Modern digital scopes: real-time sampling or equivalent-time sampling - Page 1
Forum section: electronics / testgear

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 08, 2014, 10:29:49 pm
content: I was wondering if all modern digital scopes today offer real-time sampling?For most digital scopes the sampling rate is divided by the number of active channels.I guess this is because they perform real-time sampling?What do most digital scopes offer today? Both options?Are their other differences in the way digital scopes sample the incoming signal, besides actual sample rate, real-time or equivalent-time sampling?Like to have a better understanding about the different possible implementations, and how they differ between Tektronix, Agilent, Rigol, etc.Rigol DS1000/2000: Do Rigol scopes offer both options?Tektronix TLS216: this scope had massive amount of 16 channels, had 2 GS/s per channel, irrespective of how many active channels you were using. How did they do that?Did that scope use real-time sampling or equivalent time-sampling?Philips PM3340: This scope offered 2GS/s back at that time. But it was equivalent-time sampling. Does anybody know if it also offered real-time sampling, and what the actual bandwidth was?

author: EEVblog
author_rank: Unknown
timestamp: October 08, 2014, 10:33:49 pm
content: I've heard the new Rigol 1000Z does not offer equivalent time sampling any more.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 08, 2014, 10:57:25 pm
content: Is it correct that you were going to review the DS1054Z?Would be cool as well to have a review of the MSO2072A, and specifically the Logic Analyzer features.

author: EEVblog
author_rank: Unknown
timestamp: October 08, 2014, 11:46:58 pm
content: Quote from: pascal_sweden on October 08, 2014, 10:57:25 pmYes, one is on the courier truck now.It's a loaner, won't be getting my own for some time, not in stock in Oz yet.

author: edavid
author_rank: Super Contributor
timestamp: October 09, 2014, 12:46:01 am
content: Quote from: pascal_sweden on October 08, 2014, 10:29:49 pmYes.QuoteDave is correct, the DS1000/DS2000 are real-time sampling only.QuoteReal-time sampling. The tradeoff is that the record length is only 2K points.QuoteBandwidth is 2GHz (-3dB). I think it does have real-time sampling at a sufficiently low sample rate, but I can't figure out what that is from the manual. I suspect it's quite slow (KSPS).http://exodus.poly.edu/~kurt/manuals/manuals/Other/PHILIPS%20PM%203340%20Operating.pdf

author: nixfu
author_rank: Unknown
timestamp: October 09, 2014, 02:00:29 am
content: As the processing power gets greater and they have much larger sample sizes, scopes will all eventually be real-time where they can capture an entire signal at the full rated bandwidth in a single pass and no longer have to sample multiple cycles "equiv-time-sampling" to get their rated resolution.From what I remember, the 1000z have a sample rate of 1Gs/sec which is a10x nyquist ratio, which is pretty darn good real time capabilities.Dave, I know you have discussed this before but you should briefly touch on this in the video when you get this 1000z loaner on the bench. Maybe talk about directly the individual capabilities of the 1000z relate to its specs compared to the general discussions on scope bandwidth, sample depth etc you have done in previous videos.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 09, 2014, 04:21:50 am
content: Quote from: nixfu on October 09, 2014, 02:00:29 amProcessing power is not really an issue since most of the "processing" is simply dumping samples in RAM. Processing power would affect the relationship between points per trigger and waveform update rate though.The biggest reason for ETS is the cost and availability of high-speed ADCs: you can put together something that can do a couple of GSPS ETS for little more than a hundred bucks but if you want 4GSPS from a single ADC, the ADC alone will cost you a lot more than that.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 05:40:17 am
content: Quote from: pascal_sweden on October 08, 2014, 10:29:49 pmYes, most scopes today are Real-Time Sampling (RTS) scopes. Some also support Equivalent Time Sampling (ETS), but not all of them.In general, ETS-only scopes are a thing of the past unless you need more than 65GHz bandwidth. Above that there are some older scope designs which go to 100GHz and which are still sold, but these will probably be replaced by RTS scopes soon.QuoteNo, not really. Yes, it's RTS, but the reason why on some scopes the max sampling rate is dependent on the number of active channels is because they use interleaving (combining a channel's ADC with the ADC of an inactive channel) to reach higher sampling rates when only one or two channels are active.QuoteAs I said above, ETS-only scopes are dead outside extremely high bandwidths, and this has been the case for more than 15 years.Most of the better scopes also offer an ETS mode as well, but not all do. Especially cheaper low end scopes often lack that feature.QuoteNot really. The basic principle behind RTS scopes is the same. Some use technologies like DBI (Digital Bandwidth Interleaving) where signal components are downmixed, but this is only used in certain highend scopes (i.e. LeCroy's 100GHz RTS scope demonstrator uses DBI).However, not all ETS implementations are the same. For example LeCroy uses something called RIS which works different than ETS modes of of other manufacturers. This pdf explains RIS and other ETS modes in more detail:http://cdn.teledynelecroy.com/files/whitepapers/wp_ris_102203.pdfQuoteProbably by using a 2GSa/s ADC per channel. Not all scopes use ADC interleaving, as this comes with its own issues when designing a scope.QuoteThe PM3340 was a 2GHz scope with 250MSa/s real-time sampling rate and14bit10bit vertical resolution which also offered 2GSa/s in ETS mode. It was a good scope at it's time (1989; I had aPM3343PM3320A back then which was the 200MHz version of the PM3340) but by today's standards it's a boat anchor. It's also difficult to fix with lots of unobtainium parts. The high vertical resolution made (and still makes) especially thePM3343PM3320A sought after for audio work, though.Edit:The scope was a PM3320A not PM3343, and had 10bit vertical resolution not 14bit.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: October 09, 2014, 06:53:40 am
content: And when you get into super high bandwidths, then you start to see really interesting solutions like optical-domain time stretching. In this case, the signal is modulated onto a chirped laser pulse and then sent through a long piece of dispersion compensation fiber. As it passes through the fiber, the ends of the pulse travel at different speeds (since they are different wavelengths) and the modulated pulse is stretched in time. At the end, it is detected and sampled by much lower rate ADCs. It is possible to get into the terasample per second range with this technique.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 09:19:39 am
content: Quote from: pascal_sweden on October 08, 2014, 10:29:49 pmA quarter of a century ago the HP54100 series offered upto 50GHz bandwidth with, IIRC, 40MS/s real-time.In such discussions it is always worth definingexactly whya fast RTS is necessary. Sometimes it is, but in many important applications ETS is sufficient. Signal integrity springs to mind, particularly when using eye diagrams which are inherently ETS!

