is it true, oscilloscope must reach at least 4x observed freq? - Page 1
Forum section: electronics / testgear

author: oxy
author_rank: Newbie
timestamp: September 12, 2022, 04:43:14 pm
content: Hi,some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz.Is it true?

author: bdunham7
author_rank: Super Contributor
timestamp: September 12, 2022, 04:46:53 pm
content: The first step in understanding this question is to accurately explain what you mean by "100MHz".

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 04:53:57 pm
content: Quote from: oxy on September 12, 2022, 04:43:14 pmAre you talking about bandwidth (100MHz) versus the sampling rate needed to achieve that in a digitising oscilloscope? By the Shannon/Nyquist/Whittaker/Kotelnikov sampling theorem you will need a sampling rate more than 2 times the bandwidth. So, by theory you need a sampling rate of 200Msps. In practice you need something a bit higher than that. Many scopes use something like 2.5x, while others use something considerably higher, like 4x or 5x.

author: Fungus
author_rank: Super Contributor
timestamp: September 12, 2022, 04:55:21 pm
content: Quote from: oxy on September 12, 2022, 04:43:14 pmNo. Theminimumneeded would be 250Mhz to observe a 100Mhz sine wave.That's why you see (eg.) 100Mhz, 4-channel 'scopes with 1GHz sample rate.

author: bdunham7
author_rank: Super Contributor
timestamp: September 12, 2022, 05:24:46 pm
content: Quote from: Fungus on September 12, 2022, 04:55:21 pmMSa/s and MHz are two different things and the way you put it is very confusing at best. The OP didn't actually specify a DSO and from the nature of his question, I don't think he's referring to sample rates.

author: m k
author_rank: Super Contributor
timestamp: September 12, 2022, 05:29:48 pm
content: Quote from: oxy on September 12, 2022, 04:43:14 pmWhat shape?

author: jonpaul
author_rank: Super Contributor
timestamp: September 12, 2022, 06:13:24 pm
content: See Claude Shannon 1948 BSTJ paper and book " The mathematical Theory of Communication"He was indeed the "Father of Information Theory.jon

author: nctnico
author_rank: Super Contributor
timestamp: September 12, 2022, 06:15:45 pm
content: Quote from: bdunham7 on September 12, 2022, 04:46:53 pmAgreed. 100MHz bandwidth means you can get a 100MHz sine wave on screen with 3dB amplitude error at most.

author: oxy
author_rank: Newbie
timestamp: September 12, 2022, 06:32:08 pm
content: Lets take the example of the picoScope Series 6000E:It has several models with bandwidths ranging between 500MHz and 1 GHz, yet all of them with sampling rate of 2.5Gs/s.As I understand the sampling rate sets over Nyquist the max. frequency that I can observe. Thus what hardware specification differentiates those oscilloscopes on the bandwidth?Thanks for ur very nice inputs!

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 06:33:50 pm
content: Quote from: oxy on September 12, 2022, 06:32:08 pmThe analogue front end bandwidth.

author: nctnico
author_rank: Super Contributor
timestamp: September 12, 2022, 06:44:10 pm
content: Quote from: oxy on September 12, 2022, 06:32:08 pmLikely they all have the same hardware but there is a software controlled filter in the input circuitry which limits the actual bandwidth. In some cases the lower bandwidth models have some extra filtering components in the input circuitry but the basic hardware design is the same.

author: bdunham7
author_rank: Super Contributor
timestamp: September 12, 2022, 06:45:19 pm
content: Quote from: oxy on September 12, 2022, 06:32:08 pmThat is entirely unrelated to your initial question. As already mentioned, the bandwidth rating is typically designated as the frequency where the amplitude response is down -3dB or to about 70%. The high frequency roll off can be due to the bandwidth limitations of the analog inputs or it can be the result of something further up the line (software filter, etc).

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2022, 08:28:49 pm
content: If as it seem sampling rate is what the thread is really about however 5x sampling vs BW is more the industry standard today.This with a shared ADC design provides 2.5x BW sampling which comfortably meets Nyquist.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2022, 08:41:01 pm
content: Quote from: oxy on September 12, 2022, 06:32:08 pmFor digital signals the period is irrelevant; only rise time is important. For a little theory and some practical measurements, seehttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/If you have a repetitive signal, the sampling frequency is separate to the signal frequency. I have a scope that measures 50ps rise times with ~40kSa/s. No, that does not violate Shannon/Nyquist! Various manufacturers have different names for the techniques, e.g. equivalent time sampling.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2022, 08:44:57 pm
content: Quote from: tautech on September 12, 2022, 08:28:49 pmSo you think that to usefully observe a 1kHz digital signal, you only need a 5kHz scope? That's nonsense, as you well know.The only thing that matters is the rise time. Digital circuits don't "care" when the next transition might occur, if ever.

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 08:47:18 pm
content: Quote from: tggzzz on September 12, 2022, 08:41:01 pmI think you mean "forrepetitivedigital signals".

author: switchabl
author_rank: Frequent Contributor
timestamp: September 12, 2022, 09:11:53 pm
content: Quote from: tautech on September 12, 2022, 08:28:49 pmYes, but the sampling theorem refers to the total bandwidth of a (band-limited) signal, not the 3dB bandwidth of the front-end. So realistically, there is nothing comfortable about it. At 2.5x you will either have to live with some non-negligible aliasing or with some non-negligible overshoot (steep filter).

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2022, 09:49:47 pm
content: Quote from: switchabl on September 12, 2022, 09:11:53 pmSure, however those that understand their instruments and could push them near their limits will use a single channel on each ADC so to be back near 5x sampling to minimize the chance of aliasing and also provide for greater accuracy measurements.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2022, 10:03:10 pm
content: Quote from: coppice on September 12, 2022, 08:47:18 pmNo, for repetitive signals. To put it anthropmorphically, the scope neither knows nor cares how you interpret the voltage waveform.

