Zooming *IN* [actually: signal reconstruction] - Page 1
Forum section: electronics / testgear

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 05:43:12 pm
content: ZoomingINwith the scope can be dangerous too, if done carelessly...:https://youtu.be/W4twnd-YQQ4Note that the KS, having *less* memory depth, automatically turns Sinc interpolation off...But an interesting thing I'd like experienced people to clarify (or our host hisself), is that Dave gives us some numbers (position 8:10 circa): "sinc int. works well when we have more than a 4 samples per period".Note that the scope is sampling at 2.5:1 ratio, and indeed the signal's reconstruction is far from perfect. This seems to give credit to Dave's rule of thumb, and, as he turns off the 2nd channel, the signal looks virually perfectly reconstructed (5 samples per period), while linear interpolation still sucks.But look at this:https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-samplingBasically it says that Tek scopes can reconstruct the signal with just 2.5 samples per period.This seems to be confirmed by Chris Rehorn at KS, who inhttps://siglent.fi/data/technical-common/Sin(x)x_Agilent.pdfsays:In reality, as long as the rules of Nyquist are not violated,an oscilloscopecan reconstruct a user’s signal identically. This reconstruction process is often referred to as sin(x)/x interpolation. Whether the sample rate is 25x the Nyquist frequency, or2.5xthe Nyquist frequency, interpolation can be used to reproduce the waveformexactlyas it appeared at the oscilloscopes input connector, removing all doubt about a signal’s behaviour between samples.And:Nyquist’s most famous theoremproposes that a signal can be reconstructed perfectly from discrete samples if the following two rules are observed:1.The highest frequency component sampled must be less than half the sampling frequency and2. Samples must be acquired in equally spaced intervalsNote thatnothing is said about an infinite number of samples, presumably because he wants to talk about oscilloscopes (hence not 2X, but just 2.5X).I'd like to know which of these hypotheses did the scope in Dave's video violate. Maybe the front end was ineffective at discarding the high frequency components due to roll off from being at maximum (3 db) frequency, as he seems to hint? If so, would you elaborate upon this particular point? Thanks.

author: Fungus
author_rank: Super Contributor
timestamp: October 21, 2022, 06:13:14 pm
content: Quote from: balnazzar on October 21, 2022, 05:43:12 pmAt around 7:30-8:00 in the video?I think the problem is that sin(x)/x is an infinitely wide function and oscilloscopes have to be practical so they only apply a small window of it (the central part of the sinc curve) in their reconstructions. Some manufacturers will have a wider window than others.ie. 2.5x is correct in theory but putting it into practice can be difficult.Dave's "4x" number doesn't come from theory, it's pulled out of thin air based onpractical experienceworking with that particular 'scope (and maybe some others), hence the overlayed comment at 8:20 that starts with "In practice..."Quote from: balnazzar on October 21, 2022, 05:43:12 pmThe 'scope's bandwidth rolloff doesn't apply when the input is a pure sine wave, there's no harmonics to break Nyquist.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 21, 2022, 06:20:58 pm
content: How in earth is this dangerous? Innacurate, yes.Try zooming a 1MHz wave sampled at 50us/div with any average scope, good luck with that!Only high-end equipment will be able to zoom-in so much with accuracy, definitely not the average sub-$600 scope, though these devices are getting better each year and I could be wrong.

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2022, 06:29:46 pm
content: Quote from: DavidAlfa on October 21, 2022, 06:20:58 pmYoutuber clickbaiters just don't care, they only want views.

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2022, 06:30:33 pm
content: I have a problem with any statement that refers to Nyquist and to 'perfect' reconstruction. The reconstruction isnever, everperfect. There are always errors such as mathematical truncation (you can't compute everything to an infinite number of decimal places...), quantization (8-bits has its limitations), noise, input amplifier distortion and group delay, width of signal memory and the sinc function as applied, and probably more things. The more samples you have, the less the reconstruction depends on any one sample or calculation so it becomes easier and easier to get a good (again--not perfect, ever!) result.As a practical matter, reconstruction from 2.01 samples per cycle would only be practical for on-paper mathematical examples calculated to many decimal places, whereas reconstruction from 100 samples per cycle will work no matter how bad the method--even just showing the dots would obviously work fine. In between those two extremes you have practical limits. With ideal conditions 2.5X is possible, but even a little bit of noise will make the results distorted and jumpy. 4-5X typically gives good results under most conditions, although again it is never perfectly correct. I don't know exactly what caused the result in the video that you mention, but I think if he had done continuous acquisition instead of a single shot you'd see a noisy, jumpy signal.