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 10:00:20 am
content: Quote from: tggzzz on October 09, 2014, 09:19:39 amEye Diagrams are certainly *not*"inherently ETS"! Some Eye Diagrams *can* be done with an ETS only scope but there's a risk that important signal components are missed.HP has written a nice document about RTS vs ETS for Eye Diagrams:http://www.hpl.hp.com/hpjournal/96dec/dec96a1a.pdfAnd this was written at a time when RTS scopes didn't have the high sample rates as today's scopes.Decades ago ETS was a crutch to overcome the insufficient real-time sampling rates of old days ADCs. But it's not 1989 any more!RTS at sufficient sample rates is better in any ways over ETS. The question should not be if/why RTS is necessary but why bother with all the drawbacks of ETS at all when today scopes have sufficient real time sampling rates up to 160GS/s.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 09, 2014, 10:32:59 am
content: I wanted to point out a company based in Sweden, SP Devices, who have patented technology to increase the sample rate by using several ADCs in parallel.They provide other technology as well:http://spdevices.com/index.php/technologyMaybe one day we will see a Rigol scope with SP Devices technology in sideIt is my understanding that Dave has good connections with Rigol Engineering. Maybe he can pass on the company details of SP Devices to Rigol. Of course if this would lead to any business, I would appreciate some credits =)

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2014, 10:46:13 am
content: Quote from: pascal_sweden on October 09, 2014, 10:32:59 amThat 'technology' is common knowledge. I doubt their patents are really new. It could be a nice off-the-shelf solution for a manufacturer of low volume specialistic data acquisition hardware.

author: Lukas
author_rank: Frequent Contributor
timestamp: October 09, 2014, 10:50:37 am
content: I guess that there's still a small market for high-bandwidth low-samplingrate ETS scopes. When you're doing analog work, you often don't need single-shot capture.Yes, technology is advancing but a 1GHz ETS scope still would be way cheaper than a 1GHz real time scope.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 11:05:54 am
content: Quote from: Wuerstchenhund on October 09, 2014, 05:40:17 amMost equivalent time sampling implementations work the way described in this LeCroy white paper. Modern implementation tend to replace the analog time to digital converter with some variation of a transition midpoint timing time to digital converter.The oldest DSO that I am aware of which supports random equivalent time sampling which LeCroy calls random interleaved sampling is the Tektronix 7D20 (70 MHz) with a 40 MS/s real time sampling rate and 2 GS/s equivalent time sampling rate released in 1983. It is the ancestor of the 2440 and TDS600 series DSOs. The contemporary HP 19860A used sequential equivalent time sampling.The 7D20 was followed by the the Tektronix 2220 (60 MHz) and 2230 (100 MHz) with 20 MS/s real time sampling rates and 2 GS/s equivalent time sampling rates which were both released in 1986 by which time HP already had the 54100A (1 GHz) with 40 MS/s a real time sampling rate and a 100 GS/s equivalent time sampling rate released in 1985.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 11:10:20 am
content: Quote from: Lukas on October 09, 2014, 10:50:37 amBased on the very limited availability of ETS scopes I guess this market is probably tiny. If it wasn't then I'm pretty sure that the major brand like Agilent/Keysight, Tek and LeCroy wouldn't all just offer a single ETS scope which has been on the market between ~10 to ~15 years and only serves for extremely high bandwidths above their RTS scopes.QuoteWell, if you need that 1GHz scope for a very limited set of applications then maybe, but then you'd probably also need a RTS scope for everything else so away goes a large part of your price advantage.Of course aside from the fact that you would already pay a premium just for the fact that ETS only scopes are very low volume products while RTS scopes are produced en masse.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 09, 2014, 11:12:58 am
content: Quote from: nctnico on October 09, 2014, 10:46:13 amThe interleaving technique as such is indeed common knowledge. The challenge with interleaving however is to correct for the manufacturing variations of the characteristics of the individual ADC, in order to obtain the optimal resolution.The patent of SP Devices relates to the estimation of the mismatch error and the reconstruction of the signal with all mismatch errors suppressed. The proprietary technology provides a background estimate of the gain, offset and time-skew errors of the ADCs without the need for any special calibration signal or post-production trimming. The signal is recreated with minimal latency.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 11:17:36 am
content: Quote from: Wuerstchenhund on October 09, 2014, 10:00:20 amGrrr. Annoyingly, you are correctQuoteI'm understand why they say that, but based on a recent experience, my preference is still for displaying isolated dots without linear (or other) interpolation. That way I can see what's being measured. I haven't tried a scope with the eyeline technique, but I'm sure that if it is implemented well it will be valuable in some limited circumstances. I wonder if other manufacturers implement it?QuoteJust so, but see below...QuoteThe issue is, always has been, and always will be, cost and size. Of course the decision points change with time

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 11:25:00 am
content: Quote from: Wuerstchenhund on October 09, 2014, 10:00:20 amI agree but not all DSOs made today have sufficient real time sampling rates to support their input bandwidth without aliasing even with input signals which are completely below their Nyquist frequency.Quote from: pascal_sweden on October 09, 2014, 10:32:59 amThere is nothing new here. Interleaved sampling is at least 3 decades old and linearity correction for interleaved digitizers is almost as old.Quote from: Lukas on October 09, 2014, 10:50:37 amI would say that most of the time it is not required for analog design work including digital signal integrity. I am actually disappointed in the current crop of 100 MHz 250 MS/s DSOs because of their lack of ETS.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 11:26:39 am
content: Quote from: David Hess on October 09, 2014, 11:05:54 amTek's Random Equivalent Time Sampling is *NOT* the same as Random Interleaved Sampling. Read the paper again. There are some important differences!QuoteThat is all well and nice but that was 30 years ago and bears not much relevance for modern day scopes.