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 10:56:06 pm
content: Quote from: tggzzz on September 12, 2022, 10:03:10 pmOK. I have read the article now. What you said is confusing. You replied to something talking about sampling rates, and gave a reply including reference to a slow sample interval. I assumed by "period" you were referring to the sampling interval. From the article you are referring to the period of a waveform to be sampled. Most digital waveforms don't really even have a period. They don't repeat, so they aren't periodic.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2022, 11:48:13 pm
content: Quote from: coppice on September 12, 2022, 10:56:06 pmWhen considering sampling signals, it is necessary to be clear what the signal is. If you have an 10kHz audio waveform modulated onto a 10MHz carrier, then to recover the audio waveform youdon'thave to sample at >20MSa/s; >20MSa/s is sufficient. You do still need a >10MHz front end and a <50ns sampling pulse width, though.If, as in a TDR, the waveform you are observing doesn't change over time, then you can sample arbitrarily infrequently. The slope I referred to has >10 times slower sampling mode that matches the speed of a thermal pen recorder

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 11:56:35 pm
content: Quote from: tggzzz on September 12, 2022, 11:48:13 pmYou only need to sample at twice the bandwidth of the signal (or at one times the bandwidth for analytic sampling). However, for that to work well you have to be sure the signal has no content outside the band of interest. Also, the jitter in the sampling aperture needs to be just as low for a 2Msps ADC being used to sample a 1MHz band around 2GHz, as it would for a 5Gsps ADC sampling the entire band up to 2.0005GHz.Scopes don't have any way to clean up a signal, and their sampling jitter is not usually any better than it needs to be for their basic needs. So, it is rarely practical to use them to sample a limited band well above DC. The jitteriness of the sampling aperture can work in your favour in some situations. It makes ETS work a lot better, for example.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2022, 12:15:14 am
content: Bandwidth and sampling rate are completely independent. The sampling rate must meet the Nyquist criteria to accurately reconstruct the waveform, but this has nothing to do with the bandwidth as defined by the -3dB amplitude response, which is why equivalent time sampling works.As pointed out by tggzzz, a bandwidth limited signal can be reconstructed with a sampling rate greater than twice the bandwidth no matter where in the frequency spectrum it is, within the bandwidth of the sampler. The sampling function itself is equivalent to RF mixing, and the circuits can be identical. RF mixers make great microwave samplers when driven with a suitable pulse through their local oscillator port. The sampling part of an analog-to-digital converter can be modeled as a down-conversion mixer.Quote from: oxy on September 12, 2022, 04:43:14 pmIf you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and with a 3dB amplitude error.A more useful oscilloscope specification is rise time, which in the general case in nanoseconds is 350 divided by the bandwidth in MHz, so 100 MHz yields 3.5 nanoseconds. Oscilloscope rise time needs to be several times faster than the rise time of the signal for an accurate display.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2022, 12:34:14 am
content: Quote from: David Hess on September 13, 2022, 12:15:14 amFTFY.It need be noted label BW can be substantially different from actual -3dB BW as is the case with the 100 MHz rated SDS2104X Plus which the 1st we received I tested with 3 yes 3 signal sources to convince myself the ~185 MHz result was actually real !Supporting that result is they are also supplied with 200 MHz probes.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 12:55:55 am
content: Quote from: coppice on September 12, 2022, 11:56:35 pmCompletely false.Imagine a sine wave that you're sampling at exactly 2x frequency.a) Youmightsample the signal exactly on the peaks/troughs in which case you'll be fine.b) OTOH youmightsample it exactly on the zero-crossing points, in which case you'll see nothing at all.You can also get every possible value in between (a) and (b), it's just dumb luck.If you sample at 99.99999% of Nyquist you'll drift slowly between (a) and (b) and see the amplitude varying on screen ("AM effect").2.5x Nyquist is the minimum to avoid this AM effect.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 02:23:23 am
content: Quote from: Fungus on September 13, 2022, 12:55:55 amModern statements of the sampling theorem are sometimes careful to explicitly state that x(t) must contain no sinusoidal component at exactly frequency B, or that B must be strictly less than ½ the sample rate.

author: Someone
author_rank: Super Contributor
timestamp: September 13, 2022, 02:33:56 am
content: Quote from: The Electrician on September 13, 2022, 02:23:23 amand to be more clear, 2.5x is a figure that provides some non-specific limit to that effect under typical band limiting/anitaliasing/reconstruction anddoes notensure errors are avoided.We dont have perfect sampling, or prefect band limited signals, or perfect reconstruction, and they all contribute (among other things) even in pure digital or simulation.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 02:59:00 am
content: Quote from: Someone on September 13, 2022, 02:33:56 amI don't think you can get major distortions at 2.5x with a band-limited signal, but that's still an "if". How would you know?Workaround: Use as few channels as possible on your DSO to keep the sample rate as high as possible. If in doubt, turn channels on/off to see if the signal of maximum interest changes shape.

