm
content: Quote from: wasedadoc on October 27, 2023, 05:33:58 pmdo siglent and rigol enough room at front end for that? or do they have that actually already? experienced people should already spotted that from tear down photos. and why the effect disappears on screen at max sampling rate? and reappears on much lower sampling rate?

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 05:50:24 pm
content: Quote from: TimFox on October 27, 2023, 05:27:37 pmWill throw this out for discussion, altho deviating from the DHO800, but seems lots of folks are involved, so maybe worthwhile.Think it can be stated that ANY system described by continuous domain IIR Response of ANY kind, be that low pass, band pass, high pass, ANY combinations of poles and zeros, or whatever can NOT exhibit the Gibbs effect on an ideal squarewave.This is stated without any proof (we left the DSP world long long ago at it's early beginnings).Will be interesting to see what some of the True DSP gurus have to say!!!BTW this is not to start/continue an argument, but to maybe add some clarity to the on going discussions.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 05:54:58 pm
content: Quote from: wasedadoc on October 27, 2023, 05:33:58 pmPlease elaborate on such, see above.Very interested in seeing if this is possible with an IIR system.Best,

author: ebastler
author_rank: Super Contributor
timestamp: October 27, 2023, 05:59:05 pm
content: To the moderators (I will report this post to aks for their consideration):Any chance the discussion from reply #1602 onwards could be moved to a separate thread? This thread is getting very long and hard to keep track of anyway, and the several pages of philosophical discussion on pulse response have nothing to do with the DHO800.EDIT: And it's done. A big "Thank-you" to the moderators!

author: mawyatt
author_rank: Super Contributor
timestamp: October 27, 2023, 06:12:52 pm
content: Probably a good idea. The discussions involving Nyquist, Gibbs, and Causality perked our interest from way back when we turned Nyquist around into an ally.Best,

author: nctnico
author_rank: Super Contributor
timestamp: October 27, 2023, 06:16:09 pm
content: Quote from: mawyatt on October 27, 2023, 05:50:24 pmI agree. Gibbs effect is about approximating a square wave by adding a series of sine waves which is what the sin x /x function starts to struggle with at some point (say a math problem). Showing a square wave in a bandwidth limited form is different; there simply is not enough bandwidth to display all the harmonics (say a physics problem). IMHO these are two entirely different issues and should not be confused.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2023, 06:39:33 pm
content: i should have provided a "literature" instead of putting it out my broken english mouth...https://resources.altium.com/p/how-gibbs-phenomenon-produces-measurement-artifactsQuoteQuote from: ebastler on October 27, 2023, 05:59:05 pmthat will be a "task" for the mods... i believe there is already long discussion on these subject in a siglent thread, i thought these all are sorted out among siglent enthusiasts, but i guess not, they came here with false "attack" on signal accuracy and aliasing and all etc...

author: Serg65536
author_rank: Regular Contributor
timestamp: October 27, 2023, 08:15:18 pm
content: Quote from: Mechatrommer on October 27, 2023, 05:32:10 amThat's definitely the sinc interpolation effect! On this scale it shows only 100 samples per screen. Ripple waves are exactly between the real samples.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 02:27:15 am
content: Quote from: Mechatrommer on October 27, 2023, 02:25:41 pmTo me that looks exactly like a bandwidth limited square wave:Image from here:https://www.nayuki.io/page/band-limited-square-wavesNothing to do with sinc function.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 07:03:03 am
content: Quote from: Fungus on October 28, 2023, 02:27:15 amDIGITALLYbandwidth limited.. it never happened on the actual circuit. but if you like "made up representation", then thats your personal preference i cant do anything about.https://resources.altium.com/p/how-gibbs-phenomenon-produces-measurement-artifactsQuote from: Serg65536 on October 27, 2023, 08:15:18 pmthats why dot or vector (sinc interpolation disabled) rendering can be useful so we know what we are dealing with. if these "educational" scope with "Default" button really meant for educational, they should have dot or vector plotting. imho.ps: wow! its possible to move a bunch of posts into another thread! amazing! i didnt know.

author: tautech
author_rank: Super Contributor
timestamp: October 28, 2023, 07:28:31 am
content: Quote from: Mechatrommer on October 28, 2023, 07:03:03 amA rarely given privilege. Enjoy.However as OP you can use the Move Topic to shift a whole thread anywhere in the forum just don't go to the Supporters Lounge or you need a Mod to rescue you....don't ask how I know !

author: ebastler
author_rank: Super Contributor
timestamp: October 28, 2023, 07:54:07 am
content: Quote from: Mechatrommer on October 28, 2023, 07:03:03 amA big thank-you to the helpful moderator who made this happen!

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2023, 08:34:27 am
content: Quote from: Mechatrommer on October 27, 2023, 05:28:08 pmIn my opinion, this statement is a bit false.But of course.When the system does not produce an alias frequency from fundamental (aka 1st harmonic) then fundamental is not aliasing.When harmonics are downconverted by ADC these down conversion products are there. And these down converted "false" signals we call a bit carelessly aliases. This is true with all harmonics, including also 1st. 1st harmonic aka fundamental.When you drive some waveform that contains harmonics that cross the fNyquist "wall" these are converted down and these all down conversions are alias frequencies from these harmonics (every harmonic is harmonic but also we can handle higher than 1st harmonic as "its own fundamental". But down conversion products they are in this case aliases from these harmonics.Take 11MHz "square" and look harmonics in system. 1st harmonic (aka fundamental) is 11MHz, 3rd harmonic is 33MHz and 5th is 55MHz, etc. (and now important thing, all harmonics, including also 1st (aka fundamental) can alias.Now sample it using 50MSa/s.There is fundamental aka 1st harmonic. 11MHz in this case not aliasingThere is now 17MHz which is alias (conversion) from 3rdThere is also 5MHz which is alias from 5th.Fundamental is not aliasing. Yes this is now of course true. Still system produces aliases from these harmonics. In this case all other harmonics are aliasing but not 1st harmonic. (but then totally other case is how we see these aliases and there is big difference if we look in time domain or or in frequency domain.)Naturally if also fundamental is aliasing then we see directly this aliasing and dramatic figures on time domain display. When harmonics are aliasing we see different square wave what is input. If we filter out harmonics what are higher than fNyquist also then we see different "square" what is our input but also different if we compare it to image what we get if other harmonics than fundamental is generating.Fundamental (1st harmonic) aliasing looks very very different than higher harmonics aliasing. Specially in oscilloscope time domain (digital oscilloscope of course because analog oscilloscope do not have any ADC based aliasing.Then... very different thing is what Sinc "reconstruction" do. It must not mix here with all fun Gibb's etc what must not mess with aliasing.Then we come to the oscilloscope time domain display.When fundamental is aliasing we can see its alias frequency. Example if fundamental is 15MHz and our fNyquist is 10MHz (sampling 20MSa/s) we can see 5MHz alias.I think we all know that ADC can be used for frequency conversion. (as it is also used somewhere) And now where is way to think it works differently for "fundamental" and harmonics. Because there is nothing but harmonics. Every harmonic can thing as it is "fundamental" and it behaves just as fundamental. (just filter fundamental out and use this harmonic as fundamental.)We can use 1GSa/s ADC so that we convert 995MHz signal to 5MHz but yes USB and LSB swapped.If we are doing even step decimation it also works same way.This is not for Siglent, not for Rigol, not for... long list of names. Brand names do not change physics or math, even if they want or hope. (except if they do mistakes or other way handle things wrong way)

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 09:47:30 am
content: Quote from: rf-loop on October 28, 2023, 08:34:27 amsorry if the "s" confuses you. i meant "fundamental" the 1st harmonic. i know your other arguments about 2nd, 3rd etc harmonics can alias, yes i agree, esp if signal has excessive ringing and overshoots. but few points to note so you can understand how i think:1) try drawing a square wave on paper, (a realistic signal you can think) with a little bit distortion in 2nd, 3rd etc harmonics... and then think yourself as sampling machine creating sampling dots a little higher rate than fundamental frequency * 2.5 and connecting lines among dots. fundamental will not alias, i think we both agree, you'll see trapeziods or triangles that still can (barely) capture data (if its like digital serial data transmission). now increase a little bit sampling rate just below 2nd harmonics / 2... yes 2nd harmonics will alias, but when you connect the dots, you can still see square waves closer to original signal and if its serial transmission, you can still recover its data content.2) now on to 2nd, 3rd etc harmonics. if they alias so badly up to a point rendering fundamental (1st harmonics) unrecognizable, then we have problem with signal integrity, reflection, mismatch and all... or analog signal distortion or oscillations so badly, yes we need higher priced DSO. if we dont have money, we can low down our cheap DSO to single channel only at max sampling rate to tackle problematic signal one by one. but since discussion is on a "fairly good" square waves, then imho aliasing will not affect much our judgement (given the bad Sinc interpolation is turned off)your points are valid on generally all type of analog signals (digital also analog), but we focus on digital signal like mcu clock presented use case earlier, andverifiedLeo Bodnar pulse with fairly good square wave and no problem with ringing and mismatch. and btw as you also agreed, Sinc can get thing much worse by fabricating what never happened there, amplifying alias that should not be, masking actual signal integrity and all as the article stated... ymmv.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 10:17:36 am
content: Quote from: Mechatrommer on October 28, 2023, 09:47:30 amBut that's not sincs fault. The problem is undersampling.Turning off sinc is NOT a good idea. Learning to interpret what you see IS.

author: nctnico
author_rank: Super Contributor
timestamp: October 28, 2023, 10:22:06 am
content: Quote from: Mechatrommer on October 28, 2023, 07:03:03 amIMHO that article is wrong about what the Gibbs effect is in DSOs. This Wikipedia article is more accurate and shows it is a math problem; when approximation a square wave using fourier series, you get a signal which doesn't exist:https://en.wikipedia.org/wiki/Gibbs_phenomenon

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 01:19:10 pm
content: Quote from: nctnico on October 28, 2023, 10:22:06 amand thats what a Sinc (or whatever digital filter there is) in scope does! Try to approximate (interpolate) what points in between actual sampled points.Turn off sinc, turn on dot mode, calculate how many points and sample rate there is. If you are lucky, and the scope is not transparent enough, you'll be surprised dho800 or low entry siglent can do 10GSps on screen on single channel smallest time div. Where do all those extra points come from? Sampled from actual event? Or simply.. approximated? If your eyes sharp enough, Lecroy sda6000 snapshot i linked earlier did 100GSps.. wow? No!But if you are unlucky, the dots are raw based on sample rate used by dso, you wont see any gibbs anyway. Try it and prove me wrong

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 01:27:14 pm
content: Quote from: Fungus on October 28, 2023, 10:17:36 ami give up! You win! do whatever you like!