author: EEVblog
author_rank: Unknown
timestamp: October 09, 2014, 11:34:02 am
content: Quote from: David Hess on October 09, 2014, 11:25:00 amI'm not really aware of any mainstream one that isn't?Tek set the benchmark 15+ years ago with the TDS200 series RTS scopes.QuoteBut 250MSPS is just enough to reconstruct the waveform if you use Sin X/x interpolation. In theory I believe that in most cases you can't get a huge amount more real information out of a 100MHz bandwidth limited signal once you pass the 2.4x mark, i.e. 240MSPS for 100MHz bandwidth. But that does depend on the filter type used etc.Either way, it think it's pretty marginal.That is likely why Rigol have simply dropped ETS on the 1000Z.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 11:37:42 am
content: Quote from: Wuerstchenhund on October 09, 2014, 11:10:20 amIMNSHO digital problems should be captured and debugged in the digital domain, and analogue problems in the analogue domain. The interesting case comes where analogue signals are being interpreted as digital signals. The classic example of that is often referred to as the "signal integrity" of digital signals.For such cases, it is possible and sufficient to:use an analogue scope to look at the eye diagram, and ensure the signal integrity is sufficientsomething will have "interpreted" that analogue waveform as a digital signal; use a logic analyser (in whatever form) to look at that the result of that "interpretation"debug that digitised signalYes, of course there are cases where that isn't possible, but I contend it would be practical for the majority of cases in which peoplethinkthey need a fast real-time scope with extremely long capture buffers.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 11:39:06 am
content: Quote from: Wuerstchenhund on October 09, 2014, 11:26:39 amGreat! Maybe you can point them out because I read the paper in detail and the only difference they identify is filtering after interleaving which is not unique to LeCroy and they do not even include that difference in their summary.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 11:42:38 am
content: Quote from: tggzzz on October 09, 2014, 11:17:36 amAhem, this document is from 1996! It's almost 20 years old! The technology that HP has offered back then is no longer relevant!And the question if you see an interpolated signal on the screen or not has absolutely nothing to do with ETS or RTS. It simply depends on your scope, i.e. can you switch off interpolation or is it forced enabled. For example, on LeCroy scopes interpolation is optional that has to be enabled by the user, as their philosophy is that a scope shall always display a signal unaltered by default. Other vendors have different mindsets.But again, this has nothing to do with ETS vs RTS.QuoteCost, well, a 100GHz ETS scope is slightly cheaper than a 65GHz RTS scope, but we're talking about prices in the region of $50k to $100k and more. Chances are that if you're working on such high complexity projects then you can afford a proper scope as well.For everything in the lower bandwidth ranges (i.e. below 5GHz) ETS isn't a sensible options. There are no ETS only scopes available in that bandwidth range, so you'd either have to settle for one of the few USB ETS scopes or hunt for a museum piece on ebay, which will be old, big, loud, noisy, power hungry, and chances are good spare parts are no longer available.I appreciate that we have many older EE's in this forum who have grown up with analog scopes and who never got really into DSOs, but really, it's not 1986 any more, and scope technology and prices have changed a lot since then. And there's a reason why scopes these days up to 65Ghz (or 33GHz if you're Tek) are RTS scopes.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 11:52:52 am
content: Quote from: David Hess on October 09, 2014, 11:39:06 amFrom the document:"...How RIS Implementations Vary Among ScopesRIS is a word coined by LeCroy. Usually, on other vendor's scopes, this mode goes by names like Equivalent Time (ET), Repetitive Equivalent Time (RET) or Sequential Sampling. Some vendors have stopped offering this mode in their high-end scopes.The main implementation difference between LeCroy and other vendors is the filtering of the final interleaved RIS trace. In other words, some vendors simply acquire the waveform segments and interleave them together. There are pros and cons to both methods: On one hand, the noise that is filtered out of the RIS waveform can only come from one source - the non-repetitive nature of the waveform or trigger and serves to degrade the rendition of the true analog waveform. On the other hand, the removal of this noise hides any non-repetitiveness that can lead to confusion unless the operation is understood.LeCroy believes that the removal of the noise and resulting higher performance and better signal fidelity is more important than the preservation of the non-repetitive variations. Confusion is avoided by understanding the scope operation. LeCroy believes this because the resulting RIS waveform is confusing even if this filtering is not applied if the waveform is not repetitive.The only solution to confusion in RIS mode is an understanding of how the mode works regardless of the scope vendor.QuoteWho else does it exactly the same then? Tek not, according to the documents I've seen and what I remember from the Tek DSOs I've used. So if you have some reference please share it.QuoteNo, they don't, because unlike say Agilent's or Tek's our-vs-theirs comparisons where the main aim is to make the own product look good, the aim of this document is to describe what RIS does, what its limits are and how the implementation varioes between vendors.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 11:59:25 am
content: Quote from: Wuerstchenhund on October 09, 2014, 11:42:38 amWell it was your choice of document, so naturally I referred to the points you were making!QuoteYes, but I was referring to the points in "your" reference.QuoteI started using DSOs when they first became available. I recognised their advantages and disadvantages, as I do with any tool. Nowadays if you have money to spend, digital is clearly the way to go.Analogue scopes still have two advantages for beginners: the UI is usually simpler to use (fewer options, not hidden behind menu buttons), and there is less chance for misleading marketing specmanship.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 12:13:29 pm
content: Quote from: tggzzz on October 09, 2014, 11:59:25 amThe point I was making was that there were some issues using ETS only scopes for Eye Diagrams, and I did highlight that this document was written at a time when RTS scopes had much lower sample rates.I certainly didn't suggest to use HP's 1996 technology instead!QuoteSee above. It seems you didn't really get the point I was making. The document describes a problem with ETS scopes at a time RTS sample rates were lower than today. If anything, the advantage of RTS over ETS has only increased since then.QuoteI disagree with both. The UI has fewer options but isn't necessarily simpler for beginners (the amount of options doesn't necessarily make a UI better, in fact, the lack of any supporting facilities like a help functionality or task-dependent user guidance can make it pretty difficult for a starter). And I remember things back old times, like when an unnamed company overestimated the capabilities of the trigger in some of their analog scopes. It wasn't all good back then.On the other side, analog scopes teach beginners practices of which many are no longer appropriate when using a proper modern DSO. I believe it is much better they learn how to handle a modern tool properly right from the start instead of learning outdated methods on a museum piece. I mean, we don't teach students of medicine how to properly set bloodsuckers or do a bloodletting any more, do we?

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 12:14:17 pm
content: Quote from: EEVblog on October 09, 2014, 11:34:02 amI was hoping someone would bring this up.I certainly agree that sin(x)/x reconstruction is completely sufficient to reconstruct an unaliased waveform.There is an EEVBlog discussion here from 2010 with a video which illustrates the problem at about 46 seconds in:https://www.eevblog.com/forum/chat/rigol-ds1000e-series-possible-errorfail-in-sin%28x%29x-interpolation/I disagree with the analysis of the cause in that discussion. The sin(x)/x reconstruction makes it more visible but what is being shown is the result of aliasing caused by non-linearity and sampling error in the digitizer (and oscilloscope front end) itself.Tek's real time DSOs suffer from the same problem to one extent or another (which Agilent loves to harp on) but higher sampling rates whether real time or equivalent time ameliorate it unless they come at the expense of greater distortion. Equivalent time sampling rates are usually so high that it becomes insignificant.The LeCroy white paper Wuerstchenhund linked to does not discuss this specifically but touches on it in connection with noise from interleaving and filtering to remove it. The technology from SP Devices that pascal_sweden linked includes linearization after digitizing which would help prevent it as well and may be the same thing LeCroy was referring to as their special filtering. I suspect a lot of high end DSOs do this to one extent or another. The interleaved ADCs Rigol is using (or at least the TI ones like the ADC08D500) include self calibration which helps to prevent this.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 12:38:45 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 11:52:52 amWe cannot know if Tektronix or Agilent does it the same way even if they discuss it publicly because LeCroy does not describe how their filter works in detail or what it is really doing. LeCroy even admits that others do filtering similar to theirs:The main implementation difference between LeCroy and other vendors is the filtering of the final interleaved RIS trace. In other words, some vendors simply acquire the waveform segments and interleave them together.Which says that some vendors do not and instead do what LeCroy is doing whatever that is.QuoteThe document reminds me exactly of a similar Agilent document which discusses distortion from interleaved digitizers and then shows how poorly a Tektronix oscilloscope performs in that regard.Distinguishing their products from their competitors is just good marketing.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 01:07:09 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 11:42:38 amI think there is some confusion here with the term ETS. The original post mentioned the Tektronix TLS216 which uses only real time sampling as far as I know and the Philips PM3340 which is asequentialequivalent time sampling oscilloscope optimized for high bandwidth.As you point out, sequential equivalent time DSOs are almost extinct having been replaced with very high end real time DSOs. However many lower cost real time DSOsalsosupport random equivalent time sampling.QuoteI have always liked DSOs at least since the Tektronix 2230.The reason that practically all DSOs use real time sampling now is that fast digitizers, memory, and the logic to support them have decreased in price until the incremental cost of RTS versus random ETS at a slower sampling rate is small. Many RTS DSOs still support random ETS. Only the lowest costs ones and the ones with very high real time sample rates compared to their bandwidth forgo it.