author: Someone
author_rank: Super Contributor
timestamp: September 13, 2022, 03:15:08 am
content: Quote from: Fungus on September 13, 2022, 02:59:00 amBecause I do this stuff for a living... you can keep putting out non-specific/vague figures but they are just that, something which might be true in some non-specified situation.There are easily found examples of 2.5x being inadequate.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 03:54:41 am
content: Quote from: Someone on September 13, 2022, 03:15:08 amPlease give us some examples where 2.5x is inadequate.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 04:07:11 am
content: Quote from: Someone on September 13, 2022, 03:15:08 amPlease educate us.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2022, 07:57:49 am
content: Quote from: David Hess on September 13, 2022, 12:15:14 amPrecisely.A good practical example is the "Tayloe mixer" found in SDR receivers.Those irritate me. 40 years ago I built an 4kHz bandpass filter with a Q of ~4000 using 10% capacitors. It was based on the N-path filter concept I found in a 1950s BSTJ paper. The concepts were so counterintuitive that I had difficulty explaining them to analogue/RF engineers.They continued to fascinate me, I felt sure they would be useful in other ways, but had no professional reason to use them. After retiring I was just gearing up to playing with them again, and found Tayloe had "beaten me to it". Rats.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 10:41:11 am
content: Quote from: David Hess on September 13, 2022, 12:15:14 amIn theory? Yes.In practical terms? Not so much. The reconstruction filter would become very unwieldy as you approach Nyquist.Quote from: Someone on September 13, 2022, 03:15:08 amI don't do it for a living but I've played around enough with digital audio synthesis to have confirmed the "2.5x limit" experimentally.ie. I've sat a potentiometer and manually dialed a frequency where I no longer observe the AM modulation effect mentioned earlier. The frequency I ended up with was right there in the 2.5x ballpark. Maybe itcouldhave been 2.4x but it's definitely not as low as 2.2x.2.5x may be a "non-specific/vague figure" but it works in practice.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 11:25:38 am
content: Quote from: Fungus on September 13, 2022, 12:55:55 amNo. Coppice is right. Nyquist says that you'll have all the information in a signal up to fsample /2 (note: UP TO). You'll need lots of computational power to reconstruct the signal though in order to visualise it. Remember: an oscilloscope is there to visualise a signal. When the sampling is done the data in the buffer needs to be post-processed so the human brain can see and indentify the signal.The factor 2.5 is just a convenient factor where an oscilloscope can use relatively simple sin x/x reconstruction and doesn't need an utterly steep anti-aliasing filter. If you throw in more computational power, you can get much closer to 2 and digitally correct for the steep anti-aliasing filter. But since the amplitude error will already be large near the bandwidth of an oscilloscope, it isn't worth the effort.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 11:38:08 am
content: just for a laugh, why don't we bring in some empirical data? a novel approach, i must confess, but do humour me just for a moment.980Hz (approx)square waveof about 8v p-p. sampled at various sampling rates...2000 sps (2x frequency):clearly this is a gross misrepresentation of the actual signal being measured! when data is scarce, a digital oscilloscope will default to showing a calming, un-contentious sine wave.5000 sps (5x frequency):this is just weird - the oscilloscope is getting seriously fooled by the input signal.10,000 sps (10x frequency):now we are starting to get there, although there is a whole load of waggling at top and bottom.20,000 sps (20x frequency):the 'evil' version of 10x.50,000 sps (50x frequency):starting to get there...100,000 sps (100x frequency):wow... at 100x (!) we have a pretty good representation of the original signal.200,000 sps (200x frequency):looks like the principal of diminishing returns has kicked in.10M sps (10,000x frequency):and this is what the scope chose when the "Auto Setup" button was pressed. looks like the scope manufacturer knows best - at least in this case!cheers,rob :-)

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 11:49:26 am
content: Quote from: robert.rozee on September 13, 2022, 11:38:08 amThat's not a 980Hz signal.That's a 980Hz signal plus a 2940Hz signal plus a 4900Hz signal plus a 6860Hz signal plus ... etc.The whole "Nyquist" thingonlyworks when the signal is bandwidth limited.Ref:https://en.wikipedia.org/wiki/Square_wave

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 12:01:13 pm
content: Quote from: Fungus on September 13, 2022, 11:49:26 ampiffle!it is a 980Hzsquare wave, and a perfectly acceptable example of what an oscilloscope is used for measuring. if the only thing you are going to measure is sine waves, then why even bother with an oscilloscope? get a frequency counter and RF voltmeter insteadcheers,rob

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 12:17:41 pm
content: Quote from: robert.rozee on September 13, 2022, 12:01:13 pmSure, but as you demonstrated so visually: It'snotsomething you can sample and reconstruct with a 2kHz sampling system.Because... it's a 980Hz sine wave plus a 2940Hz sine wave plus a 4900Hz sine wave plus a 6860Hz sine wave ... to infinity.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 12:38:12 pm
content: Quote from: Fungus on September 13, 2022, 12:17:41 pmthe device generating the square wave did not make use of an form of infinity, either theoretical or actual.now a square wave can berepresentedby an infinite sum of sine waves, but it is not made out of them. it is made with just a little fellow switching a light switch: on... off... on... off... on... off... very fast.the infinite sum of sine waves you talk about only exists in one's imagination.cheers,rob :-)

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 12:58:10 pm
content: Quote from: robert.rozee on September 13, 2022, 12:38:12 pmFun fact: Your ears can't tell the difference between a 10kHz square wave, a 10kHz sawtooth wave and a 10kHz sine wave. They all soundexactlythe same!

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 01:03:28 pm
content: Quote from: Fungus on September 13, 2022, 12:58:10 pmi thinkyourears might need checking...cheers,rob :-)

author: wasedadoc
author_rank: Super Contributor
timestamp: September 13, 2022, 01:05:53 pm
content: Quote from: robert.rozee on September 13, 2022, 12:38:12 pmAperfectsquare wave can be represented by that infinite sum. In practice no generator can make a waveform that has zero risetime. It may be close enough to zero to make no engineering difference in the situation under examination but the risetime will be non-zero and in the frequency domain that corresponds to no harmonics above some point.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 01:27:19 pm
content: No. Google 'Gibbs phenomenon'

author: switchabl
author_rank: Frequent Contributor
timestamp: September 13, 2022, 01:30:16 pm
content: Quote from: Fungus on September 13, 2022, 12:55:55 amThe familiar version of the sampling theorem as published by Shannon is his seminal 1948 and 1949 papers assumes a function f that "contains no frequencies higher than W cps". At first sight this might seem to include your counter-example (sine at frequency W), however Shannon is writing for engineers, not mathematicians, so he glosses over some details. The proof has an implicit assumption that f is square-integrable which in fact excludes signals containinganypure sines. Note that there are ways to generalize this using more advanced mathematical machinery but you may or may not be content to just claim that there are no pure sines in the real world anyway.On the practical side, it is also important to realize that the sampling theorem is inherently infinite time. It assumes that you have sampled the signal for an infinite duration. If you take a finite portion of a band-limited signal you are effectively applying a window function and thereby broadening the spectrum beyond its original bandwidth.¹ Together with any sampling jitter this will give you a hard limit that is higher than 2x but probably not by much.More importantly, the sampling theorem also assumes you are usingallthe samples for reconstruction all the time but you probably aren't because a) it would use a lot of processing power and b) it doesn't work for real-time applications (the ideal sinc-filter is non-causal and has infinite delay). Depending on the number of taps you use, frequencies close to the Nyquist limit will still alias close to DC. This is what you observe. However, this is a trade-off and the limit of 2.5x is an arbitrary choice of the implementation you are using. You can get a lot closer to 2x if you use more than a couple of samples for interpolation.¹ It will technically no longer be band-limited at all (you cannot have a signal that is both time- and band-limited). But let's ignore that here, as long as it drops below our noise floor, we don't really care.