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2023, 01:38:12 pm
content: Quote from: Mechatrommer on October 28, 2023, 01:19:10 pmPlease, explain this part in bold.Do you know what you are saying or was it just what you just believe. My opinion is that religion is not for this forum.I have used and examined many (nearly all) Siglent SDS1000 models including the 1000X HD and all 2000 (2000, 2000X, 2000X Plus, 2000X HD) models... also some 6000 models.Based on these my experience and knowledge, your statement is not a general truth.Be it anyone, it would be nice to stay as close to the truth as possible.

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2023, 01:45:16 pm
content: Quote from: Fungus on October 28, 2023, 10:17:36 amIn many cases turning Sinc off (least momentarily and in some cases)is very good idea. But also, sadly, impossible in some oscilloscopes.But hey, just keep your bullshit generator running, you'll get good numbers in the message counter if you get any pleasure out of it.

author: mawyatt
author_rank: Super Contributor
timestamp: October 28, 2023, 01:47:57 pm
content: Quote from: Mechatrommer on October 28, 2023, 09:47:30 amNote a proper squarewave has no even harmonics, even a terrible rise/fall time "squarewave" at limit, a triangle wave if you will, has little or no even harmonics (triangle has no even harmonics). A pulse waveform can have even and odd harmonics of varying intensity depending on duty cycle, in the limit of "zero" pulse-width becomes a repetitive "impulse".We mention this because it seems many folks are a bit confused about this topic.Best,

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 01:59:34 pm
content: Quote from: rf-loop on October 28, 2023, 01:38:12 pmi dont own siglent, i only assume. But please! Show me gibbs effect in dot mode in siglent scope, a good scope should tell the truth about where its sampled points are located. What religion got to do here? Are you trolIing? empty talk without proof is that..

author: nctnico
author_rank: Super Contributor
timestamp: October 28, 2023, 02:06:09 pm
content: In dot / vector mode, you won't get the Gibbs effect because it comes with sin x /x reconstruction. Still, the sin x/x reconstructed signal is the closest you'll get to seeing something that resembles the real signal when you are getting near the nyquist limit. There is a tradeoff to be made by the user.

author: mawyatt
author_rank: Super Contributor
timestamp: October 28, 2023, 02:08:46 pm
content: Quote from: rf-loop on October 28, 2023, 08:34:27 amMany seem to forget an ADC is actually like an Impulse Sampler in the time domain (Zero order hold), or close approximation of such. This is true because the ADC produces a digital output rendering of the analog input signal at precise time interval sample points.This along with Nyquist, produces frequency translation similar to a classic heterodyne mixer from the RF world. We utilized this Nyquist "Feature" with custom ultra-high speed Sampling CCD Convolvers (Chirp Z Transform) which operated in various Nyquist "Zones" back in 1980 for a handheld RTSA we developed.The reverse is also true as one can employ Nyquist to "Produce" output signals from a DAC in various Nyquist frequency "zones", very powerful techniques indeed!!Best,

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 02:28:45 pm
content: Quote from: Mechatrommer on October 28, 2023, 01:27:14 pmWhat do you propose the 'scope should show instead?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 02:33:16 pm
content: As you (rf loop) also agree, this is not siglent vs rigol war.. this to increase awareness.. if you dont nitpick, you should see there i put rigol name there as well.. not for tug war, but thats 2 name we usually own, and S people usually come and degraded cheaper R based on delusion. I can as well ask you to do it on r&s, lecroy, tektronix et al, for the sake of technical/theoritical/academic discussion.. but probably hi end brands implements interpolation a bit differently.. please also do so whats on your bench and convenient to you.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 02:35:22 pm
content: Quote from: Fungus on October 28, 2023, 02:28:45 pmwhat good proposal is? if only for you to deny? And you keep putting 2 contradicting statements in one post. Amazing!

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 05:52:38 pm
content: here i tried to replicate2N3055's capture of Gibbs Effectin my DS1054Z. i dont have fast rise time 16MHz that can produce 10mVpp, but since the focus is on Gibbs Effect (whether its real or not?), the closest i can get is Leo Bodnar's pulse 10MHz @ 1Vpp. and then i figured i was actually struggling to get as low sampling rate i can get similar to 2N3055, i have to turn all channels ON and have to go up to 2us/div, reduce memory depth down to minimum 3Kpts/ch, then only i get 125MSps, but at this time/div, the cycles are too densed together to give any useful info for the purpose. so i have to dial back down 20ns/div and thats at 250MSps, thats the closest i can get. we cant set sampling rate to our heart content in Rigol, apart from reducing mem depth and increase time/div... other than that its automatic, maybe to avoid nasty Gibbs? who know?turn on Sinc, vector plot (sinc_on.jpg)turn off Sinc, dot mode (while running/capturing) (dot_on_run.jpg)turn off Sinc, dot mode (Stopped to better see points) (dot_on_stop.jpg)turn off Sinc, vector plot (sinc_off.jpg)Gibbs effect succesfully produced in (sinc_on.jpg), but when in dot mode (dot_on_run.jpg or dot_on_stop.jpg) Gibbs dissappear, if you try imaginatively (if you dont want to try it on paper) to connect straight line between point to point in dot mode, you wont see any Gibbs effect. Gibbs only appear when Sinc interpolator is turned on in scope. do you understand what i meant before posting?so tell me Mr sharp trained eyes Fungus (and others who have sharp eyes or brain) which signal do you like? or think its true happening on circuit?.. sinc_on.jpg? sinc_off.jpg? or dot_on_stop.jpg? or all true? because when i turn on Sinc, suddenly something on front end DSO pcb clicked and activate 8th order non-causal Chebysev?and can any S or other brand DSO to do this test? and demonstrate here? cheers..

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 06:20:28 pm
content: Quote from: nctnico on October 28, 2023, 02:06:09 pmno... see above...

author: gf
author_rank: Super Contributor
timestamp: October 28, 2023, 07:20:51 pm
content: Quote from: Mechatrommer on October 28, 2023, 05:52:38 pmI guess that your signal is undersampled. Then It is expected to happen. Consider it an aliasing artifact. A properly bandwidth-limited signal is reconstructed almost exactly by sinc reconstruction, but if the signal fed into the ADC violates the sampling theorem, then perfect reconstruction from the samples fails and the reconstructed signal looks different from the original signal you fed into the ADC.

author: gf
author_rank: Super Contributor
timestamp: October 28, 2023, 07:57:50 pm
content: Quote from: Fungus on October 28, 2023, 02:27:15 amThe relation is the inverse Fourier transform of the frequency response of a linear phase brickwall filter. It is a sinc function in the time domain. IOW, applying such a brickwall lowpass filter in the frequency domain is equivalent to a convolution with a sinc function in the time domain.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 08:21:22 pm
content: turn off Sinc...You can turn off sinc on a DS1054Z when there's 4 channels enabled but it replaces it with a different filter. You'renotseeing raw data.