author: EEVblog
author_rank: Unknown
timestamp: October 09, 2014, 01:33:15 pm
content: Quote from: David Hess on October 09, 2014, 12:14:17 pmTek on the issue:http://www.tek.com/dl/55W_17589_2.pdfI have not read it all, but they basically summarise that there is no practical difference between Sin X/x and ETS.From that Lecroy paper:QuoteBottom line is that all the manufacturers seem to say a similar thing, and I have a more detailed paper mathematically proving 2.4x minimum in some way (can't find it now). So for all but the most critical applications, a 250MSPS 100MHz scope with Sin X/x should be just fine.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 02:06:41 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 12:13:29 pmMy experience of teaching beginners is that having everything visible and logically arranged on the front panel helps guide them towards making appropriate use of the features. The problem with "hidden" options is you don't know they are there until after you've looked for them.As for manuals, well the acronym "RTFM" is there for a good reasonT'was ever thus. As for DWIM techniques (sorry, "task dependent help"), to some extent they are necessary as a result of having by hidden controlsBut there are no absolutes here, and the comments only apply to beginners.QuoteStrawman argument! T'was ever thus!QuoteNeither do we teach them to drive cars using a Bugatti Veyron (I think that's the latest hot car). No, we teach drivers on gutless new clunkers.

author: edavid
author_rank: Super Contributor
timestamp: October 09, 2014, 03:47:13 pm
content: Quote from: EEVblog on October 09, 2014, 01:33:15 pmYou missed David Hess's important point that this is only valid for an unaliased waveform. Since the Rigol DS1000Z doesn't have a sharp enough antialiasing filter in front of the ADC (as shown by the bandwidth measurements that people have posted), the 250MSPS sample rate is not adequate to support 100MHz bandwidth.

author: edavid
author_rank: Super Contributor
timestamp: October 09, 2014, 03:50:42 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 05:40:17 amThe PM3340 manual shows that it has a 10 bit ADC, not 14 bit.Are you sure it was 250MSPS? It's hard to believe they could build even 10bit 250MSPS ADCs back then.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 04:03:23 pm
content: Quote from: David Hess on October 09, 2014, 12:38:45 pmIt doesn't mean other vendors do the same. It is a non-exclusive statement, i.e. that the author doesn't know if other vendors do the same.At least the big names don't seem to use filtering in their ETS modes.QuoteOnly that Agilent often stretches the truth to the extreme and goes to immense lengths to make other products look bad.QuoteIt is, but there is a difference between construing a synthetic situation to make the competitor look bad and stating some facts which are easy to check. This document doesn't even say that the competition is inferior, all they say is that there are differences and that the user should know how the specific ETS implementation works independent of who manufactured the scope.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 04:08:55 pm
content: Quote from: David Hess on October 09, 2014, 01:07:09 pmThat's exactly it

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 04:28:23 pm
content: Quote from: tggzzz on October 09, 2014, 02:06:41 pmTrue, but that is no different for a DSO. What you call "hidden menu" (which btw aren't really hidden on most scopes, it's often more of a case of 'can't be bothered to learn how to use my tool properly') is usually only required if you need advanced functionality.Quote"RTFM" applies to any tool, even an analog scope (where else would you learn about it's limitations?).QuoteNot really. You seem to think that vendors only got creative with their specifications when DSOs came along (and your signature supports that impression) but that doesn't conform with reality, which is that the specs of your average big brand DSO are as reliable as they were for analog scopes.QuoteI'm sorry but like in most cases where someone comes up with a car analogy this one is silly, too. It doesn't matter if you drive a Yugo or a Veyron, all cars have a steering wheel and the same set of basic controls (throttle, brake, clutch for stick shifts, blinker, gear/transmission lever). Both cars work exactly the same (the energy from a piston combustion engine goes to the transmission and from there to the wheels). A Veyron might be of much higher performance than a Yugo but essentially you drive it the same, especially on public roads. In short: if you can drive a shitty Yugo perfectly then you can drive a Veyron, too. No need to re-learn driving.If it has to be about cars, a better analogy would be teaching a driving student using horse and buggy.That's not necessarily true with analog scopes and DSOs. The operating principles (direct analog display vs sample and digital storage) are completely different, which means there are different things to consider when using a DSO than if an analog scope was used. This also means that treating a DSO like an analog scope often won't cut it and give poor/false results. Some basic controls are the same on analog and digital scopes but that's about it, and as soon as you want to have a closer look at a complex signal your analog scope leaves you with maybe some primitive CRT storage mode and (if you're lucky) some basic cursors with readouts while a good DSO offers you a full suite of signal analysis, maths, decoding etc. So unlike a driver that moves from a Yugo to a Veyron, even an EE who has many years or decades of experience in using analog scopes will have to learn quite a bit of new stuff when moving to a DSO, and much more if it's one of the more advanced ones (although the hardest part is giving up old habits from the analog scope days).When I train apprentices and students I want to prepare them to be able to make the best use of tools they will be working with when designing the Next Greatest Thing(tm), not to become curator in a T&M museum. I can see that an analog scope can still be useful for many simpler tasks, and there's nothing wrong with that. But in this day and age it no longer should be the entry for someone who wants to learn using an oscilloscope properly.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 04:50:51 pm
content: Quote from: edavid on October 09, 2014, 03:50:42 pmI had a look at some old documents. The scope I had was a PM3320A, not PM3343 (not sure the latter even exists).I couldn't find the specs on a quick search but you're probably right that it was 10bit only.QuoteYes, 200MHz bandwidth and 250MSa/s sample rate.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 05:34:50 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 04:50:51 pmI remember wanting a PM3382/PM3384/PM3392/PM3394. The PM3394 is a 4 channel, 200 MHz, combination analog/DSO with a 200 MS/s real time sample rate which is shared in somewhat between channels. They support real time and random equivalent time sampling. Feature wise they seemed better than the Tektronix equivalents.The Philips PM3340 is a sequential digital sampling oscilloscope with a 2 GHz input bandwidth, 10 bit vertical resolution, and based on the specifications generates a 512 point record in less than 25 milliseconds which works out to a sampling rate greater than 20 kSamples/second.It has built in delay lines after the trigger pickoffs so it can capture the triggering edge unlike most sampling oscilloscopes. This will be what actually limits the input bandwidth.There was a short discussion on EEVBlog about it here:https://www.eevblog.com/forum/buysellwanted/philips-3340-2ghz-scope-%28uk%29/