author: mawyatt
author_rank: Super Contributor
timestamp: September 13, 2022, 01:44:36 pm
content: Quote from: tggzzz on September 13, 2022, 07:57:49 amInteresting, we employed the N-path filter concept back in ~1980 to pick out signaling "tones" displaced within multiple 4KHz bandwidth "sections" displaced at microwave frequencies, the filter had a bandwidth of 10Hz. These multiple (thousands) 4KHz sections were down-converted using Nyquist as our "friend" rather enemy, employing Microwave RF Downconversion by means of Nyquist sub-sampling where we knew exactly where each multiple displaced 4KHz section would be and where the tones should be.So in a way Nyquist can be helpful but not in the DSO discussions here!!It's amazing that it took another ~35 years to discover the N-Path Mixer, or Polyphase Mixer, which is fundamentally the N-Path Filter without the up-conversion back end sectionBest,

author: BillyO
author_rank: Super Contributor
timestamp: September 13, 2022, 01:57:39 pm
content: I wonder if the OPs question is well answered here? I would be good if he could return and try to answer the outstanding issues with his first question.He may not know the difference between BW and SR.

author: switchabl
author_rank: Frequent Contributor
timestamp: September 13, 2022, 02:08:52 pm
content: Quote from: robert.rozee on September 13, 2022, 11:38:08 amThis may be a nice illustration of Gibb's phenomenon but it is fortunately not what happens if you sample with a properly matched anti-aliasing filter. It is what happens if you just ignore the Nyquist criterion. And then apply sinc-based interpolation anyway. Did you have to turn that on manually or is that actually the default? If so, that would seem to be either a bug or a strange design choice indeed.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2022, 02:15:17 pm
content: Quote from: mawyatt on September 13, 2022, 01:44:36 pmYes indeed. I'm irritated that I didn't have a chance to (re)invent the Tayloe mixer. I would have had a fighting chance if I'd been working in the right field. Oh well.My use was simply to reduce thermal and environmental noise when using a large area BPW92 photodiode to measure the loss in early installed multi mode fbres. When asked why I didn't use a PLL, I said I couldn't predict how long it would take to lock up, which would have been a problem.I still like to think I could find a use for ASP, I.e. analogue sampling/signal processing.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 02:50:26 pm
content: Quote from: robert.rozee on September 13, 2022, 01:03:28 pm(Yoda voice)So sure, you are.PS: What you're saying is that you can hear 30kHz sine waves - square waves are the sum of sine waves.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 02:53:32 pm
content: Quote from: switchabl on September 13, 2022, 01:30:16 pmSure there are. There's just a lot of harmonics mixed in so isolating them is difficult.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 03:03:45 pm
content: Quote from: switchabl on September 13, 2022, 02:08:52 pmthe default behavior of a Siglent SDS1204X-E. i'd imagine most other similar modern DSOs would default to the same.Quote from: BillyO on September 13, 2022, 01:57:39 pmhe asked a simple question, and in return received a whole jumble of complex technical explanation that went straight over his head - and that was, indeed, irrelevant to the context in which his question sat. i'd imagine he is long gone, and will likely not be seen on the forums again.to answer the OP's question:as the (broadly speaking) frequency of the signal you're sampling heads towards 1/2 of the scope's sampling rate, the image displayed by your DSO will approach a smooth undulation / sine curve. if you wish to observe fine detail in a complex waveform, you need to ensure that your DSO has a sampling rate at least several times the 'frequency' of that detail - as per the earlier posted screenshots.if you are just wanting to observe a simple RF signal (such as AM modulation on an RF carrier), then you can get away with a sampling rate that is a tad over 2x the carrier frequency. just be wary that a DSO will 'smooth over' signal features that approach the scope's limits.cheers,rob :-)

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 03:07:46 pm
content: Quote from: switchabl on September 13, 2022, 02:08:52 pmIt's mathematically correct. What else would it do?

author: switchabl
author_rank: Frequent Contributor
timestamp: September 13, 2022, 03:32:42 pm
content: If the scope has a 200 MHz front-end and the sampling rate is set to 10 kSa/s like in the example, it is not mathematically correct (unless the user makes sure the signal itself is < 5kHz BW or it does decimation). Keysight scopes e.g. switch to linear interpolation in such a scenario which I think is a sane default. At the very least it gives you an immediate visual cue that you are sub-sampling. I guess the only mathematically correct option would be dot mode?EDIT: I am not sure if the Siglent behaviour is a Lecroy-ism or if this is actually more widespread. It seems strange to me.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 04:08:46 pm
content: Quote from: switchabl on September 13, 2022, 03:32:42 pmAnd are therefore less valuable when you're trying to learn signal theory.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 04:25:16 pm
content: Quote from: switchabl on September 13, 2022, 03:32:42 pmIMHO switching to linear interpolation isn't a good solution either. It is entirely possible that the signal you are looking at still meets Nyquist at a lower sampling rate. The oscilloscope simply doesn't know that. This is a typical case where the user needs to know what he/she is doing.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 04:35:35 pm
content: Quote from: nctnico on September 13, 2022, 04:25:16 pmYep.The correct thing to do would be to put a (discreet) warning message on screen, not to try and "out-think" the user.A usermightknow what they're doing or they might be trying to demonstrate/explore the limits of signal theory.

author: 2N3055
author_rank: Super Contributor
timestamp: September 13, 2022, 04:35:56 pm
content: Quote from: switchabl on September 13, 2022, 03:32:42 pmOn first image, scope is set for 500 us/div (7 ms total on screen) and 2kS/s. That is only 14 points on the screen, although scope shows memory setting of 14 kpoints.On other images timebase and memory depth is show the same but sample rate is different?How was variable sample rate with fixed memory length and same timebase achieved? SDS1104X-E has manual sample rate setting? I don't have it, so I'm asking for clarification and manual is not saying...Sample screen shown are certainly not how scope would behave by default.Scope is by default trying to keep sample rate as high as it can. Also that is smallest memory settings, only 14kpoints of 14Mpoints available, so 1000 times worse than possible.Also it has both linear and sin(X)/X interpolation, and point mode that does no interpolation whatsoever. And would probably show this signal nicely.I can demonstrate wrong signals with any scope if I deliberately set it wrong...I understand that post was meant to demonstrate sampling, but this is not scope normal behaviour. Robert had to specifically set it to show these effects.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 04:39:11 pm
content: Quote from: 2N3055 on September 13, 2022, 04:35:56 pmDoes that mean it could zoom out a bit?