author: 2N3055
author_rank: Super Contributor
timestamp: October 28, 2023, 08:23:21 pm
content: Quote from: Mechatrommer on October 28, 2023, 05:52:38 pmI repeatedly politely asked you to stop using my images and FALSELY claim what I said they mean and ascribe to them completely wrong conclusions you are spewing.I explained what they are, how they were made and what they mean.Your weird explanations are just that. Weird.You have some grasp about math theory behind and some grasp of how sampling and DSO internals works. Just enough to arrogantly think you know it all. Better than all of the rest of us, including, among others, PhD who designed ADC for living, some people that designed scopes, and other very good engineers.....Stop doing it and listen to explanation why are you wrong. You are behaving like flat earth people. There is no conspiracy, not everybody except you are stupid. It is simple. You are wrong.Gf very nicely, eloquently and very concisely said what is going on.That being said.You seem to be confused by dot mode.You arecorrectthat dot modecan(in some cases) better show signal shape when signal ispartiallyundersampled.Do you understand why?As an example I can show 10 MHz square wave signal, sampled at very low 25MS/s in dot mode. Signal is band limited with 200 MHz filter, so there are no frequencies (odd harmonics up to 19th in this case) above that. Simple edge trigger. Scope is my Siglent SDS6000.Reference image is same signal, just sampled at 5GS/s.Signal sampled with 25MS/s is perfectly reconstructed.Can you explain why?Let's start with that. Then we will proceed from there. Explain that first.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 08:37:34 pm
content: Quote from: Mechatrommer on October 28, 2023, 05:52:38 pmI still have no idea what you're trying to prove, but:When I see a signal like that my first thought is "square wave". My second thought is to turn off some other channels to get a better picture.At no point do I think that's what's happening on the circuit - a circuitcannotpredict the future as that signal implies.Nor do I think "sinc is evil, I wish I could turn it off".Sinc artifacts are useful because "pre-ringing" is a visible warning that your sample rate is too low. They also give you a rough idea of how much "post-ringing" can be ignored.My DHO800 even has a measurement called "preshoot" to measure the value.IIRC your DS1054Z has it too.This is the same signal with only CH1 enabled. Preshoot is gone sonowI can start to believe what I see on screen.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 08:51:06 pm
content: Quote from: Fungus on October 28, 2023, 08:37:34 pmpredicting actual event from false event? ok good for you, thats new. thats not how we work... but then i believe you've stated you want to avoid such artifact as much as you can by lowering scope's BW or in need of more sampling rate?... you can stick with 100MHz BW and be safe, cheers.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 08:52:19 pm
content: Quote from: gf on October 28, 2023, 07:20:51 pmyes! so far so good!... i hope people like N03* and tau* get it as you do.... i believe some sharp people already understand this... now on to why i raised this issue? because i saw arguments that can be confusing to other people such as Fung* that lead to what i believe a poor decision or reasoning....argument 1) DHO900 is not properly designed because at its rated BW (230MHz) can alias when all channels turned on (312.5MSps)my answer:1) its true, but why need to push DHO900 to see at its rated BW when you want 4 channels? you know its only 312.5MSps, lets just do < 125MHz circuit. if you want to include circuit integrity or rule out possible aliasing in digital circuit, lets go down further to 30-12Mbps digital system/clock. thats what the limit is (need proper knowledge though, newbies can be trapped, who cant?) why expect more than this? why want to push DHO900's metric to be on par with other more expensive scope? i say this as DHO804 owner that upgraded to DHO924... from cheapest properly designed, to save money from hack but improperly designed scope?2) 230MHz BW limitation has its use instead of cutting it off in circuit further with analog LP filter to the "proper" nyquist BW of 125MHz will cripple some of our ability to debug circuit. we can use 1 or 2 channels "properly" on that BW... oscillations or unexpected circuit behavior will be hidden from users before the behavior even reach the ADC, if BW crippled in first place. in DHO900, we can see fuzzy or nasty display (aliasing) indicating something went wrong. in properly cutoff DSO, we are happy with sharp and thin traces thinking the circuit works, in fact its already clamped down by analog front end LP filter. when tested with hi end scope, or suddenly circuit behave erratically during production, we will scratching head as to why?3) and this is relating to this topic... even if properly designed BW scope, there is no stopping it from experiencing aliasing, and worse, interpolated Gibbs effect. esp when you are probing much slower "square digital" frequency at lower sampling, maybe by accident lowering mem depth or time/div large enough. as my "properly designed" DS1054Z and N03*'s scope demonstrated. maybe the argument is only at DSO's max sampling rate, as answered in (1) why push it? and expect more than $400 scope? members in forum already give behavioral testing in tear down and comparison thread, and hence readers can make decision to buy. no need bashing to prove your point, we have some use for it (230MHz). but if argument is rigol didnt give proper explanation in spec sheet of DHO900 (about LA, sample rate and stuffs), then i agree.argument 2) Rigol scope representation is not accurate, it hides details of overshoots and undershoots here and theremy answer:its been lengthy posts from me try to prove that here, that even with your "shiny properly designed" scope can show false details that you think "accurate", and if Gibbs Effect is the sign of improperly designed scope as N03* tried to emulate and prove, then your scope is also not properly designed due to not cutting off frequencies above nyquist limit. how we know? because what you showed and claimed as accurate is in fact a Gibbs Effect that was never there in real circuit. and why Gibbs Effect is an indication of improperly designed scope? because N03* says so, because he demonstrated hard enough so.... here again, we go back to where we began.. cheers.

author: tautech
author_rank: Super Contributor
timestamp: October 28, 2023, 09:17:48 pm
content: Let me again present 2 GHz DSO 10GSa/s in DOT mode.BW and sampling rates are everything.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 09:18:47 pm
content: Quote from: Mechatrommer on October 28, 2023, 08:51:06 pmI'd say it's pretty darn close in your example image.Quote from: Mechatrommer on October 28, 2023, 08:51:06 pmI've thought about it.Right now I'm at 200Mhz because I figure 1.25GSa/sec. is plenty for two channels at that bandwidth.Dropping down to two channels from three/four when there's any doubts about what's on screen doesn't seem too painful.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 09:23:54 pm
content: Quote from: Mechatrommer on October 28, 2023, 08:52:19 pmI agree that you shouldn't push it but limiting the circuit isn't practical when you have pulses.The practical solution is to monitor the preshoot and turn off some channels when you have a lot of it and you need a better view of the circuit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 09:32:28 pm
content: Quote from: Fungus on October 28, 2023, 09:23:54 pmif i got preshoot, i'll turn off Sinc, if there is significant difference, i know its artifact, but if not, then i can guess the signal is real, or either undersampled, here we need to think carefully and if needed rig up experimental setup or simply, go find 10GSps scope. and turning off channels in case of suspicious is what exactly we do.. make sure there's written 1GSps too on screen. the keyword is, Know Thy Tools...Quote from: tautech on October 28, 2023, 09:17:48 pmgood! thats "more accurate"but thats not apple to apple anymore with DHO800/900... cheers.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 09:58:08 pm
content: Quote from: Mechatrommer on October 28, 2023, 09:32:28 pmThis still needs explaining though:

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 10:00:58 pm
content: Quote from: Fungus on October 28, 2023, 09:58:08 pmthats where turning Sinc off, or plot in dot mode, and dial down to proper timescale can give extra clue as to why. circuit debug is not simply looking at one screenshot and be happy. if ringing is same freq or more as sampled dots, thats probably interpolation artifact. but if one cycle of ringing sampled by 10 points or more and you see almost straight lines between dots then that probably real.

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2023, 10:03:34 pm
content: Quote from: Mechatrommer on October 28, 2023, 10:00:58 pmThatisin dot mode...(according to tautech)

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 10:05:44 pm
content: Quote from: Fungus on October 28, 2023, 10:03:34 pmthen that probably causal... a real signal. who knows whats inside semiconductors IC? that one is not as fancy as other examples presented before. someone mentioned high order "physical/analog/real" filter that can cause apparently non-causal ringing, but i have not be presented with one real life example, and i dont know if the scope has it. or what it does internally in DSP. probing with other scopes, compare and peer reviews may help.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 28, 2023, 10:23:00 pm
content: here talking about improperly designed scope? 6GHz BW SDA6000 @ 10GSps, what? (bought used probably phased out from company and out of cal)feel free to pick which one to believe, i would say... close enough... if you still got concerned, really you need more BW and sample rate, and a new one!

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2023, 01:09:29 am
content: Quote from: Fungus on October 28, 2023, 10:03:34 pmNo.The scope menu clearly shows the scope is in Dot mode.However this was arunningcapture, not one from Stop mode where many fewer dots get shown.Yet, even with a capture we can toggle to Vector or Sinc etc modes.There's good reason why we show full menus so to accurately display scope settings.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 07:04:51 am
content: Quote from: tautech on October 29, 2023, 01:09:29 amanalyzing those fewer dots may reveals whether the scope showing sampled data only, or sampled data along with interpolated data, like Lecroy snapshot is combination of vector and dots, but the dots are interpolated data.since at it, Rigol DHO800/900 cannot turn off Sinc, i searched last night.. this ispoint on them (not to forget the otherpoints to be fair, but thats OT, Fungus starts to cringei suspect) so i have to download and analyze data in PC as screenshot shows... lets analyze this "improperly designed" scope..1.jpg = the dso respond to a "pulse" (a reality approximation to a theoretical "step") here we dont see Gibbs, only under-damped front end respond2.jpg = reducing sample rate by enabling all channels, bingo! Gibbs Phenomenon. newbies need to be educated to avoid this delusion. or even long time users can usually forgot the fact. if not careful3.jpg = increasing time scale to see how Gibbs represent differently4.jpg = increasing time scale more, the "imaginary" part of Gibbs disappear, maybe got folded back (aliased) during interpolation process?5.jpg = this is where downloadable data to PC comes to rescue. vector plot without fancy Sinc shows how the signal was sampled. no Gibbs! this what i tried to explain.6.jpg = look closer at sampled data vectoredwith PC data available, one can do further fancy interpolation (or countless amount of analysis types) if needed, but i'm not a fancy guy, so vector plot is enough for me. i'll do interpolation and judgement in my head myself. imagine even if raw/sampled data pulled from scope is the one with interpolation/Gibbs, user will not be able to further analysis correctly, such as when plotting our own FFT (7.jpg.. there's bug or misconception i know Fnd'tl should be the 1st Hm) imagine a process... sample data -> fft -> sw bw limiting -> ifft -> pull data to PC -> fft -> display.... the resulting end FFT will looks funny i suspect.i've been searching for siglent programmability and sw/program example in pc to no available. since ds1000e or ds1000z i think. but recently i see siglent is following the path of using ni-visa driver as its API, now its a good step imho for me long time using ni-visa in rigol. lets hope there will be good sw to post process raw data from siglent's scope. as for now, i got cheap dho800/900 to play with, and its 12bits is the main reason i bite the bullet getting it, to further push boundary down the noise floor of my FFT. one funny thing i realized is i eventually hit the limit of my fft library memory points. it cannot do more than 25Mpts FFT analysis, maybe Windows is limiting memory quota for my program/library thread. so i cannot utilize fully the 50Mpts DHO900 memory. even more so for siglent with much larger memory.since its the lowest price and already familiar with the workflow thats why DHO800 is the chosen one. siglent? maybe later when i'm rich enough to spend money for minor minor things. back to my sw, i was planning to add LA/serial decoder (from long memory data) functionality 7 years ago, which the main drawback in DS1000Z toyish serial decoding function. but then i lost enthusiasm during development and leaved my VisaDSO as it be. since i dont use serial decoder much, and i already have cloned salae 8 channels if i really need the job done. if minor upgrade/fix thats not requires very long time to do, then i come back to VisaDSO.. like few weeks ago.with diy PC SW, there is no limit to where your imagination can go... (ahaks the marketing tone BS)... fwiw.