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 05:54:42 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 04:28:23 pmOf course, as I previously noted the words I have nowemphasisedin my original statement.QuoteYou are falsely inferring things I didn't write. Again, note the words I have nowemphasisedin my original statement.QuoteMy mechanical analogy is as stupid and unhelpful as your previous medical analogy. By "too", are you referring to your medical analogy?QuoteI've snipped a long diatribe about somethingyouclaim is silly, for the simple reason that all the analogies in this thread are misleading and unhelpful. Your elaborating them does not change that.QuoteIf you knew my career history, you would realise just how off-beam your presumptions are.You seem to be making a habit of feeling something warm, grey, wrinkled, 2ft(!) in diameter, and hypothesising an elephant.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 06:16:11 pm
content: Quote from: edavid on October 09, 2014, 03:47:13 pmMy more significant point is that the situation is actually worse than that and I linked to that EEVBlog discussion and video to show exactly why:https://www.youtube.com/watch?feature=player_detailpage&v=W7Opur1Xbvs#t=44Assume for the moment that the signal source is perfect or the analog antialiasing filter is perfect and no frequency components higher the the Nyquist frequency are present at the input to the digitizer. If the signal source is close to but below the Nyquist frequency, then nonlinearity and sampling clock error in the digitizer will produce harmonic distortion andthatwill be above the Nyquist limit. They will also produce mixing between the input signal and sampling clock which will produce sidebands and some ofthosewill be above the Nyquist frequency.The result is shown in the time domain in the above video as what I like to call "wobbulation". I suspect the same thing on one of the Rigol oscilloscopes with an index graded display makes for waveform thickening which is mistaken for noise. An older Rigol in envelope mode will show the same apparent noise.A simple way to verify this is to single shot capture a clean high frequency sine wave and run an FFT on it which will display spurs in the passband caused by aliasing.Page 12 of this Agilent application note discusses this:http://www.newark.com/pdfs/techarticles/agilent/EvaluationgOscilloscopeRatesFidelity.pdfQuote from: EEVblog on October 09, 2014, 01:33:15 pmI do not disagree with this provided that the oversampling ratio is high which is what ETS provides or the sin(x)/x interpolated waveforms are averaged. The very end of the Tektronix application note you linked says this very thing:... An interesting point is that the average of interpolated waveforms (*) and the average of ET mode waveforms produce a virtually identical result! This tends to contradict misconceptions that sin(x)/x interpolation does not accurately reproduce high speed digital signals.I have taken advantage of both techniques as appropriate on my old Tektronix 2440 which suffers greatly from interleave distortion by design although it is not particularly worse than modern DSOs operating at similar (500 MS/s) sample rates. It is just annoying and something I would look to minimize if I was in the market for a newer DSO.(*) Sin(x)/x interpolated waveforms.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 08:34:27 pm
content: Quote from: tggzzz on October 09, 2014, 05:54:42 pmIt seems you've now resented replace arguments in an adult discussion with childish diatribe, aggressiveness and personal attacks. Although it's unfortunate (but not completely surprising), to keep the peace in this forum I will simply end the discussion with you.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 09, 2014, 08:58:03 pm
content: Quote from: David Hess on October 09, 2014, 05:34:50 pmYes, Philips/Fluke ("The T&M Alliance") did have some nice scopes at that time. I remember the PM338x/339x models. We had some at work (together with some PM305x/PM307x analog scopes), and most of our engineers seemed to prefer the Philips scopes over the Tek 465's we also had (I did).QuoteI vaguely remember that the PM3340 had a very low real-time sample rate, but at that time that was pretty much standard for (sequential/random) ETS scopes.I think it's a shame that "the T&M Alliance" didn't work out.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2014, 09:54:19 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 08:58:03 pmThis era was before my time so the best I could do was drool over the product brochures and catalogs. I was happy enough to have a Lavoie LA-265A (30 MHz dual trace delayed sweep) which is a clone of a Tektronix 545A. Since it has leather handles on top, I consider it portable although portability is limited to crushing things with its weight.I remember advertising at the time comparing the PM338x/339x models to the Tektronix 2232 series with the Philips oscilloscopes having twice the sample rate, twice the bandwidth, twice the number of channels, and math/FFT support. I see them show up on Ebay occasionally in what looks like reasonable shape.I have been told that Hitachi and/or Philips analog oscilloscopes had sharper traces than Tektronix did on their 465 series but this was after Tektronix had started using a scan expansion mesh in their CRTs for greater deflection sensitivity which would explain that. Personally I have not noticed a significant difference but I have not had a Fluke or Hitachi available to make a direct comparison.QuoteThis was certainly true of the sampling oscilloscopes and of course continues to be the case. The Tektronix 7854/7T11A runs at about 50 kSamples/second in analog or digital mode and the 7854 alone (400 MHz bandwidth) runs at about 500 kSamples/second (10 bits though) although the former could be considered "real time" in analog mode while the later was not even close despite having 10 times and sample rate and an order of magnitude less bandwidth. Nobody would mistake the 7854 for a modern DSO.Their follow on high bandwidth 11k series digital sampling oscilloscopes ran at about 200 kSamples /second.The contemporary real time digital storage oscilloscopes were 40 to 100 MSamples/second but quickly rose to 500 MSamples /second and faster in the 2440, 11k, and later TDS series DSOs. I think they all supported random equivalent time sampling as required except maybe for some of the oddball TDS models.QuoteI never learned the story behind that. I knew there was some relationship between Fluke and Philips but never any details.Now of course Fluke, Tektronix, and Keithley are all owned by Danaher Corporation. I have heard interesting stories about that like how the Tektronix DMM916 line of handheld multimeters was making the Fluke 97 series look bad so Tektronix received an offer that they could not refuse and discontinued them.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2014, 09:56:40 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 08:34:27 pmPot. Kettle. Black.But ending this to-and-fro is the right decision.

author: edavid
author_rank: Super Contributor
timestamp: October 10, 2014, 12:04:11 am
content: Quote from: David Hess on October 09, 2014, 09:54:19 pmhttp://www.fundinguniverse.com/company-histories/fluke-corporation-history/Quote

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 10, 2014, 05:05:01 am
content: Quote from: edavid on October 10, 2014, 12:04:11 amThanks for the link. It was very interesting to read.QuoteIt's a real shame that Fluke didn't continue Philips benchtop scopes. I guess they only really wanted the portable gear like the ScopeMeter.