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2022, 06:36:35 pm
content: Quote from: bdunham7 on October 21, 2022, 06:30:33 pmYep this is all scope newbie stuff learned shortly after getting a scope and AWG and pushing limits like we do.Can't afford a HF AWG then get some old RF gen and start pushing the limits and learn all this basic stuff first hand.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 06:47:40 pm
content: Quote from: Fungus on October 21, 2022, 06:13:14 pmUnderstood, thanks.Quote from: Fungus on October 21, 2022, 06:13:14 pmBut no awg is capable of providing a true pure sine wave...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 06:50:06 pm
content: Quote from: DavidAlfa on October 21, 2022, 06:20:58 pmDangerous not in physical sense,of course. I poses a danger to measurements, up to the point that you see a completely different signal. That's a bit worse than just inaccurate.

author: Fungus
author_rank: Super Contributor
timestamp: October 21, 2022, 06:50:37 pm
content: Quote from: tautech on October 21, 2022, 06:29:46 pmRelax, this one wasn't an anti-Siglent video.

author: Fungus
author_rank: Super Contributor
timestamp: October 21, 2022, 06:53:46 pm
content: Quote from: balnazzar on October 21, 2022, 06:50:06 pmThis is where (eg.) the Rigol MSO5000s 8GHz sample rate is a big advantage. It's 6x Nyquist where most scopes aim for 2.5x.It's not always about "noise". Other numbers matter, too.

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2022, 07:23:00 pm
content: Quote from: Fungus on October 21, 2022, 06:50:37 pmFFS Fungus, that's kids stuff. Maybe you need get a RF gen and find this stuff out for yourself.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 09:30:03 pm
content: Quote from: Fungus on October 21, 2022, 06:53:46 pmThis is where (eg.) the Rigol MSO5000s 8GHz sample rate is a big advantage. It's 6x Nyquist where most scopes aim for 2.5x.It's not always about "noise". Other numbers matter, too.True....But if it had been just a bit less noisy...

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2022, 09:36:05 pm
content: Quote from: Fungus on October 21, 2022, 06:53:46 pmSure, it's probably better if you happen to need a solid response to 350MHz on more than two channels at once.

author: Bud
author_rank: Super Contributor
timestamp: October 21, 2022, 09:47:23 pm
content: Quote from: balnazzar on October 21, 2022, 05:43:12 pmThere is a flaw in this statement because it gives no guidellines as to how low the amplitude of a high frequency component is considered practically sufficient for satisfactory visual reproduction of the waveform on the oscilloscope screen. Based on the rules quoted above, a 20GHz s scope would not be able to perfectly reconstruct my 1kHz square wave because the spectrum of my square wave is infinite.

author: tautech
author_rank: Super Contributor
timestamp: October 21, 2022, 10:07:11 pm
content: Quote from: Fungus on October 21, 2022, 06:53:46 pmThey do.Turn on all 4 channels and look what happens to your 8GSa/s.

author: Fungus
author_rank: Super Contributor
timestamp: October 21, 2022, 10:18:13 pm
content: Quote from: Bud on October 21, 2022, 09:47:23 pmCorrect, although whether or not you'd be able to see the difference on screen is another matter.PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.

author: Fungus
author_rank: Super Contributor
timestamp: October 21, 2022, 10:19:31 pm
content: Quote from: tautech on October 21, 2022, 10:07:11 pmUmm... I get 2Ghz per channel?ie. 6x Nyquist, the number Ialready postedabove.

author: switchabl
author_rank: Frequent Contributor
timestamp: October 21, 2022, 11:55:15 pm
content: Quote from: Fungus on October 21, 2022, 10:18:13 pmI would suggest that you measure the rise-time with both scopes and then you may want to reconsider this statement...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 22, 2022, 12:05:28 am
content: Quote from: Bud on October 21, 2022, 09:47:23 pmGood point.. I'm looking forward to reading more people's comments on that...

author: bdunham7
author_rank: Super Contributor
timestamp: October 22, 2022, 12:25:16 am
content: Quote from: Fungus on October 21, 2022, 10:18:13 pmI don't think that makes sense mathematically. At the rising and falling edges all of the (odd) harmonics add, although they don't add at their peaks so it gets complicated and hard to visualize or make up simple hand-wavy explanations.. You don't see the individual harmonics, you see their sum. So harmonics 127 + 129 add up to more than 1 LSB, as do harmonics 247-265, even though individually they wouldn't.