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2023, 07:10:59 am
content: Quote from: Mechatrommer on October 29, 2023, 07:04:51 amWant some special screenshot ?Just ask.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 07:20:27 am
content: Quote from: tautech on October 29, 2023, 07:10:59 amit will not hurt to post related stuffs. if you can plot as close as possible as Lecroy capture (vector + dots) or dots only maybe can add value to the thread or the "marketing"but for me, data gathered are already adequate, and my points are conveyed hopefully. cheers.

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2023, 07:34:20 am
content: Quote from: Mechatrommer on October 29, 2023, 07:20:27 amDots are the raw sampling points, nothing else. This I can prove.One complete period from the 10 Mhz Bodnar enough ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 07:49:52 am
content: it will not hurt to prove, but i trust you already. this is basic stuff which everyone already know. please do so if its not too much hassle for you (i hate asking people to do more than what i need) cheers.

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2023, 08:26:15 am
content: 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.Please stop this BS about dot interpolation.

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 09:10:43 am
content: Quote from: Mechatrommer on October 28, 2023, 10:05:44 pmSubstrates that can predict the future?

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 10:13:02 am
content: Quote from: Fungus on October 29, 2023, 09:10:43 amCome on, guys. A digital FIR filter (which may cause pre-ringing) cannot see into the future either when it processes a stream of digital amplitude values. It introduces a delay. The main edge appears delayed on its output, while some pre-ringing effects appear with less delay. No magic required.

author: gf
author_rank: Super Contributor
timestamp: October 29, 2023, 10:16:05 am
content: Quote from: tautech on October 29, 2023, 08:26:15 amDoes run mode superimpose multiple traces on the screen, even though Persistance is off?Apparently it does. How many traces?QuoteIn 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?

author: rf-loop
author_rank: Super Contributor
timestamp: October 29, 2023, 10:44:55 am
content: It goes (least in Siglent) just as in theoryLook Fungus... Sinc is predicting... hehSorry these very small real raw dots is bit difficult to see... specially after GIF animation processed.I have used Zoom mode for get more low samplerate just for make this thing more visible and also because at this time just now I do not have suitable signal source with enough fast edges so that can demonstrate this using higher speed. But how ever it do not change math.Acquisition and after then oscilloscope stop and visible only last single acquisition.After then changed modes: Dots only, Sinc, Linear.Siglent do not have this kind of linear mode where is added interpolated extra dots between real dots. Only line what connects sample dots.Sadly there is also not display mode where real dots can highlight when Sinc or Linear interp.In some situations it is very good if we have opportunity to turn Sinc off when ever, run time or stopped or captured to history buffer and independent of what are other settings (sadly there is also oscilloscopes where this is not possible).

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 11:03:41 am
content: Quote from: rf-loop on October 29, 2023, 10:44:55 amHappily: I don't know any where you can't have a sample rate far above the bandwidth of the input BNC.

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 11:10:57 am
content: 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

author: gf
author_rank: Super Contributor
timestamp: October 29, 2023, 12:11:57 pm
content: Quote from: Fungus on October 29, 2023, 11:10:57 amNot 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 12:26:22 pm
content: Quote from: tautech on October 29, 2023, 08:26:15 amyour tone is a bit too high for a representativedidnt 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...

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 01:08:33 pm
content: Quote from: Fungus on October 29, 2023, 09:10:43 amlooking 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.

author: 2N3055
author_rank: Super Contributor
timestamp: October 29, 2023, 01:29:41 pm
content: Quote from: Mechatrommer on October 29, 2023, 12:26:22 pmMy 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 01:51:51 pm
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 01:55:57 pm
content: Nope. Your scope is showing the real signal. Not 'virtual' Gibbs ears. The dead-giveaway is that the signal doesn't change between vector, dot and sin x /x mode.

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 01:57:34 pm
content: Quote from: wasedadoc on October 29, 2023, 01:51:51 pmJust a note: The trick of turning on three channels on a DS1054Z to enable the menu item that says "sin(x)/x on/off" doesn't really work.It replaces sin(x)/x with a different filter, but it's still a filter. You do NOT get raw data by doing that.It's much better to turn off channel three and get double the sample rate.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 01:59:41 pm
content: Quote from: 2N3055 on October 28, 2023, 08:23:21 pmsorry i miss this post... i was distracted by other postsfrom eyeballing your picture, sampling at 25MSps, nyquist limit is 50MHz, but you claimed 200MHz limited. by theory, its should already violates nyquist law and unable to reconstruct, yet you showed perfect reconstruction, somethings not right. is your scope can switch down to 50MHz BW at front end? i'm unable to make conclusion, not enough data and something not add up, such as....in your other "properly designed" scope, even 100MSps sampling on "properly cutoff" 200MHz scope, it cannot reconstruct 1MHz square accurately, we got nasty Gibbs! can you answer why? before i can make firm conclusion. here comparison:1) sampling 10MHz square, 25MSps = perfect2) sampling 1MHz square, 100MSps = badwhy? please elaborate. i suspect you hide some setup i'm not aware of... you didnt explain anything in your post where i usually linked.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:05:08 pm
content: Quote from: 2N3055 on October 29, 2023, 01:29:41 pm10GSps quad channel, 20GSps dual channel to be accurate, and i've disabled that feature in screenshot, you can see. where did you get SDA6000 upsampling from 5GS/s to 10GS/s? its also in the setup picture cant you see?Quote from: 2N3055 on October 29, 2023, 01:29:41 pmwho's insulting who? and who got insulted? (by facts) are you trolling so this thread can get locked? by involving your unstable emotion here? answer academically please.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:11:35 pm
content: Quote from: wasedadoc on October 29, 2023, 01:51:51 pmyes! i was about to sketch similar picture for Fungus to understand, but since you provided proven one, thanks! i didnt rule out the possibility of it existing... so thats why i asked for evidence. if we have yellow trace only, we could have mistakenly assume its non-causal Gibbs effect, but actually its causal real Gibbs. from picture its about 1us delay. mind to tell what filter is that? or if that feasible to put on DSO front end?

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 02:13:46 pm
content: Quote from: Mechatrommer on October 29, 2023, 02:11:35 pmAgain, what wasedadoc is showing are not Gibbs ears!

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 02:18:49 pm
content: Quote from: nctnico on October 29, 2023, 02:13:46 pmWhat are they then?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:19:40 pm
content: Quote from: nctnico on October 29, 2023, 02:13:46 pmdont be pedantic friend... if someone give your kid an elephant toy and they call it ohh its an elephant! and then you say, its not an elephant, elephant is in jungle, you screw it man..ok fine! it just looks like Gibbs ear, but not Gibbs ear... or else what proper term for it? ever-increasing-magnitude-apparently-seems-non-causal-ringing? cheers.

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 02:25:05 pm
content: Quote from: Mechatrommer on October 29, 2023, 02:11:35 pmThose aren't Gibbs ears.(because the circuit is NOT ringingbeforethe pulse occurs)

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:34:23 pm
content: Quote from: Fungus on October 29, 2023, 02:25:05 pmhere attached for your sharp and trained eyes... input is blue, but given we dont know it exists. similar if its like in the IC, who knows? yes its not Gibbs, but we could be mistakenly think is...

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 02:39:14 pm
content: Quote from: Mechatrommer on October 29, 2023, 02:19:40 pmFor the umpteenth time: Gibbs ears are never in the actual signal. They are a digital filtering artefact from sin x / x that can show up on the display of a DSO. But Gibbs ears are never present in a real world signal you feed into a DSO. So you can't make Gibbs ears appear by performing any kind of operation on a real signal.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:44:07 pm
content: Quote from: nctnico on October 29, 2023, 02:39:14 pmi created this issue remember? so i ought to do the reading, thanks for emphasizing that for mebtw elephant toy is not an elephant... cheer.

author: Fungus
author_rank: Super Contributor
timestamp: October 29, 2023, 02:53:40 pm
content: Quote from: Mechatrommer on October 29, 2023, 02:34:23 pmNo we couldn't, because weknowthat "Gibbs" doesn't happen in a circuit.Pre-ringing might, but that's not "Gibbs".

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 02:58:46 pm
content: Quote from: nctnico on October 29, 2023, 02:39:14 pmI dont think that is correct. If you build a higher-order, linear-phase lowpass filter and send a square wave through it -- frequency chosen such that e.g. the fundamental and the 3rd and 5th harmonic pass the filter -- what would you expect as the filter output signal?You might use something like the LTC1064-7, for example:https://www.analog.com/media/en/technical-documentation/data-sheets/lt10647fb.pdf

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 02:59:42 pm
content: Quote from: Fungus on October 29, 2023, 02:53:40 pmi know, but it happened on monitor, monitor as you saw in the attached picture is not circuit. i can show you what never happened and existed in this real world on your monitor

author: 2N3055
author_rank: Super Contributor
timestamp: October 29, 2023, 02:59:48 pm
content: Quote from: Mechatrommer on October 29, 2023, 01:59:41 pmStating that I'm hiding things is offensive, especially because it is your refusal to try to read and understand what other people are saying.I'm not hiding a thing, you pompous baboon...As I explained one is made with dot mode, other one is with Sin(x/x) interpolation...Do you, in your infinite wisdom and omnipotent knowledge know why it was possible?