author: EEVblog
author_rank: Unknown
timestamp: October 10, 2014, 05:32:51 am
content: Quote from: edavid on October 09, 2014, 03:47:13 pmI knew about David's point and was ignoring it for the argument that a scopeshouldhave an adequate filter in place.I have not seen the result for the Rigol, but ok, if it doesn't then the issue becomes relevant.

author: marmad
author_rank: Super Contributor
timestamp: October 10, 2014, 04:45:34 pm
content: Quote from: David Hess on October 09, 2014, 11:25:00 amWhat "crop" are you referring to? The single DS1000Z model from Rigol, sold as three FW-delineated versions?Quote from: David Hess on October 09, 2014, 06:16:11 pmYes, we already know you weren't happy with the DS1000E you bought. It still doesn't explain how whatever nonlinearity and sampling clock errors you feel were present in the 5x dual-ADC-chip DS1000E series were magically transferred to the redesigned, single quad-ADC-chip DS1000Z model.Quote from: EEVblog on October 10, 2014, 05:32:51 amI'm not sure how a low-cost 100MHz DSO could have an adequate enough filter for a 125MHz Nyquist frequency. A Gaussian frequency response would only be around -5db at 125MHz, and even with a flat-response, it would only be approx. -9db at 125MHz. Perhaps the 50MHz model's frequency response is adequate. The DS1000Z series is, for all practical purposes, a 1/2 channel 50/75/100MHz DSO and a 3/4 channel (50?)25MHz DSO.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 10, 2014, 06:38:49 pm
content: Isn't the BW 100 MHz for ALL 4 channels?The digital sample rate is divided when you use more than one channel, but my understanding is that the BW is 100 MHz for ALL 4 channels.Note that analog BW and digital sample rate are two different things.Analog BW = 100 MHz on ALL channels, even if you use all 4 channels at once.Digital sample rate is 1 GS/s when you use 1 channel, 512 MS/s when you use 2 channels, 250 MS/s when you use 4 channels.For a 100 MHz BW, 250 MS/s is enough to represent the original signal as it complies with the Nyquist theorem.The Nyquist theorem states that a signal must be sampled at a rate greater than twice the highest frequency component of the signal to accurately reconstruct the waveform; otherwise, the high-frequency content will alias at a frequency inside the spectrum of interest.More detailed information about the relation between Bandwith, Sample Rate and the Nyquist Theorem is available here:http://www.ni.com/white-paper/2709/en/What can an anti-aliasing filter do to impact this? I like to read this in a formal white paper to get better understanding.

author: marmad
author_rank: Super Contributor
timestamp: October 10, 2014, 07:03:42 pm
content: Quote from: pascal_sweden on October 10, 2014, 06:38:49 pmDigital oscilloscopes can be subject to sampling alias errors. Sampling alias errors occur when the signal has frequency content beyond the Nyquist frequency (which is 125MHz @ 250MSa/s). As mentioned inthis Agilent paper:"...why aren’t oscilloscope sampling rates exactly 2x higher than the oscilloscope’s bandwidth? This is because out-of-band signals may only be attenuated by 10 dB beyond the band-edge of the instrument. Said differently, the frequency response of the oscilloscope does not roll-off infinitely fast and some buffer room is used on the sampling rate to minimize aliasing."This is the reason the majority of DSOs sample at 4-10x greater than the maximum bandwidth. Unfortunately, the frequency response of the DS1000Z series does not roll-off fast enough to minimize aliasing when sampling at 250MSa/s.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 10, 2014, 07:11:28 pm
content: The analog input path in the frontend attenuates, amplifies, filters, and/or couples the signal to optimize the digitization by the ADC.How does the analog frontend look like in the Rigol? What sacrifices are made by Rigol in comparison with higher end scopes?Is this where the high cost comes from in the higher end scopes, or is the majority of the cost driven by the ADC quality as such?Which types of filters are used in Rigol scopes? Which types of filters are used in higher end scopes?

author: edavid
author_rank: Super Contributor
timestamp: October 10, 2014, 07:26:01 pm
content: Quote from: pascal_sweden on October 10, 2014, 07:11:28 pmRigol doesn't publish any information about their designs, so the little that is known comes from reverse engineering. It's thought that they use an off the shelf PGA chip with a simple 1 or 2 pole lowpass filter.Higher end low bandwidth scopes use faster ADCs (or more of the same speed ADCs), and higher end high bandwidth scopes use a combination of custom multipole analog filters and DSP to get closer to the Nyquist limit.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2014, 07:30:25 pm
content: Quote from: marmad on October 10, 2014, 04:45:34 pmThe current crop includes the index graded and high waveform aquisition rate DS1000Z and DS2000A series including their MSO cousins which Rigol treats separately. That is a lot more than three models but I would consider them to be two basic implementations. Many (all?) of their predecessors support ETS.QuoteI never bought any Rigol oscilloscope but that had nothing to do with ETS or graded index display support or aliasing in the digitizer. It was a result of Rigol's misleading documentation and sales representatives as well as some brief evaluations. This was before the DS1000Z and DS2000A series were available which I might have considered.Why would you expect the DS1000Z (or DS2000A) digitizer to be free of these errors? Almost all DSOs display them to one extent or another (*) and even if the DS1000Z (or DS2000A) ADC was free of them, it is still at the mercy of its clock source which is usually a significant source.I am suspicious that Rigol left ETS out on these models because the sampling performance could not support it but it could have been do to market segmentation or cost. The graded index display and high waveform acquisition rate would tend to mask the aliasing as noise so it would not be apparent anyway making ETS less useful.QuoteEven a high-cost DSO is not going to have an adequate enough analog antialiasing filter and doubt such can practically exist at any price except when it is not needed. The ways to combat this include more linear digitizers which have less clock error and higher sampling rates.I have never argued that analog antialias filtering in any DSO was inadequate because instead I have argued that analog antialias filtering in any DSO is useless if only because it makes the transient response and bandwidth vary with sample rate. At low sample rates with an aliased input signal, it produces just as deceptive a display as aliasing would if not more deceptive.(*) I have seen some high end DSOs which did not but never got to test one specifically for it. I tried to replicate wobulation on my 2232 which was handy and could not but only because it has no way to disable ETS. Aliasing produced by sampling error was apparent but not in a particularly revealing way.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 10, 2014, 07:31:59 pm
content: QuoteI have a 3382 which is the 100MHz 2+2 channels model. I rather like it as an analogue 'scope and the digital side is occasionally useful but a bit limited in sample rate and memory depth (8k samples/channel IIRC).I've had my eye out for a 3394 but have only been able to spot them in the wild for excessive amounts of money

author: nctnico
author_rank: Super Contributor
timestamp: October 10, 2014, 08:21:33 pm
content: Quote from: David Hess on October 10, 2014, 07:30:25 pmEven a high-cost DSO is not going to have an adequate enough analog antialiasing filter and doubt such can practically exist at any price except when it is not needed. The ways to combat this include more linear digitizers which have less clock error and higher sampling rates.The worst case 250Ms/s samplerate for the DS1000Z could be sufficient with a very sharp anti-aliasing filter and lot's of math to reconstruct the signal that close to the Nyquist frequency. With my own (math intensive) algorithms I achieved proper signal reconstruction up to 0.45fs (112MHz at 250Ms/s).I did some anti-aliasing tests on my Siglent SDS2024 and it seems that the anti aliasing filter works pretty well. There is only a tiny bit of aliasing.