author: noisyee
author_rank: Contributor
timestamp: October 22, 2022, 01:48:01 am
content: Quote from: balnazzar on October 21, 2022, 05:43:12 pmIt seems Dave feeding some not so ideal sine wave to the scope so it cause artifact by interpolation.To my own experience, you can hardly benefit from interpolation if you push to the 2.5X ratio limit since in reality pure sine wave don't exist, needless to say the front end itself will distort your signal. At 2X analog BW, attenuation is usually less 20dB. 2nd harmonic can easily cause aliasing and ultimately cause that kind of twisting effect in the video by interpolation.Dave's rule of thumb is more practical while other statement is true mathematical.A good practice is always use a little bit "over-killed" instrument if possible. You will save a lot of time by no needing to carefully verify your test setup.

author: noisyee
author_rank: Contributor
timestamp: October 22, 2022, 02:21:45 am
content: Quote from: Bud on October 21, 2022, 09:47:23 pmThat's correct because ideal square wave have zero rise and fall time while 20 GHz scope doesn't. In signal integrity, frequency is not that important than rise and fall time.This statement is mathematical correct but not particularly useful.Quote from: Fungus on October 21, 2022, 10:18:13 pmI don't think so because:a. harmonics of a square adds together to form the edge which is far larger than 1LSBb. you can see less than 1 LSB signal in FFT because of the noise floor can do the dither for you

author: Fungus
author_rank: Super Contributor
timestamp: October 22, 2022, 12:27:19 pm
content: Quote from: bdunham7 on October 22, 2022, 12:25:16 amYeah, you're right. Those tiny harmonics can add up to a lot if there's a large number of them and they're all in phase (which they will be on the edge of a step input).Mental model adjusted.

author: 2N3055
author_rank: Super Contributor
timestamp: October 22, 2022, 01:22:41 pm
content: Quote from: balnazzar on October 21, 2022, 09:30:03 pmTrue....But if it had been just a bit less noisy...Really, you should read a bit before making completely inaccurate statements.Max ZOOM ratio (acquired any way) is ratio of available memory. Scopes with same memory and different sample rate will drop at same sample rate after you just change several timebase positions.Actually we can argue that scope with higher sample rate will see it's higher sample rate only on few fastest timebase positions.

author: Fungus
author_rank: Super Contributor
timestamp: October 22, 2022, 01:28:37 pm
content: Quote from: 2N3055 on October 22, 2022, 01:22:41 pmSo it's best to start out with as high a sample rate as possible...?Another point in favor of the MSO5000.

author: 2N3055
author_rank: Super Contributor
timestamp: October 22, 2022, 01:41:28 pm
content: Quote from: Fungus on October 22, 2022, 01:28:37 pmAre you making a point not think at all before writing an response?I know you can do better than that..Max sample rate has no bearing to detail acquired when sampling large amount of data if sample rate had to drop because memory is limit... On high memory length to detail ratios, memory will be the limit.Lets presume you have 2 scopes, both with 1Mpoints of memory, one with 10GS/s and other with only 1GS/s.At 10 GS/s you spend 10X more memory than at 1 GS/s per second. Okkay?So if you have 1 Mpoints of memory, at 10GS/s you use up all memory when sampling 100us worth of data. If you want to sample 1ms worth of data , both of those scopes will have to sample at 1GS/s.. and will be the same. You will have same detail on both. That is why we say that long sampling memory is important, to keep native sample rate as long as possible. And that is why Keysight Megazoom is good "analog emulation" scope but loses even to DS1000Z when you need to sample longer period and zoom in to detail...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 22, 2022, 02:39:22 pm
content: At this point i feel obliged to ask: why manufacturers don't release scopes with more mem depth. They seem to be on the verge of generous with other specs (see indeed rigol with sub-1k$ 350 MHz 8 GSa/s), but memory depth is almost always the weak point of accessible scopes..

author: bdunham7
author_rank: Super Contributor
timestamp: October 22, 2022, 03:03:43 pm
content: Quote from: balnazzar on October 22, 2022, 02:39:22 pmHow much do you want? Current model entry level DSOs have an astounding amount of memory compared to older models, even those from A brands.

author: Bud
author_rank: Super Contributor
timestamp: October 22, 2022, 03:07:04 pm
content: Quote from: 2N3055 on October 22, 2022, 01:41:28 pmNot true for event-based sampling. Keysight with its segmented memory can sample and store only when triggered. With that, you can have information captured for much longer if events are spaced apart in time than a scope with larger conventional memory, which will get filled in a few milliseconds.

author: Fungus
author_rank: Super Contributor
timestamp: October 22, 2022, 03:44:32 pm
content: Quote from: balnazzar on October 22, 2022, 02:39:22 pmI assume it's because they'd have to up the processing power to match.(...so all the EEVBLOG forum haunters don't spend every waking hour pointing out howslowthey are)I think some teardowns have seen much more RAM inside them than actually gets used.Also: Note that they don't have 256Mb or 512Mb of RAM or some "computer" number. It's always a nice round number that the processor can handle.