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 03:01:14 pm
content: Quote from: Fungus on October 29, 2023, 02:53:40 pmPrey tell, what is the difference? If we filter a square wave to let only its fundamental and a couple of harmonics pass, and do so with a linear-phase lowpass filter -- in which way is the output signal not a direct demonstration of the Gibbs phenomenon?

author: rf-loop
author_rank: Super Contributor
timestamp: October 29, 2023, 03:23:09 pm
content: Quote from: 2N3055 on October 29, 2023, 02:59:48 pmAnd, to Mechatrommer.This dots mode, SARI (Sequential Acquistion Random Inteleaving), (not same as LeCroy RIS ut its litlle cousin) have explained and handled in many threads over years when we have handled these things about Siglent scopes.So you need not even think yourself, just look old threads. They ara allthere and multiple times and places.Also many years agohere.Sorry sides are finnish language but if you look images 9 and 10...(note signal frequency and samplerate)perhaps you hit it.And note. this old oscilloscope was quite slow in zoom mode and tb (but also this slow update give it better visible what happen therein random interleaved sequential acquisition (interleaving happen due to wfm/s speed where in same frame is overlayed many acquistions. In image 10 there is red highlighted ONE acquisition... and if scope is now faster etc there can be so many acq on one image frame that it looks nearly continuous line.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 03:24:51 pm
content: Quote from: 2N3055 on October 29, 2023, 02:59:48 pmi wrote "i suspect"Quote from: 2N3055 on October 29, 2023, 02:59:48 pmoic, so you are comparing elephant to apple then... i got distracted again... in that case... my answer is...1) there is no Sinc in dot mode, no wonder! there is no "reconstruction" actually happened in there, its just try to overlap REAL sampled data too many times, of course no Gibbs effect! read nctnico post. or even literatures i've provided earlier. Gibbs only happened when you turn on fancy interpolation such as Sinc. when we talk about "reconstruction", it means Sinc interpolation. not overlapping real data to many times, thats statistical. different animal! no Sinc, no "reconstruction" (interpolation) period. sorry if you dont understand that earlier.2) now 2nd pic Sinc is turned ON (5GSps), but 1st pic not (25MSps), thats why i said comparing elephant to apple. my answer is... at 2nd picture, at 5GSps is 0.2ns samples interval, that means 500 points for each cycle, you showed 5 cycles, meaning 2500 points, that means more dots than your monitor can handle, lets just assume for simplicity the dots are 1080 dots filling your monitor. even if there is Gibbs, its too small to see the interpolation on sub-pixel leveldo you have any idea what you are posting?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...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)or like this as an example except i need it at 200ns/div same as gibbs effect setup above.. except the settings i've mentioned.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 03:44:19 pm
content: Quote from: nctnico on October 29, 2023, 02:39:14 pmNonsense. Digital filtering and sin(x)/x are not the only ways to create Gibbs ears. I no longer have access to an analogue Cathode Ray Oscilloscope, but before 1980 (when Digital Sampling Oscilloscopes were but a gleam in someone's eye) I could observe the same effect when passing video test signals through a low pass filter and looking at the result on a Tektronix waveform monitor. (Similar to one in the photo.)

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 03:56:03 pm
content: If you are low-pass filtering, you are removing higher harmonics. The ringing on a bandwidth limited square wave are not Gibbs ears. Just less harmonics. Because Gibbs ears are a digital signal processing artefacts from using sin x /x reconstruction, you can't get these on an analog scope because the whole digital signal processing step isn't there.

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 04:06:49 pm
content: Quote from: nctnico on October 29, 2023, 03:56:03 pmMy definition of the Gibbs Phenomenon or "Gibbs ears" is the following:The effect of approximating a signal with sharp jumps (e.g. a square wave) by its partial Fourier series, i.e. by its fundamental frequency and a limited number of harmonics.Such an approximate representation can be generated in the real world in an additive way (by adding sine functions) or in a subtractive way (by starting with a square wave and removing higher harmonics via low-pass filtering). In either case, the resulting output is an example of the Gibbs phenomenon in my book, and will show the characteristic "ears".Your definition of the Gibbs Phenomenon seems to be different from mine. Could you state what it is, and where you got it from?

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 04:12:12 pm
content: Read the Wikipedia article carefully. It says that a square wave can not be constructed from a fourier series because a square wave is a discontinuous function. As a result you get artificial ringing near the edges. The discontinuity gets you artificial pre-ringing when using sin x /x reconstruction.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 04:16:14 pm
content: Quote from: nctnico on October 29, 2023, 03:56:03 pmtracing back history, it came from Fourier Series itself Jean-Baptiste Joseph Fourier (1768–1830), and then the Gibbs Phenomenon discovered by Henry Wilbraham (25 July 1825 – 13 February 1883)... so, since i guess Sinc is using some of Fourier derivative, so the effect as well... Sinc is too math complicated for me, but i have FFT library that can easily produce this effect by band limiting in digital (FFT) and do the inverse FFT. i dont care whether if its a "conceptual", "theoretical", or "mathematical" property, as long as we can mimick it in reality, then we call it that... just as singularity, we call black hole is a singularity, not because its the truth singularity, but thats a reality we dont understand about where all the mathematical formulations, scientifical theory and logics collapse. i dont have time with this literalism

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 04:17:25 pm
content: Quote from: nctnico on October 29, 2023, 04:12:12 pmyes you can get perfect square with fourier series, except the order N = infinity...

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 04:19:19 pm
content: Quote from: Mechatrommer on October 29, 2023, 04:17:25 pmNope.

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 04:19:46 pm
content: Quote from: nctnico on October 29, 2023, 04:12:12 pmSo how is this different from what I said? A square wave cannot be exactly represented as afiniteFourier series. If you truncate the series somewhere, you will see the Gibbs effect. The ringing near the edgesisthe Gibbs effect.A real-world sqaure wave is, of course, already an approximation of a "perfect" square wave, due to its limited risetime. Nevertheless, a sharp low-pass filter will remove higher harmonics and hence will truncate the Fourier series further. The resulting approximation of a square wave will show the Gibbs effect. (Provided that the low-pass filter treats the phases of the incoming harmonics in the right way.)Edit: Typos

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 04:23:05 pm
content: @nctnico -- somehow we have our wires crossed here, and it may just be a matter of assuming different definitions. I have given my definition of the Gibbs Phenomenon; I think it would really be helpful if you stated yours.Quote from: ebastler on October 29, 2023, 04:06:49 pm

author: 2N3055
author_rank: Super Contributor
timestamp: October 29, 2023, 04:23:23 pm
content: Quote from: Mechatrommer on October 29, 2023, 03:24:51 pmI give up...

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 04:24:46 pm
content: Quote from: nctnico on October 29, 2023, 03:56:03 pmNo. Just waiting for someone else who does have a completely analogue scope to produce the same result as I have shown.In the "additive method" used by mathematicians, Gibbs ears appear when you start with a fundamental frequency and add a non-infinite number of harmonics of suitable amplitude and phase.Fourier says that a real world square wave is a fundamental frequency plus many harmonics. Where "many" is enough to make the wave appear square.The low pass filter removes the higher harmonics. It is important that the filter is linear phase. That means the fundamental and the remaining harmonics suffer equal delay. What remains in this "subtractive method" is the same as the "additive method". They both result in Gibbs ears.

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 04:25:41 pm
content: Quote from: ebastler on October 29, 2023, 04:19:46 pmIt is not about truncating the series, you can never approach a perfect square wave using fourier series to start with. If you truncate the fourier series, you are left with a bandwidth limited signal.

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 04:28:35 pm
content: Quote from: nctnico on October 29, 2023, 04:25:41 pmI keep reading "that is not the Gibbs effect, because that is not the Gibbs effect" from you.Please state your definition, thanks!And the statement in bold is just plain wrong, under any definition, sorry. Look at the limit for pushing the number of Fourier components towards infinity, and measure e.g. an RMS deviation between the square wave and its Fourier series. What would you claim the limiting value is?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 04:29:54 pm
content: Quote from: nctnico on October 29, 2023, 04:19:19 pmthere are differences between theoretical science and applied scienceapplied scientists try to make theoritical applicable. one of it is FFT. knowing there are limitations what a real world can do, or what we can achieve, we know where to stop as "good enough". theoritical by itself is useless if it cannot be applied. imho ymmv.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 04:36:34 pm
content: Quote from: nctnico on October 29, 2023, 01:55:57 pmThat is not a dead-giveaway. The square wave frequency is 100kHz. The sample rate is 250MHz. That means 2500 samples for every period. There would not be any visible change between modes.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 04:41:26 pm
content: Quote from: nctnico on October 29, 2023, 04:25:41 pmAnd how did the mathematicians produce the Gibbs ears? They added a limited number of harmonics. Are you disputing that is not "bandwidth limited"?

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 04:51:28 pm
content: Quote from: Mechatrommer on October 29, 2023, 02:11:35 pmPhoto shows filter output when sine wave input frequency repeatedly swept linearly from 100kHz to 3MHz. (That range does not occupy entire screen width. It should be obvious where the two frequency extremes are.)