author: marmad
author_rank: Super Contributor
timestamp: October 10, 2014, 08:29:35 pm
content: Quote from: David Hess on October 10, 2014, 07:30:25 pmThe DS2000A is not a "100MHz 250MSa/s" DSO, thus doesn't fit your original description. Not only that, but it's a completely different design involving a different ADC, different processor, different architecture, etc; i.e. extremely dissimilar internally (much closer to the DS4000 then the DS1000Z). Again, your original "100MHz 250MSa/s" description only applies to a single base model: the DS1000Z (and, of course, all it's permutations: -S, MSO, etc).QuoteNone of the new generation of high level (>= 64) intensity graded DSOs have ETS, beginning with Agilent's 2000 / 3000 X-Series. The lack of it on Rigol's later-released UltraVision DSOs is as likely to be attributed to copying the Agilent X-Series' list of features as it is to anything else.QuoteI never said that I expected them to be free of all errors, but you seem to be implying errors which have yet to be proven. Logically, I would think that using four internally-interleaved ADCs would have less interleave distortion (and perhaps a lower noise level) than when using multiple externally-interleaved ADCs, but I haven't seen any evidence either way.QuoteAgain, none of the new generation of intensity graded DSOs have it (Agilent, Siglent, etc), so I can't see how one could logically deduce anything about Rigol's performance from the fact that the feature is missing on their new DSOs too.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2014, 09:37:08 pm
content: Quote from: grumpydoc on October 10, 2014, 07:31:59 pmI have seen the 3394 at an affordable price a couple of times but was always leery about attempting to repair and maintain one.The analog operation makes for a very effective sanity check and also usually makes up for the lack of index grading in digital storage mode. In a very effective way, both modes of operation complemented each other in this type of oscilloscope. It was many years and a lot of dollars before DSOs included DPO like functionality which could replace or attempt to replace an analog oscilloscope.I do not know about Philips but Tektronix continued to make this type of oscilloscope until 1994 and it outlived its close analog only cousins by 3 years. Looking at the production dates make me suspect that they only stopped making them because they stopped making lower bandwidth 100 MHz CRTs at the same time. Higher bandwidth CRTs and analog oscilloscopes held on for at least two more years but maybe they just had extras in stock.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 10, 2014, 09:50:27 pm
content: Quote from: marmad on October 10, 2014, 08:29:35 pmThat's not entirely correct. The new LeCroy Wavesurfer 10 which only came out recently offers 256 intensity grades and does offer ETS (RIS), although only up to 50GSa/s. The other new Wavesurfer (3000) also offers ETS (50GS/s), and I'm pretty sure it offers at least 64 intensity levels (couldn't find it in the spec). Since the WS3000 is a Siglent SDG3000 the same is probably true for the Siglent variant as well.The WS3000 roughly competes with the Agilent/Keysight DSO-X3k and the WS 10 with the DSO-X-4k Series.This is in line with the older generation of LeCroy scopes. The predecessor WaveSurfer Xs-B which is still sold also offers 256 intensity grades and ETS (RIS) up to 200GS/s. Same is true for many other LeCroy scopes going back to at least 2005 (including my WaveRunner 64Xi). It might be longer but I couldn't find data about the number of intensity levels on the even older scopes.

author: marmad
author_rank: Super Contributor
timestamp: October 10, 2014, 10:21:31 pm
content: Quote from: Wuerstchenhund on October 10, 2014, 09:50:27 pmSorry, I meant the new generation oflower cost, intensity-graded DSOs; I would assume the feature would continue to be present on some new intensity-graded DSOs. I shouldn't necessarily even have included the Agilent 3000 X-Series in my above post - but they just happen to share the lack of ETS with their lower-cost 2000X sibling.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2014, 10:50:19 pm
content: Quote from: marmad on October 10, 2014, 08:29:35 pmI was not as precise as I could have been. I should have been more inclusive and said "the current crop of 100 MHz 250 MS/s and 300 MHz 1 GS/s DSOs". The DS2000A series is going to suffer from the same aliasing problem to a somewhat lessor degree unless its digitizer performs worse.QuoteAfter thinking about it, I have a better explanation; these oscilloscopes which support intensity grading including the Rigol DS1000Z and DS2000A series and Agilent ones you mentiondohave ETS but not in the way it is generally thought of and the manufacturers are not advertising it as such.During acquisition, the trigger to sample clock delay is detected in a way similar to how a transition midpoint timing TDC works (or they use an analog TDC but the result is the same unless there is aliasing); this essentially involves a real time reconstruction filter similar if not identical to a sin(x)/x reconstruction filter before the digital trigger. The trigger to sample clock delay is used to align the waveform acquisition record with the display record.The difference between this an an older DSO without an index graded display using ETS is that a histogram is generated in real time and later transferred to the display. An older non-DPO DSO would instead transfer the waveform acquisition record to the display and then optionally generate the histogram and graded index display.Both methods result in equivalent time sampling whether they call it that or not. With some difficulty, it should be possible to derive the ETS sampling rate from the specifications but in practice it is high enough not to matter to the user who would not be able to see the effects of ETS in the presence of index grading.QuoteEvery DSO with a real digitizer suffers from these errors and they become more serious as the sample rate approaches the Nyquist frequency. All someone has to do is measure a clean sine wave which is close to but below the Nyquist frequency (Technically it does not have to be below the Nyquist frequency and it is not even desirable for it to be so to make this measurement.) on a DS1000Z or DS2000A in single shot mode and and they will become apparent just like in that video I linked to.Externally interleaved ADCs do usually perform worse in this respect and the internally interleaved ones should be better if only because they generally implement some form of self calibration to minimize it. Both are subject to external clock jitter however. The specifications of the internally interleaved converters that TI makes do show that more distortion occurs when interleaving is used but the difference is small. Unfortunately they do not include a complete comparison between modes in their datasheets.QuoteSee above. They did not actually leave it out and neither did Agilent or Siglent. You have convinced me that they still implement ETS but not that the aliasing issue is moot.Actually, this convinces me that the aliasing issue is more important because it occurs before the reconstruction filter and trigger and this may explain some very odd demonstrations LeCroy made in the past where they showed their very low jitter "digital trigger" which did not look to be very low jitter.

author: pascal_sweden
author_rank: Super Contributor
timestamp: October 10, 2014, 10:56:48 pm
content: Interesting thread which I started, which brought up a lot of knowledge up to the horizon.Although my contribution itself is limited, I initiated the post, so some contribution in that respect