author: 2N3055
author_rank: Super Contributor
timestamp: September 13, 2022, 04:53:54 pm
content: Quote from: Fungus on September 13, 2022, 04:39:11 pmNo it probably means he captured this at extremely long timebases like 1sec/div and then changed timebase back to 500us/div with stopped acquisition to provoke this effect. Like I said, definitely not how you would use the scope..While i understand he did it to explain sampling, without explaining what was done, people here started discussing how scope is performing poorly. Any scope would do this even Keysights mentioned if driven this way..

author: switchabl
author_rank: Frequent Contributor
timestamp: September 13, 2022, 05:29:49 pm
content: Quote from: nctnico on September 13, 2022, 04:25:16 pmIt is not ideal but I think it makes sense as a default. The oscillscope can't know, so it shouldn't guess but instead use the safest option. I am not saying you shouldn't be able to override it if you want to. If you have a large touch-screen, it might be nice to have a little warning symbol pop up that you can tap to change the setting or something. I don't know if anyone actually has that.I mean if you actually forced a super low sample-rate manually, I guess you had it coming and are probably getting what you wanted. But if just did a single-shot at a long timebase, the sample-rate dropped automatically and then you zoom in, chances are you didn't want to see massive ringing that isn't actually there.Quote from: 2N3055 on September 13, 2022, 04:53:54 pmNo, I realize what he did. I was just surprised it doesn't turn off sinc-interpolation when you do that (I am sure you could do that manually). With the Keysights this actually happens more often because they don't have a lot of sample memory, so I know they switch to linear interpolation and don't do this.

author: Fungus
author_rank: Super Contributor
timestamp: September 13, 2022, 05:55:29 pm
content: Quote from: switchabl on September 13, 2022, 05:29:49 pmTo be fair: There was a caption that said"2000 sps (2x frequency)"above the image.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 13, 2022, 06:03:15 pm
content: Quote from: 2N3055 on September 13, 2022, 04:53:54 pmexactly: use timebase to set acquisition rate (2x frequency, 5x, 10x, 20x, 50x, 100x, 200x), allow buffer to fill, stop, then adjust timebase to set 500us/div. in all cases 14kpts of buffer was more than sufficient.this simulated a DSO measuring a 100MHz (as specified by original poster) square wave, with a maximum sample rate of 20Gsps, and unencumbered by any bandwidth limitations on the analog front-end.i would have used an actual 100MHz signal and my 20Gsps DSO, but it is currently in at the shop having the oil changed and the fluffy dice re-calibrated...cheers,rob :-)(image from:https://en.wikipedia.org/wiki/Fuzzy_dice#/media/File:1958_Ambassador_4-d_hardtop_fuzzy_dice.jpg)

author: 2N3055
author_rank: Super Contributor
timestamp: September 13, 2022, 07:44:02 pm
content: Quote from: switchabl on September 13, 2022, 05:29:49 pmIf you are doing long single capture you don't set scope for smallest memory setting.... He could have set scope for 14MPts and have 1000X better sampling rate. If you want to capture all possible glitches use peak detect mode.. For single captures like that I set scopes for all it has (200-250 Mpts, would use more if available, although it gets to the point of diminishing returns after a while) and I can get lots of data at full sample rate...As for linear/sinc interpolation, that is a matter of visual aesthetics in this case.. Both are showing nonsense...And as I said, you could set Siglent for dot mode and get "sort of" ETS and it would actually show signal correctly (if signal is repetitive, of course) .. Or it would show single dots and give you info on sparsity of data. So you know that you should up the sampling rate...

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 07:55:41 pm
content: I figured out what robert.rozee was doing a couple of hours ago, and did the same thing on my Agilent DSO5054 scope. I set the timebase to 50 seconds/division and used the probe cal signal as input. I went back to watching the Queens last trip to London while wating for the buffer to fill up. I then switched the timebase to 500 uS/div and set the display to dot mode. Here's what that looks like; the dots are really hard to see, but they're there:]Here it is with Agilent's linear interpolation turned on:If you can see sharp corners in linear interpolation mode where there should be smooth curves, the signal is probably sub-sampled. Of course, some sharp corners should be there, such as the corners of a square wave, but if a sine wave looks like it's made of only a few straight lines, that's what I'm referring to.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 08:08:16 pm
content: The signal isn't sub-sampled. Just turn sin x/x reconstruction on and you'll see the actual waveform. Linear (vector) display is one of the most useless features of a DSO IMHO.Quote from: 2N3055 on September 13, 2022, 07:44:02 pmNo, not at all. Both methods will show a waveform that connects all the sample pointsby definition. Remember an oscilloscope is there to visualise a waveform. It makes little sense to turn sin x/x off when there are few sample points to work with; it is there to help your brains to see and interpret the signal. However the assumption is that the oscilloscope is setup in a way the signal is sampled correctly (either through a high enough samplerate or using ETS on a periodic signal).

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 08:42:44 pm
content: Quote from: nctnico on September 13, 2022, 08:08:16 pmThat signal is quite sub-sampled. The rise time of the square wave is 180 nS; no details of the rising and falling edges are visible. The sample rate is 8 Ksa/s and there is substantial frequency content well above 4 kHz.This scope is old enough that it doesn't have sin x/x interpolation. As switchabl mentioned the older Agilent scopes typically use linear interpolation, and it isn't at all useless. If your choices are dot mode and linear interpolation, linear interpolation is just fine. When your signal is oversampled, you can't tell the difference anyway. This scope has rated max sample rate of 4 Gsa/s, and analog bandwidth of 500 MHz, so it's 4 times oversampled at rated bandwidth, and much more than that for lower bandwidth signals.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 09:08:48 pm
content: Quote from: The Electrician on September 13, 2022, 08:42:44 pmFirst of all, I strongly doubt the older Agilent scopes don't have sin x/x reconstruction. This is a pretty basic functionality for a DSO. Even my old CRT based Iwatsu DS-8617 DSO had sin x/x interpolation. And for a good reason, with 4 times oversampling you won't get a true representation of a waveform using vectors (linear) reconstruction.Secondly, feeding a high frequency waveform and then using a too low samplerate is just 'user error'. It proves nothing.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 09:10:22 pm
content: The Agilent DSO5054 does not have sin x/x interpolation.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 13, 2022, 09:12:50 pm
content: The Agilent DSO5054 does not have sin x/x interpolation. It's only at 500 MHz that the oversampling rate is 4x. At lower frequenies, for example audio and low RF, the oversampling rate is much higher.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2022, 10:06:27 pm
content: Quote from: robert.rozee on September 13, 2022, 03:03:45 pmThat is misleading to the point of being false.Replace "frequency of the signal" with "the highest component frequency of the signal" and that would be more nearly correct.QuoteCompletely wrong in several important respects, as noted earlier in the thread - and in more detail in standard textbooks.