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 04:57:22 pm
content: Quote from: nctnico on October 29, 2023, 03:56:03 pmIf the ears are as you claim "a digital signal processing artefacts from using sin x /x reconstruction" please explain why they are only present on one trace.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 29, 2023, 05:16:33 pm
content: Quote from: wasedadoc on October 29, 2023, 04:57:22 pmyou misunderstand him... to him the real signal that you captured is not Gibbs, since Gibbs only is the one calculated out of Sinc interpolator. however similar they look like. you do know what a pedantic and literalist are right? there is no point continue arguing round and round. i'm afraid 2N3055 will miss my challenge posts earlier..

author: ebastler
author_rank: Super Contributor
timestamp: October 29, 2023, 05:55:38 pm
content: Quote from: Mechatrommer on October 29, 2023, 05:16:33 pmYes, that seems to be the problem. We just get circular arguments here: "No, what you show is not Gibbs, since according to my definition it is not Gibbs, but I won't tell you my definition."I feel strongly reminded of the "Is digital always binary?" thread in the Beginners section, with nctnico playing the radiolistener part. And I think I should draw the same conclusion as in that other thread and stay out of this.

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2023, 06:29:12 pm
content: Quote from: ebastler on October 29, 2023, 05:55:38 pmThere is no 'my' or 'others' definition. Again, the Wikipedia article is super clear on this! You simply can't create a perfect square wave using fourier series. Not even with an infinite number of harmonics.This page shows the effect of sin x/x and the Gibbs ears related to the Nyquist limit very clear:https://www.embedded.com/filling-in-the-blanks-in-digital-oscilloscope-waveforms/https://www.embedded.com/filling-in-the-blanks-in-digital-oscilloscope-waveforms/

author: TimFox
author_rank: Super Contributor
timestamp: October 29, 2023, 07:02:16 pm
content: 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 29, 2023, 08:11:15 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2023, 11:57:44 pm
content: Quote from: gf on October 29, 2023, 10:16:05 amSeveral.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 amDot mode = true sample points beforeanyinterpolation.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 seeandhow we might capture it.Run and Print shows some overwrites in dot modewhereasStop 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 30, 2023, 07:26:59 am
content: Quote from: 2N3055 on October 29, 2023, 04:23:23 pmsorry 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/#msg5140329now 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/#msg5140950here 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...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)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/#msg5139081anyone? 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 anINSULT, to the designer and brand, and to the users too... and not trying to defend your point in maturely and calmly manner isRUDEat least thats the culture around here (my place, not this forum)... imho.

author: gf
author_rank: Super Contributor
timestamp: October 30, 2023, 09:50:18 am
content: Quote from: TimFox on October 29, 2023, 07:02:16 pmThat'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 theconvergencewhen 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 30, 2023, 12:50:57 pm
content: btw i was stumbled last night with my own code as to why my data introduces Gibbs Effect in "Color Grade" mode where in "Vector Mode" there is none! just now i debug my code just to find out i actually implemented Lanczos Resampler Library which is a derivative or implementing Sinc function in it (a library that i copied from internet that i have no clue about the internal math implementation in it)https://en.wikipedia.org/wiki/Lanczos_resamplingi've just forgotten my own code 7 years agoLanczos Resampler (interpolator) will be activated whenever data (sample points) count on display is less than screen pixel width size. when there are enough or more points, Lanczos will not be activated. so here i think also a good example of the resampling algorithm that produces Gibbs Effect, where in reality there is none. fwiw...ironically pretty similar result as in herehttps://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5140329/#msg5140329where i thought "the "imaginary" part of Gibbs disappear, maybe got folded back (aliased) during interpolation process?" i wonder if those scopes are actually implementing Lanczos Resampler? who knows? you can get it online free.

author: Martin72
author_rank: Super Contributor
timestamp: November 01, 2023, 10:43:10 pm
content: I had just discovered pictures of my Bodnarpulser on the Rigol DHO4204 which I had on loan, I put them here.One channel active, 4GSa/s maximum sample rate, the signal looks "good".Two channels active, 2GSa/s sample rate, the signal deforms a bit - but is "faster"(rise time)...4 channels active, but still 1GSa/s samplerate, the signal looks a bit "blurred" in two places and the rise time becomes slower again.Why is the bandwidth highest at 2GSa/s....

author: bdunham7
author_rank: Super Contributor
timestamp: November 02, 2023, 12:05:43 am
content: Quote from: 2N3055 on October 26, 2023, 09:31:30 pmWhich models of Siglent have an analog 'brick wall'? Mine seem to trail off for quite some distance. I've never attempted to accurately characterize them, but I'm shocked to hear them described as 'brick wall'.I was a bit surprised at this discussion because unless I missed something, the various concepts seemed to be mixed together--perhaps like they are in real life. Here's a few thoughts.Gibbs is one thing, and it can be eliminated from the picture by simply turning off interpolation and looking at the dots. The trigger is still interpolated, of course, but that's not too important for this issue.If you consider undulations after the transition to be 'ringing' (whatever the cause) and before to be 'pre-ringing', then the question arises as to the causality issue, which I think was central toMechatrommer'squestion. The simple answer is that at a very basic theoretical level, causality is a complete non-issue simply because the display does not appear on the screen until a long time after the actual event, long meaning compared to the timescale of the signal. You can demonstrate pre-ringing in an entirely analog oscilloscope! The obvious culprit in a CRO is the delay line along with the delay line compensation circuit, but group delay is a feature of many analog filters and can show up as pre-ringing. So after you've eliminated Gibbs if you still see a bit of pre-ringing, there you are.Finally, an RLC filter may be underdamped without really making that much of a difference in the ultimate BW and rolloff. That will cause actual ringing due to oscillation, but not pre-ringing. Practically I have no idea whether you can separate this out from group delay for a real-life example.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 02, 2023, 05:32:00 am
content: Quote from: Martin72 on November 01, 2023, 10:43:10 pmi guess because it produces cleaner less jittery Gibbs (Sinc interpolation) at 2GSa/s (200pts/screen 500ps/pts)? what surprises us is why the properly bw limited scope can still produce gibbs artifact at 10x sampling rateiirc DHO800/DS1000Z dont produce this artifact at max sampling rate 1GSa/s. btw real machine got jittery here and there, internal clock not locked what not resulting sampling at not perfectly consistent interval. making practical application deviates from theory, last night i read about yaigol problem.

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2023, 07:01:41 am
content: "Yaigol" was fixed by a firmware update about six months after launch.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 02, 2023, 09:48:27 am
content: Quote from: Fungus on November 02, 2023, 07:01:41 amyeah but who have time verifying new scope model?

author: rf-loop
author_rank: Super Contributor
timestamp: November 03, 2023, 08:04:36 am
content: Quote from: Martin72 on November 01, 2023, 10:43:10 pmI have one "question"...When there is 10ns/div and 4Gsa/s (40 sample points / div) do we really need Sin(x)/x (aka Sinc) specially when oscilloscope is also 200MHz BW.Think about it.(yes I know where one other oscilloscope turn Sinc off (without indicating it) when some t/div is selected...and Sinc do not give any real advantage. Even when Sinc is still selected)Also if I design oscilloscope I will really think carefully when Sinc give some advantage and when not...Just for thinking..

author: gf
author_rank: Super Contributor
timestamp: November 03, 2023, 11:07:40 am
content: Quote from: Mechatrommer on November 02, 2023, 05:32:00 amKeep in mind thatany filterin the signal path with a sharp enough cut-off can introduce overshoot/ringing near edges, and if the filter happens to be a linear phase filter, then this ringing is symmetric (pre + post). This is not limited to sinc reconstruction filters, and it is not limited to undersampled signals.In Bodnar_Dual.png (2GSa/s) I do not think that the pre- and post-ringing comes from an interpolation filter. An upsampling filter from 2GSa/s has a cutoff at 1GHz (or slightly below), and its ripples are supposed to have a period in the order of 1ns. But the period here is rather 2ns (or even a little bit more), which suggests that we see the result of a FIR filter with a cut-off < 500MHz. However, don't ask me what's the objectiveAnd since the trace is pretty clean, I'm quite confident that it doesn't suffer from aliasing (at least not significantly), i.e. very likely it was properly bandwidth limited before sampling.OTOH, I'm pretty confident, that the signal in Bodnar_Quad.png (1GSa/s) wasnotproperly bandwidth limited before sampling and that the apparent (let me call it) "reconstruction jitter" is caused by aliasing.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 03, 2023, 01:09:55 pm
content: Quote from: gf on November 03, 2023, 11:07:40 amshow me in any rigol DSO, or any other "better" competitive DSO from teardown photos that have this "sharp enough cut-off can introduce overshoot/ringing near edges". one member here proved that filter exists, but not for DSO. even if it is, it will not fit from teardown photos evident, we dont see fancy inductors capacitors 3x 5x parellel networks over there. prove me wrong, and err... also care to explain why at 4GSa/s snapshots above got no overshoot/undershoot.pre or post ringing?Quote from: gf on November 03, 2023, 11:07:40 amnope! until evident provided. i've provided a few, its your turn... here's the challenge below if you miss it (on any brand scope thats capable both Sinc ON and vector/dot (Sinc OFF) mode. its like nobody understand this thread and i'm just talking to the airin fact the evident that there is no pre-ringing is just right above the 2GSa/s image, ie capture at 4GSa/s. how amazing people can miss it!https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5142249/#msg5142249Quote from: rf-loop on November 03, 2023, 08:04:36 amnot just when developing DSO, but PC SW that utilize sampled data, or even in graphics 2D 3D rendering will deal with this decision matters. like my SW just above. i let Sinc to take place when sampled points is less than screen size divide by 2.. this is developers decision.. maybe if (data_count < screen_size / X) then do Sinc, else do vector plot.... X can be 2,3,4,5? it depends on developers/designers... but user should decide if need Sinc ON? or want Sinc OFF? in any condition...