author: rf-loop
author_rank: Super Contributor
timestamp: October 22, 2022, 03:45:22 pm
content: Quote from: Bud on October 22, 2022, 03:07:04 pmBit different thing this segmented memory acq.And it is also limited to available amount of memory and also limited to max 1000 segments least in some Megazoom IV infinii vision X series 1000X to 6000X.In many scopes there is more memory - also for segmented acquisition of course. Example even in Siglent SDS1004X-E up to 109Mpts and lot of more than max 1000 segment.Many oscilloscopes today have some kind of segmented memory acquisition - even in Rigol. But, also more memory for it. So what is point... most of modern normal scopes have it.

author: 2N3055
author_rank: Super Contributor
timestamp: October 22, 2022, 04:42:49 pm
content: Quote from: Bud on October 22, 2022, 03:07:04 pmThat is absolutelly not what we are talking about in this topic. True, but not a topic.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 22, 2022, 05:38:42 pm
content: Quote from: Fungus on October 22, 2022, 03:44:32 pmAs I thought. So, the bottleneck, in the end, is always processing power...

author: tautech
author_rank: Super Contributor
timestamp: October 22, 2022, 06:35:09 pm
content: Quote from: bdunham7 on October 22, 2022, 03:03:43 pmFTFY

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 23, 2022, 12:43:15 pm
content: Quote from: tautech on October 22, 2022, 06:35:09 pmThat's true. If only lower-status brands would care to do a lot of other things better, they'd become the new A-brands.

author: 2N3055
author_rank: Super Contributor
timestamp: October 23, 2022, 01:50:47 pm
content: Quote from: balnazzar on October 23, 2022, 12:43:15 pmThat is weird and false statement.Siglent and Rigol are for years not doing MUCH better than ANY A brand in area of less than 2000 USD scopes.They have same quality and reliability and loads of features more than big brands.It is only that there are many people out there who expect unrealistic things, disrespecting how much effort goes into making things.And when, for instance, Siglent makes new SDS2000X HD that is better in its class than anything from A brands, they complain it is expensive.. While when Keysight asks you same money for a scope that just looks funny next to it than it is money "well invested" into good brand...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 23, 2022, 05:16:04 pm
content: Quote from: 2N3055 on October 23, 2022, 01:50:47 pmFirst of all, you shouldn't mark as false statements other people's opinions.A few days ago a SDS5000 got stuck in my hands and never recovered. Now it shows the spash screen every time you tried to boot it. I was just probing a small circuit stimulated at 200mV.Second, I sometimes I use scopes from A-brands (Agilent) at the lab, and they are quite different (not sure about the price range because they are no longer in production). The difference is remarkable.Third, this thing that one tells how he thinks and you scream disrespecting here and there has to stop. Some people think that Siglent and Rigol are built down to a price, while KS, Tek, etc are built to last.Other people think that they are equivalent, and the big brands rip you off by making you pay for their name. And even if it was exactly so, nothing wrong with that, because paying a premium for a brand that is old and has built a solid reputation across decades is perfectly normal in any kind of manufacturing domain. Is it OK to pay 1 million for a Ferrari when a Dacia can bring you from A to B all the same (well, kind of..)? It's up to each individual to decide. It's not that people who buy the Ferrari are idiots. Or if you want a more realistic analogy, put a BMW in place of the Ferrari.Stop patronizing people for their opinions. You should just tell them that you think differently, and articulate your thoughts.

author: Fungus
author_rank: Super Contributor
timestamp: October 23, 2022, 06:05:01 pm
content: Quote from: balnazzar on October 23, 2022, 05:16:04 pmIf only there was some sort of explanation:

author: noisyee
author_rank: Contributor
timestamp: October 24, 2022, 01:37:47 am
content: Quote from: balnazzar on October 22, 2022, 02:39:22 pmNowadays most scope equips with more than enough memory. Few Mpts to tens of Mpts is a good trade-off between response speed and available sampling rate at long time base.I had used some top class scopes with hundreds of Mpts memory, but only enable it at specific case. Like debugging high-speed data bus, in which case I want to capture as many packets as possible while keeping enough sampling rate. The response time is horrible: just take a cup of tea while waiting the scope processing it's data. So you MUST relay on proper trigger set-up, not a good luck, to capture what you want which again is a time consuming work.So why bother significant slow down while in most case you don't need GSPS at ms/div. Needless to say entry level scopes don't have the power to analysis full data at large memory depth. You don't want to capture a large frame and then zoom in to find what you want, trust me.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 24, 2022, 01:02:22 pm
content: Quote from: noisyee on October 24, 2022, 01:37:47 amIndeed! (About both your statements).