author: nctnico
author_rank: Super Contributor
timestamp: September 13, 2022, 10:25:21 pm
content: Quote from: The Electrician on September 13, 2022, 09:10:22 pmIt has. It says in the datasheet but not in the manual. It switches automatically between linear and sin x/x mode when linear interpolation is not going to work to produce a waveform that resembles the signal at the probe tip. BTW: I used to own an MSO7000A which is basically the same hardware platform as your DSO5054 so I have some hands-on experience with these models.

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 01:04:26 am
content: Quote from: BillyO on September 13, 2022, 01:57:39 pmYes, the thread title and opening post are vague enough that the usual suspects are making some big argument out of thin air:"perfect band limited signal in infinite time capture can be perfect"vs"real world signal with real world oscilloscope does not work that way"Even "sin(x)/x" is not a fully specified filter, as many (most?) implementations are truncating/windowing the extents.2.5x is a magic number associated with "sharp" yet still practical antialiasing filters that has been widely misunderstood. It all depends on the input (antialiasing) and output (reconstruction) filters in the specific implementation/use-case.... remembering that most scopes dont have those sharp filters and use gaussian/bessel filters or something close to that for which the "rule of thumb" is 4x or 5x sample rate to bandwidth (attached below since Keysight have started personalinfo-walling all their documents).

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 01:11:17 am
content: Quote from: The Electrician on September 13, 2022, 03:54:41 amPlease provide the band limited signal source that has zero energy above the measurement frequency and I'm sure it would be difficult. But real world signals are not perfectly band limited....This can go around and around as many times as you like. It's an entirely different argument which has derailed the thread immediately into complete nonsense.What are the errors that you are trying to avoid/limit? what is the accuracy of the measurement (and what is the measurement) you are trying to make? What are the antialiasing and reconstruction filters being used?With that sort of information you could start to answer what the actual bandwidth to sample rate is required. Its not some simple every situation works at 2.5x or everything is fine once you go past 5x, it depends on the specifics.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2022, 01:32:13 am
content: Quote from: Someone on September 14, 2022, 01:11:17 amI'll take that as a no.Quote from: Someone on September 14, 2022, 01:11:17 amNo, but in the real world there are somevery simplecases that fail below 2.5x. 2.5x is a good starting point.FWIW: I was observing the AM modulation effect below 2.5x using ahardwarereconstruction filter.

author: BillyO
author_rank: Super Contributor
timestamp: September 14, 2022, 02:03:06 am
content: Quote from: Someone on September 14, 2022, 01:04:26 amThat's what I see too. If the OP is a newbie to all this and does not even know the difference between BW and SR, how is this discussion gong to help them? More than likely it will send them away. There seems to be a certain amount of elitism on this site, and I can understand that, but if you are not willing to address a new members basic questions with patience then maybe go off to the the "I know everything better than you" forum and hash out the nuances there.We need more people in this field rather than out of it and contemplating f'd up concepts of ignorance such as flat earth and anti-science. The experts are the ones we need supporting STEM and helping people elevate themselves and not alienating those taking baby steps. This is something I struggle with too, so..Sorry for the rant.

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 02:07:51 am
content: Quote from: Fungus on September 14, 2022, 01:32:13 amAgain, this can keep going around. You stick with 2.5x as the magic figure yet with none of the other simultaneous conditions/requirements specified or quantified. Complete nonsense. If you want to talk about 2.5x sample rate vs frequency being some accurate or significant number, then you need to work through the rest of the maths that justifies that. "hardware reconstruction filter" says nothing of its form/length/characteristics and is something that is usually undocumented in an oscilloscope.Observingsomethingwhichlooks"ok" to you, is not quantifying anything. When you are the one being so insistent on 2.5 as some magic quantification. 2.5x is a massive gross simplification thatlooksok in most cases for a single frequency sine wave.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2022, 02:32:19 am
content: Quote from: Someone on September 14, 2022, 02:07:51 amMe? I'm not the only one. Rigol/Siglent/etc. have been building it into their oscilloscopes for many years and people are happy with the results.What number wouldyousuggest as a workablecompromisefor building real devices?

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 03:05:28 am
content: Quote from: Fungus on September 14, 2022, 02:32:19 amAs I keep saying, there is no single figure. It depends on what the desired outcome/performance requirement actually is. These things can be quantified and discussed, but instead the shortcut/"rule of thumb" is shouted as some absolute (by you and others) who actually have no understanding of its basis. The short and oversimplified answer is easy to communicate, and is dominating the discussion. While the complex and actual answer is buried by the noise.

author: BillyO
author_rank: Super Contributor
timestamp: September 14, 2022, 03:50:31 am
content: Point made. Too bad.

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2022, 04:04:04 am
content: Quote from: oxy on September 12, 2022, 04:43:14 pmTechnically not true, but a good rule of thumb. It depends on the front end bandwidth filter shape and the interpolation used. IIRC there is a Tek paper somewhere that goes through the math for x2.4 is the minimum or something for a gausian shape front end.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:27:28 am
content: Quote from: nctnico on September 13, 2022, 10:25:21 pmPost a link to said datasheet, please.Here's what I get with a sine wave at the probe tip. The scope didn't live up to the data sheet's promise: "It switches automatically between linear and sin x/x mode when linear interpolation is not going to work to produce a waveform that resembles the signal at the probe tip."