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 03, 2023, 01:24:48 pm
content: here again i upgraded VisaDSO v1.1.3 to enable interpolated Sinc/Lanczos on single shot capture...see the difference. Leo Bodnar Pulse captured by DHO924 then data downloaded to PC... Sinc OFF (no Gibbs Effect) the boring straight line vector drawing mode, and Sinc ON (with Gibbs Effect) for noob's eyes satisfaction curvy plot. granted this is a scope that violates Nyquist at max sample rate and 4 channels, but for DHO4000 above, 2GSa/s single channel... its puzzling.

author: rf-loop
author_rank: Super Contributor
timestamp: November 03, 2023, 01:27:34 pm
content: Quote from: Mechatrommer on November 03, 2023, 01:09:55 pmAs you can see not everyone...

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 03, 2023, 01:37:43 pm
content: Quote from: rf-loop on November 03, 2023, 01:27:34 pmthere corrected for me. i know you in arena that keep rf work again and againusuallypeople less talk because doing more work... cheers.

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2023, 02:47:51 pm
content: Quote from: Mechatrommer on November 03, 2023, 01:09:55 pmIf you look carefully at Dave's teardowns you'll see quite a few scopes have LC filters in front of the ADC to previous aliasing. However, these filters are designed so their phase response doesn't cause (excessive) ringing. For obvious reasons as the designer of a DSO wants to introduce as little 'distortion' to the visible signal as possible.

author: Martin72
author_rank: Super Contributor
timestamp: November 04, 2023, 12:57:37 pm
content: Quote from: rf-loop on November 03, 2023, 08:04:36 amI'll read through this first before I start thinking(link to a pdf):https://cdn.teledynelecroy.com/files/whitepapers/wp_interpolation_102203.pdfMaybe that will help me think.

author: gf
author_rank: Super Contributor
timestamp: November 04, 2023, 02:05:23 pm
content: Quote from: Martin72 on November 04, 2023, 12:57:37 pmThe Simple Test described in this paper was also my reconstruction validity criterion when I wroteQuoteHowever, there is admittedly a catch: If the sampled signal happens to be filtered (e.g. if a bandwidth limit is applied), then the check described in the paper indicates whether the filtered signal was reconstructed properly. It does not indicate whether a decent reconstruction would have been possible from the unfiltered samples.Quote from: Mechatrommer on November 03, 2023, 01:09:55 pmWell, let's assume it comes from the interpolation filter. Why would you design an interpolation filter that cuts off 50% of the recoverable bandwidth? Sure, in practice you have to sacrifice a little, but why so much? Maybe I am missing some information to understand the intentionAs a result, the interpolation filter does not only reconstruct, but additionally it acts as a bandwidth limiter. And the latter, of course, can still introduce ringing, even if the reconstruction alone at (almost) full bandwidth would possibly not do that.EDIT: Of course, we do not know if we are dealing with two separate filters here (one for bandwidth limiting, and a second for reconstruction), or a single filter that combines both. At the end, we can only observe the combined effect.

author: nctnico
author_rank: Super Contributor
timestamp: November 04, 2023, 02:22:34 pm
content: Quote from: gf on November 04, 2023, 02:05:23 pmBut you have to keep in mind that sin x / x is not always implemented correctly. I have come across a few oscilloscopes which had broken sin x / x implementations. An easy check is to feed a sine wave into an oscilloscope with a frequency of 2.5 times less than the samplerate (fsample / 2.5). You should be able to get the sine wave on screen properly. If you get the wrong amplitude or a distorted sine wave, then the sin x / x implementation is incorrect.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 04, 2023, 02:31:17 pm
content: Quote from: gf on November 04, 2023, 02:05:23 pmi'm not sure how internal Lanczos/Sinc filter is designed/coded, possibly cut off more bw to cut cpu processing speed, there's been try and error by the people who know it, for example change Lanczos kernel width or what not, we will have more or lesser gibbs effect. and secondly, we have to take into account the imperfection of sampling machine in real world.. non consistent sampling interval... a little bit of ADC noises etc. if we read theory, they seems to be interesting, given the limited BW and enough parameters, "you should be able to reconstruct waves PERFECTLY!", but i guess in practice there will be catches here and there to save resources. ymmv.

author: mr ed
author_rank: Regular Contributor
timestamp: November 04, 2023, 08:27:00 pm
content: Anything inbetween the samples is made up data. It's - art. Pick your interpolation. Or rather, it's done for you by the manufacturers. Any digital scope when the sampling gets thin will show some artifacts of some sort. Then the BW limiter adds confusion. Even with it off, the pcb wiring will still have some minor parasitics.I hate SW options. Hack away!

author: Fungus
author_rank: Super Contributor
timestamp: November 04, 2023, 11:04:09 pm
content: Quote from: mr ed on November 04, 2023, 08:27:00 pmNope. It's correct data, providing the bandwidth conditions are met.

author: nctnico
author_rank: Super Contributor
timestamp: November 04, 2023, 11:17:55 pm
content: Quote from: Fungus on November 04, 2023, 11:04:09 pmIndeed. It is signal reconstruction, not interpolation. Signal reconstruction is only there to make a signal visible for humans (as a visual aid) in case the sample points are too far apart on the screen to see the shape. However, when dealing with steep edges, the sampled data represents a step function (infinite risetime) which can not exist in the real world. So there sin x /x will produce pre and post ringing (Gibbs ears).

author: Martin72
author_rank: Super Contributor
timestamp: November 04, 2023, 11:29:59 pm
content: I had read through the whitepaper I linked to and connected the bodnar pulser to the scope again, in this case the DHO804(100Mhz).The signal from the bodnar is 10Mhz, so it should be interpolated cleanly, regardless of whether linear interpolation or sinx/x is used.But it doesn't, even at 1.25GSa/s you can already "discover" something and in the worst case, the 312.5MSa/s, it already looks pretty blurred.This can only have something to do with the very fast rise time of the signal(~40ps).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.(Why the "roof" looks so "wobbly", I'll have another look tomorrow).Now I also understand the behavior of the DHO4204, thanks for the food for thought@rf-loop.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 05, 2023, 12:16:05 am
content: 3ns sample interval... 0.04ns risetime, how many points? 3ns sample interval... 9ns risetime, how many points? Just for thinking.. too bad dho cannot show dots... harder to think... 2GSa/s also Gibbs

author: nctnico
author_rank: Super Contributor
timestamp: November 05, 2023, 12:25:59 am
content: The only thing that matters where it comes to getting Gibbs ears (as in reconstruction artefacts) is when the signal your are measuring becomes a step response in the sampled aka digital domain. You really need to think about what a sampled signal looks like in the digital domain and how that depends on the samplerate.

author: bdunham7
author_rank: Super Contributor
timestamp: November 05, 2023, 12:30:24 am
content: Quote from: Mechatrommer on November 05, 2023, 12:16:05 amIs that true--it does not have a dot mode?

author: Martin72
author_rank: Super Contributor
timestamp: November 05, 2023, 12:45:43 am
content: Yepp, no dot mode and no interpolation choice.Neither the smallest (DHO800) nor the largest (DHO4000) model.

author: wasedadoc
author_rank: Super Contributor
timestamp: November 05, 2023, 12:50:32 am
content: Quote from: Fungus on November 04, 2023, 11:04:09 pmIt is only correct data if the bandwidth conditions are metandthe 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.

author: Fungus
author_rank: Super Contributor
timestamp: November 05, 2023, 01:24:46 am
content: I didn't say it was perfect, I said it was correct.To be "perfect" you need an infinite amount of samples.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 05, 2023, 09:16:58 am
content: Quote from: bdunham7 on November 05, 2023, 12:30:24 amyep. 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.

author: mawyatt
author_rank: Super Contributor
timestamp: November 05, 2023, 01:38:21 pm
content: 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,

author: gf
author_rank: Super Contributor
timestamp: November 05, 2023, 05:04:32 pm
content: Quote from: Martin72 on November 04, 2023, 11:29:59 pmIs 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.QuoteNo, 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.QuoteThe 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.QuoteWhich is indeed an indication for a reconstruction problem at this bandwidth / sample rate combination.QuoteIMO It is not. Again, the superimposed persistent traces are not "blurred" (ilke in 312MSa.png), which suggests that there is no interpolation/reconstruction issue.QuoteCheck 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.QuoteI 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.

author: Martin72
author_rank: Super Contributor
timestamp: November 05, 2023, 05:15:01 pm
content: QuoteNot bad, not bad...It´s the 70Mhz model hacked to 100Mhz - But for whatever reason, I had measured 200Mhz (-3dB).As for the rest, thanks for that, I'll have to look into it further.I think it's important to know this, or to know it more precisely.

author: Fungus
author_rank: Super Contributor
timestamp: November 05, 2023, 05:25:58 pm
content: Quote from: Martin72 on November 05, 2023, 05:15:01 pmThe 100Mhz isn't an officially available option so who knows what it's doing internally. They're all capable of much more than 200Mhz.Let's hope Rigol doesn't (or can't) address this issue.

author: Martin72
author_rank: Super Contributor
timestamp: November 05, 2023, 05:28:14 pm
content: QuoteIt´s not a bug, it´s a feature..Good for us.

