Products > Test Equipment
Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)
Fungus:
--- Quote from: Mechatrommer on October 28, 2023, 05:52:38 pm ---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?
--- End quote ---
I 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 circuit cannot predict 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 so now I can start to believe what I see on screen.
Mechatrommer:
--- Quote from: Fungus on October 28, 2023, 08:37:34 pm ---Nor do I think "sinc is evil, I wish I could turn it off". It's actually helpful to see the "pre-ringing" because it gives you a rough idea of how much "post-ringing" can be ignored.
--- End quote ---
predicting 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.
Mechatrommer:
--- Quote from: gf on October 28, 2023, 07:20:51 pm ---
--- Quote from: Mechatrommer on October 28, 2023, 05:52:38 pm ---Gibbs effect successfully produced in (sinc_on.jpg), but when in dot mode (dot_on_run.jpg or dot_on_stop.jpg) Gibbs disappear, 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?
--- End quote ---
I 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.
--- End quote ---
yes! 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 there
my 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.
tautech:
Let me again present 2 GHz DSO 10GSa/s in DOT mode.
BW and sampling rates are everything.
Fungus:
--- Quote from: Mechatrommer on October 28, 2023, 08:51:06 pm ---
--- Quote from: Fungus on October 28, 2023, 08:37:34 pm ---Nor do I think "sinc is evil, I wish I could turn it off". It's actually helpful to see the "pre-ringing" because it gives you a rough idea of how much "post-ringing" can be ignored.
--- End quote ---
predicting actual event from false event? ok good for you, thats new. thats not how we work...
--- End quote ---
I'd say it's pretty darn close in your example image.
--- Quote from: Mechatrommer on October 28, 2023, 08:51:06 pm ---you can stick with 100MHz BW and be safe, cheers.
--- End quote ---
I'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.
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