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2022, 07:33:39 am
content: Quote from: EEVblog on September 14, 2022, 04:04:04 amI searched the Tek web site and found a couple of places where it says "2.5x":eg.https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-sampling(see appendix for math)This onesays:Code:Oscilloscope Sample Rate > 2.5x highest frequency component of signal (For sin(x)/x interpolation)Oscilloscope Sample Rate > 10x highest frequency component of signal (For linear interpolation)So that explains the "x10" rule that the old fogies occasionally mutter around here when they see Rigols/Siglents - they were using 'scopes with linear interpolation(!)

author: gf
author_rank: Super Contributor
timestamp: September 14, 2022, 08:47:44 am
content: Quote from: The Electrician on September 14, 2022, 04:27:28 amWhat was the timebase and sample rate for the screenshot?Maybe the automatic interpolation is only available at a couple of fastest timebase settings, where the sample rate is already at maximum, so that the resolution in samples/div cannot be increased any more by increasing the sample rate.(That's at least what my cheap Chinese scope does.)

author: wasedadoc
author_rank: Super Contributor
timestamp: September 14, 2022, 09:05:02 am
content: Quote from: Someone on September 14, 2022, 02:07:51 amIf the specific combination of sampling rate and reconstruction filter gives an acceptable result on that single frequency sine wave it will also give an acceptable result on any lower frequency single sine wave. Then by the superposition principle it will also give an acceptable result on any combination of such sine waves. That isanywaveform which does not contain frequencies above that sampling rate divided by 2.5.The 2x is the theoretical limit for perfect reconstruction of the original. As the sampling rate is decreased down towards 2x the reconstruction filter requirements for perfect reconstruction become more and more demanding. In practice a sampling rate of 2.5x is a good compromise between practically realisable filter complexity and accuracy of reconstruction.The Nyquist limit (half sampling frequency) applies when the input signal can contain any frequencies up to that limit. There are situations when the input signal spectrum is not continuous. For example has "gaps" such as monochrome TV signals where the energy is largely confined around integer multiples of the TV line frequency. (Those gaps were used by the NTSC composite colour system by arranging that most of the energy of the modulated colour subcarrier is confined around n+0.5 multiples of the TV line frequency.) In the case of non-continuous spectrum signals it is quite possible to employ sub-Nyquist sampling with the alias components folding back into the vacant areas. And it is possible to unfold them later and reconstruct the original signal.

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2022, 09:57:24 am
content: Quote from: The Electrician on September 14, 2022, 04:27:28 amI'm pretty sure you can find that yourself. And yes, sin x/x is supported on the DSO5054 so either you found the 'sweet' spot where it doesn't work or your settings are incompatible. I don't recall my MSO7104A ever showing a signal like yours (I checked some of my old screenshots to be sure).

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 10:21:23 am
content: Quote from: wasedadoc on September 14, 2022, 09:05:02 amCool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise withunspecifiedcriteria. What is actually important? amplitude accuracy? relative phase? waveform shape?I interpret the OP's question as much more toward the balance of fundamental frequency of a clock/signal and the bandwidth limit of the scope, since both are expressed in MHz and nowhere is sampling mentioned.So all this chest beating by people who want to talk about Nyquist (without actually pointing to how it applies to oscilloscopes and their antialiasing + reconstruction) is off topic, and in many points wrong. Band limited signals are pure imagination, they dont exist. Perfect antialiasing filters dont exist, any real sampled signal has errors that should be quantified if you want to start putting some limit on their effect.

author: adam4521
author_rank: Regular Contributor
timestamp: September 14, 2022, 11:01:42 am
content: Yes, sin x/x on Agilent/Keysight only on short time bases/maximum sample rate. If there is any decimation going on (eg you are zoomed in on a longer capture) it will join the dots with straight lines.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2022, 11:06:40 am
content: Quote from: Someone on September 14, 2022, 10:21:23 amPractical experience has found a good balance: Sample at 2.5x the analog bandwidth of the oscilloscope's input circuitry and use sin(x)/x reconstruction to display it on screen.It's good enough for Australians.PS: Most DSOs can get double/quadruple the basic sample rate by limiting the number of enabled channels.

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2022, 11:58:46 am
content: Quote from: Fungus on September 14, 2022, 07:33:39 amI don't think so. The "old fogie x10 rule" started with the Tek TDS210, it was the first "real time" digital scope that looked and acted like an analog scope, and it had 1GS/s for 100MHz bandwidth and also SinX/x. So x10 kinda became the defacto standard that gave everyone the "warm fuzzy" that digital scopes were now fast enough to look and feel like a "real scope" when you turned on the sample dots and could see how many samples it was taking.

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2022, 12:02:37 pm
content: Quote from: Someone on September 14, 2022, 10:21:23 amRemember that the input wave shapegets changedby the input bandwidth response of the scope front end. Different scopes and models have different input antialiasing filters and responses.This is why the shape of the input filters matters in these dicsussions. Gaussian response is usually assumed unless otherwise specificed.Tek mention a x5 rule here:https://www.tek.com/en/documents/primer/evaluating-oscilloscopes

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2022, 12:11:00 pm
content: The Tek x2.5 reference with the math explained for Sinx/x:https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-sampling

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2022, 02:40:01 pm
content: Quote from: EEVblog on September 14, 2022, 12:02:37 pmGaussian response is long gone. The typical anti-aliasing filters are much steeper. If you want a Gaussian response, then you'll need to set the bandwidth limiter. IMHO this is one of the reasons higher end scopes have multiple bandwidth settings so the user can choose to have maximum bandwidth or a slow (first order) roll-off.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2022, 02:59:07 pm
content: Quote from: nctnico on September 14, 2022, 02:40:01 pmPersonally I like having the option to turn off interpolation and simply see dots representing samples.