author: Martin72
author_rank: Super Contributor
timestamp: November 05, 2023, 06:09:43 pm
content: @gf:QuoteI had forgotten the "not"...

author: gf
author_rank: Super Contributor
timestamp: November 07, 2023, 04:16:48 pm
content: Quote from: Martin72 on November 04, 2023, 11:29:59 pmI looked through the whole thread and could not find a 625MSa/s screenshot for the DHO800.All screenshots are either 1.25GSa/s or 312.5MSa/s.Could you please also post a screenshot of a Bodnar signal taken at 625MSa/s?Does it also show overshoot, and blur/jitter due to aliasing?

author: Martin72
author_rank: Super Contributor
timestamp: November 07, 2023, 06:48:46 pm
content: Hi,Just made a few pictures...

author: Martin72
author_rank: Super Contributor
timestamp: November 07, 2023, 06:56:51 pm
content: Here again a comparison between the original 70Mhz version and now, the "unleashed" version, which has 200Mhz bandwidth (although you can only hack on 100Mhz and only 100Mhz are displayed, maybe the reason why after the last firmware update the bandwidth option is no longer displayed).

author: Fungus
author_rank: Super Contributor
timestamp: November 08, 2023, 08:49:02 am
content: Quote from: Martin72 on November 07, 2023, 06:56:51 pmAs noted earlier: The "70MHz" version of the DHO800 has 125MHz measured bandwidth.I think Rigol's marketing department is under-labelling the DHO800 to fool people into buying the DHO900.(Which makes perfect sense - they probably make 3x profit in return for a $1 edge connector!)

author: Martin72
author_rank: Super Contributor
timestamp: November 10, 2023, 05:17:05 pm
content: Just for fun, bodnarpulse on a lecroy WR9054, 500Mhz, 20GSa/s.

author: iMo
author_rank: Super Contributor
timestamp: November 10, 2023, 05:33:37 pm
content: Quote from: Martin72 on November 07, 2023, 06:48:46 pmYou may see with 3ns/point (312.5Msa/s) the sinc function tries to interpolate the points such the "sinc waves" periods fit that 3ns between the points..

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 10, 2023, 07:08:21 pm
content: right! playing with sim tonight to design 3rd order elliptic filter (analog/physical) filter (about 60MHz cutoff)... no gibbs effect on sim. so there must be a special one to build a physical filter that can cause "gibbs like" effect. fwiw..

author: wasedadoc
author_rank: Super Contributor
timestamp: November 10, 2023, 10:38:40 pm
content: Quote from: Mechatrommer on November 10, 2023, 07:08:21 pmCan you plot the phase versus frequency?

author: gf
author_rank: Super Contributor
timestamp: November 10, 2023, 11:45:22 pm
content: Quote from: Mechatrommer on November 10, 2023, 07:08:21 pmI do see overshoot/ringing caused by attenuation of higher harmonics. Why do you classify it as "no Gibbs"? Symmetry is not the criterion. Gibbs ringing introduced by a filter when applied to an ideal square wave can be either symmetric or asymmetric, depending on the filter's phase response.The only difference between the filters that produce the step responses in figure1 and figure2 is their phase response. Both filters have the same magnitude response (figure3), i.e. attenuation of harmonics is the same. Figure1 is minimum phase and figure2 is linear phase. A causal, stable IIR cannot have linear phase, though.

author: nctnico
author_rank: Super Contributor
timestamp: November 11, 2023, 12:10:26 am
content: I still don't agree with attributing ringing due to low pass filtering a non-perfect square (IOW: a square wave as it exists in the real world) wave to Gibbs effect. The way I read the mathematical description is that Gibbs ears occur because you can't create a perfect square wave from adding sine waves (a Fourier series) which in turn is because in the mathematical world a square wave is not a continuous function. You could call Gibbs ears a 'residu'. However, a discontinuous square wave can exist in the digital world after sampling a square wave or other waveforms with steep edges in relation to the samplerate. When the sampled signal is brought back to the real world using sin x/x reconstruction (for example), you'll get Gibbs ears as a displayed artefact.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 11, 2023, 03:26:09 am
content: Quote from: gf on November 10, 2023, 11:45:22 pmi only focus on pre ringing part, the unrealistic one. the post ringing overshoots are normal, so i will not question that part. Not just in electrical signal, but also normal in physical control system n mechanical. Your car doesnt get bumpy before hitting the bump, otherwise thats paranormal.

author: bdunham7
author_rank: Super Contributor
timestamp: November 11, 2023, 04:58:42 am
content: Quote from: Mechatrommer on November 11, 2023, 03:26:09 amAs I pointed out earlier, the causality argument is a trap, a red herring, a.. well, whatever. There is no theoretical causality issue if the display on the screen doesn't appear until long (relatively speaking) until the event is over. In the case of just the analog filter, this means there could be 'pre-ringing' to the extent that the portion of the signal that represents the bulk of the step is delayed in the filter while other components get through faster. This can be demonstrated with with real components, although I don't have an all-analog example that would give you nice pronounced and symmetrical Gibbs ears.

author: gf
author_rank: Super Contributor
timestamp: November 11, 2023, 09:01:01 am
content: Quote from: Mechatrommer on November 11, 2023, 03:26:09 amMinimum phase filters don't pre-shoot. The traditional all-pole filters like Bessel, Butterworth, Chebycheff type1 are minimum phase, IIRC. Butnot everyIIR (or analog) filter is mininimum phase. If you linearize the phase response of a (say) Butterworth filter with additional allpass filters you can even approximate a linear phase response in the analog domain.See alsohttps://cdn.teledynelecroy.com/files/whitepapers/group_delay-designcon2006.pdf

author: iMo
author_rank: Super Contributor
timestamp: November 11, 2023, 09:21:28 am
content: As it has been discussed earlier, the pre-ringing has nothing to do with hardware.The scope samples a couple of samples, and in order to draw a "continuous solid yellow signal" on the LCD display it uses sinc interpolation in math (it creates additional LCD points based on the "sinc" function). You may see it on previous picture in the replay #220 (the Bodnar 312.5MSa/s) where there were only 20 samples taken (3ns from sample to sample, 60ns wide screen, therefore 20 points to interpolate) and the graphical function based on the "sinc" interpolated those 20points into a "continuous picture" you see on the screen. Therefore you see the "pre-ringing". With more points (samples) taken you do not see the pre-ringing.You cannot get "pre-ringing" when the electrical signal has not entered the filter.Thus the "sinc" here has nothing to do with a ringing signal, "filtering in hw" or "dsp". It is a graphical interpolation method/tool they have chosen to create continuous visual lines on the LCD screen. You may interpolate number of bananas exported each month within a year and you will get the same pre-ringing and post-ringing with the sinc interpolator when having 12 numbers..PS: below the picture, the display even says "16points" taken in total, 3ns apart over the entire screen width (info on top)..Look at the nodes where the wave changes its phase - aprox six times (20ns/3), 3ns apart, mind the time scale below - there are the "6 real physical points sampled", all yellow between them has been "created artificially by the graphical Sinc interpolation".The ringing before and after the rising edge is made by the sinc graphical interpolation such you get a solid line instead of only 16 points placed on the LCD screen..

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 12, 2023, 05:30:12 pm
content: Quote from: wasedadoc on November 10, 2023, 10:38:40 pmi just got home, see attached. not sure what it tells. but anything larger than 40MHz is more than 180deg phase?

author: bdunham7
author_rank: Super Contributor
timestamp: November 12, 2023, 06:22:56 pm
content: Here is a rather lame attempt to throw something together with assorted junk within reach on my messy bench. First screenshot is a square wave from an FY6900 through a Mini Circuits BLP70+. Second is the same except a small modification was made. Obviously I'd have to tweak my modification to getnicer lookingsymmetric results.Edit: I did tweak the modification a bit to look 'nicer', but I obviously still don't have a symmetrical Gibbs ears appearance. Note that even 'real' Gibbs ears are not always perfectly symmetric.https://www.minicircuits.com/pdfs/BLP-70+.pdf

author: wasedadoc
author_rank: Super Contributor
timestamp: November 12, 2023, 06:36:35 pm
content: Quote from: Mechatrommer on November 12, 2023, 05:30:12 pmTo see if linear phase or not, plot both axes as linear, not log frequency.Large phase angles per se are not necessarily a problem. Think about it. To preserve wave shape all frequencies must be delayed by the same amount oftime. That means phase angle increaseslinearlywith frequency. Going past 180 or even past 360 degrees is not an automatic no-no.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 13, 2023, 05:29:40 am
content: Quote from: wasedadoc on November 12, 2023, 06:36:35 pmunfortunately tina-ti doesnt have linear scale frequency plot. but eyeballing it, the curvy shape could be linear phase when plotted on linear scale x-axis? here's the group delay plot, so i guess its linear phase up to 3MHz? or with some compromise up to 40MHz? see attached.https://audiojudgement.com/group-delay-explanation-measurement-audio/

author: ebastler
author_rank: Super Contributor
timestamp: November 13, 2023, 06:10:02 am
content: Quote from: Mechatrommer on November 13, 2023, 05:29:40 amClick the "Log" button in the lower right, next to the "Resolution" entry field.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 13, 2023, 06:28:43 am
content: Quote from: ebastler on November 13, 2023, 06:10:02 am

author: mawyatt
author_rank: Super Contributor
timestamp: November 21, 2023, 05:22:14 pm
content: Here's something that folks might find interesting. Gibbs Ears created on DHO824 with SDG2042X AWG utilizing Sinewave and Harmonics feature.Best,