author: The Electrician
author_rank: Frequent Contributor
timestamp: September 14, 2022, 03:14:49 pm
content: Quote from: nctnico on September 14, 2022, 09:57:24 amThe only data sheet I can find is:https://www.keysight.com/us/en/assets/7018-08449/data-sheets/5989-6110.pdfIt doesn't say "It switches automatically between linear and sin x/x mode when linear interpolation is not going to work to produce a waveform that resembles the signal at the probe tip." Why is there no mention of an exception where this doesn't work; and I certainly wouldn't call a failure to perform as claimed a "sweet spot".The data sheet I found says on page 13: "Sinx/x interpolation (single shot BW = sample rate/4 or bandwidth of oscilloscope, whichever is less) with vectors on and in real-time mode". It doesn't say a word about linear interpolation, which I've demonstrated that the actual oscilloscope can do. If the scope can do both, why don't they mention both? Perhaps the mention of "Sinx/x interpolation" was something they intended to do when the data sheet was written, but they chose not to include in the actual hardware when it came time meet a deadline and get something to market.If you don't post a link to a data sheet that says "It switches automatically between linear and sin x/x mode when linear interpolation is not going to work to produce a waveform that resembles the signal at the probe tip.", your credibility is zero. You're claiming something you haven't backed up.Then there is the user manual:https://www.keysight.com/us/en/assets/9018-01896/user-manuals/9018-01896.pdfWhich says on page 187: "When enabled, Vectors draws a line between consecutive waveform data points." No mention anywhere of sin x/x.And there is the performance of an actual DSO5054 which I show in Reply #80

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2022, 04:14:14 pm
content: Quote from: tggzzz on September 14, 2022, 02:59:07 pmSome people even turn the screen persistence way up then enable dot mode and pretend they can see the real signal building up on screen.

author: gf
author_rank: Super Contributor
timestamp: September 14, 2022, 04:59:14 pm
content: Quote from: Fungus on September 14, 2022, 04:14:14 pmIf the fundamental frequency is not an exact integral fraction of the sample rate and if the trigger point is interpolated between the adjacent samples, then it is not impossible.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2022, 08:33:51 pm
content: Quote from: Fungus on September 14, 2022, 04:14:14 pmWhy "pretend"? In some cases that is what happens.Linear (r higher order) interpolation is a fiction designed to help those that never mentally completed "join the dots" puzzles as a kid

author: Someone
author_rank: Super Contributor
timestamp: September 14, 2022, 10:47:32 pm
content: Quote from: nctnico on September 14, 2022, 02:40:01 pmGaussian analog anti-aliasing filters aren't disappearing (as they are cost effective), the work is being shifted to digital. Tek front end ASIC comparison below.A modern scope has multiple different sampling rates with all sorts of filtering going on along the chain, its far removed from the simple models people keep trying to attach to it.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2022, 03:18:07 am
content: Quote from: tautech on September 13, 2022, 12:34:14 amSome oscilloscopes are weird. Even old high bandwidth instruments often break the Gaussian response -3dB rule, and the 0.35 rule does not apply to them either. For other instruments, the specified bandwidth was more of a guideline than a rule, and the instrument was *at least* that fast. Modern examples of weirdness include the Rigol 1000Z series where full power bandwidth varies with the V/div setting.QuoteTektronix specified their oscilloscopes for -3dB bandwidth *at the probe tip*, so a 200 MHz Tektronix oscilloscope with a 200 MHz Tektronix probe yielded 200 MHz. At least for them, the probe bandwidth specification represents the highest frequency which the probe will reproduce faithfully and not the -3dB bandwidth.Quote from: Fungus on September 13, 2022, 10:41:11 amIn a subsampling bandpass application, the input signal bandwidth is tightly constrained by a high selectivity bandpass filter, so the difficulty of reconstruction is considerably relaxed. Ideally aliasing products will be close to or below the noise floor.QuoteI find 2.5x times to be optimistic when sin(x)/x reconstruction is required. The effect you mention can be modeled as the result of non-linear mixing between a pure sine wave and the sampling clock, with aliasing of some of the mixing products. The reconstruction then has multiple solutions and each pass shows one of those solutions. When a fast edge is applied, the same thing happens even when the reconstruction filter correctly filters the higher frequency aliasing products because the mixing products are still there close to the aliasing frequency. I find the result annoying in the extreme.The solution is to use a faster sample rate. Sometimes there is no substitute for high bandwidth and fast sampling rate.Quote from: The Electrician on September 13, 2022, 08:42:44 pmFor Tektronix you have to go back to 1990 and the 2232 series of DSOs for the last of their DSOs which did not support sin(x)/x reconstruction, and those are what I consider to be the first "modern" DSOs, at least from Tektronix. Their 2430 series first made in 1986 had sin (x)/x reconstruction and I think all of their instruments after that, other than the 2232 series, did.Honestly I have never missed sin(x)/x reconstruction on the 2230 and 2232. Usually equivalent time sampling removes the need, and cases where single shot acquisitions need to be made with full bandwidth signals are rare.

author: radiolistener
author_rank: Super Contributor
timestamp: September 16, 2022, 03:53:45 am
content: Quote from: oxy on September 12, 2022, 04:43:14 pmNo, for 100 MHz signals, the oscilloscope bandwidth should be about 1 GHz (10 times).This is because you're needs to see at least 10 harmonics of square wave in order to see waveform distortions. Otherwise you will see just a sine and cannot see signal distortions and glitches.You can still use 400-600 MHz bandwidth oscilloscope, but it's ability to see 100 MHz waveform details will be very limited. For example, you can see if signal exists, and can see if it's clean sine. But cannot see if it has glitches or distortions.Note, that oscilloscope bandwidth is applied for a clean sine wave. If you're dealing with other kind of waveform it consists of high order harmonics and oscilloscope should be able to see it, otherwise your scope input will works like low pass filter and all what you can see is just a 100 MHz sine despite the fact that signal waveform is actually square wave, triangle or other type.

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2022, 09:42:22 am
content: Quote from: radiolistener on September 16, 2022, 03:53:45 amOscilloscopes are time domain instruments; spectrum analysers are their frequency domain equivalent. The principal use of an oscilloscope is to show waveform shape, especially a waveform's risetime, overshoot etc.If you concentrate on the fidelity of displaying the risetime and ignore frequency domain characteristics, all of the frequency domain "specifications" are seen to be misleading and ambiguous rules-of-thumb.

author: BU508A
author_rank: Super Contributor
timestamp: September 16, 2022, 10:09:21 am
content: Screenshot from a datasheet of the DSO5054A:Datasheet source:http://www.testequipmenthq.com/datasheets/Agilent-DSO5054A-Datasheet.pdf

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 15, 2022, 05:49:06 pm
content: Just to say that I found this thread to be quite useful and informative. Not all the details examined are clear to me, but it's a very good starting point for further investigation.Especially the stuff about sampling rate and accurate signal reconstruction (which is not what the OP originally asked, I think).Thanks!