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2014, 11:25:21 pm
content: Quote from: pascal_sweden on October 10, 2014, 07:11:28 pmThe analog front ends have gotten a lot simpler because of integration. You can get an idea of what is involved by looking at the datasheet for the TI LMH6518 although it doesnotshow the high input impedance buffer or high input impedance attenuator switching which precedes it. I would be most interested in seeing how that later is done now although I know how I would do it which almost matches some photos from teardowns I have seen.For a general idea of what is before the first integrated low impedance amplifier, check out chapter 7 of "The Art and Science of Analog Circuit Design" which discusses almost modern oscilloscope vertical input amplifiers. At the end of the chapter the author, Steve Roach, speculates about the designs we are probably buying now:http://goo.gl/R44h5lBased on approximately when various Rigol models were released and their performance, it looks to me like low end DSOs have been following the trend in increasing integration of ADCs, FPGAs, and wideband integrated analog electronics intended for ATE (automated test equipment). Where older high performance designs would have taken 2 or 4 separate interleaved ADCs and memory channels to meet their sample rate goals, newer models have that all integrated into one ADC and one FPGA.QuoteHigh bandwidth high sample rate ADCs are not cheap and better ones cost more. Some designs interleave several ADC for higher sample rates although that is more commonly done now in an integrated manner if possible.The analog design before the digitizer is not trivial either and becomes increasingly difficult at wide bandwidths for all kinds of arcane reasons.As far as the distribution of costs, I suspect that is rather closely guarded and largely irrelevant to the price you have to pay when buying one.QuoteIn all of the designs I am familiar with, simple bandwidth limit filters are implemented not for antialiasing but for noise reduction. The integrated front-end amplifiers which are part of the analog signal chain like the TI LMH6518 include selectable single pole filters.I have only seen analog antialiasing filters in DSOs that I would consider toys.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2014, 07:18:54 am
content: Quote from: EEVblog on October 09, 2014, 01:33:15 pmDave, I had a thought about that Tektronix paper (*) in light of my post above about the similarities between traditional ETS and what the current Rigol oscilloscopes and others are doing with digital triggering and this EEVBlog conversation going on here:https://www.eevblog.com/forum/testgear/new-rigol-ds1054z-oscilloscope/315/There is a critical difference between traditional ETS using an analog time delay counter and averages of a sin(x)/x reconstructed waveform when digital triggering is used after sin(x)/x reconstruction which is common in DSOs now.If I use an DSO which supports traditional ETS with a waveform that is above the Nyquist frequency, the TDC (time delay counter) returns the delay between the trigger and sample clock effectively raising the sample period to the resolution of the TDC. On a Tektronix 2230 for instance, this allows a 20 MS/s digitizer to operate at an equivalent of 2 GS/s but limited of course to repetitive waveforms.On a modern DSO which relies on reconstruction, usually sin(x)/x but it could be something else, before the digital trigger, a measurement is made which returns the delay between the trigger and sample clock again effectively raising the sample period to the resolution of the TDC but now the TDC uses something like transition midpoint timing which is a complete digital process implemented in logic. The manufacturers are not calling this capability ETS and maybe the following is a good reason.If the waveform is above the Nyquist frequency, the reconstruction is going to completely fail causing the trigger to fail as well. In this case, averaging after sin(x)/x reconstruction is useless because of aliasing before the trigger.If triggering occurs on a fast edge which is not bandwidth limited and so contains significant frequencies above the Nyquist limit, aliasing during sin(x)/x reconstruction before the trigger is going to corrupt the trigger to sample clock measurement although the results will be less dire. I think averaging will still work but the histogram of the signal is going to show a lot of excess noise produced by aliasing and corruption of the trigger.(*) Thanks for pointing this paper out to me. I know I have read it in the past but lost track of it and have been searching for it in a desultory manner ever since.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: October 11, 2014, 09:22:22 am
content: Quote from: marmad on October 10, 2014, 10:21:31 pmI guess with the DSO-X3k it's difficult as technically it shares a lot with the DSO-X2k but competes in a higher price bracket.The interesting thing with these new LeCroy scopes is that the ETS sample rate has dropped to only 50GSa/s while previous models offered 200GSa/s. Either there's a technical reason for the drop (i.e. slower embedded processing on the new scopes vs full PC platform on the older ones), or it's simply an indication that ETS has lost relevance in this day and age.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2014, 03:43:36 pm
content: Quote from: Wuerstchenhund on October 11, 2014, 09:22:22 amMy guess is that they decided they did not need 5 picosecond timing resolution in the external TDC and 20 picoseconds was sufficient. There are lots of possible design reasons this may be the case including noise considerations. One common one on old DSOs is where the display record only has say a maximum timing resolution of 50 picoseconds but the TDC has a 10 picosecond resolution.The lack of external ETS in low end DSOs may simply represent a lower cost point and market segmentation. All digital ETS is *cheaper* to implement once you have some spare FPGA or ASIC logic handling decimation or DPO functions. Of course it is not needed at all if you have a sampling rate sufficiently high above your analog bandwidth.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 11, 2014, 10:17:02 pm
content: Quote from: David Hess on October 10, 2014, 09:37:08 pmPrices in the 'states seem more reasonable, I've seen a few for $500 or less but shipping to the UK is prohibitive. Someone locally had several which he sold through ebay but wanted about £750 for a 3394B (about US $1200). At that price several modern 'scopes can be bought new. I did offer £375 which I thought fair but no dice. It took him a while but in the end it looks as though he sold them for not far off what he wanted - proving him right and me wrong on price.I have to say that I don't really understand this; it's a nice 'scope and I'd quite like to own one but not for the same price as a Rigol DS1104Z-S or nearly the price of a Siglent SDS1204CFL.As to repair - yes, well, there is a lot of unobtanium in these 'sopes so if they fail it might be difficult to find spare parts, and Philips designs could be idiosyncratic. It's probably no worse than getting a new U800 for a 2445 or 2465 though. Service manualsareavailable on the net, at least.Maintenance is perhaps not so much a problem. If you have the equipment to calibrate a 2465 then a 3394 shouldn't be too much of a challenge - in fact there's an "auto cal" function which serves for keeping in trim on a day-to-day basis - certainly I've found my 3382 very accurate once warmed up.

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2014, 10:52:51 pm
content: Quote from: grumpydoc on October 11, 2014, 10:17:02 pmWhen I was looking, the DS1000Z and DS2000A series of Rigol oscilloscopes did not exist. Having decided that their other models were unsuitable, I rebuilt a pair of Tektronix 2230s for about $80 each and a 2232 for $120 instead.QuoteCalibration is definitely a problem although I would have needed the equipment to do transient response calibration anyway whether I had a new DSO or not and that is the hardest one to do.I never really considered the 24xx series analog oscilloscopes because I already had a 2247A and a fast 7000 series mainframe. The Philips oscilloscopes were not common enough for me to risk having to find a parts donor.

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 17, 2016, 05:03:43 pm
content: Quote from: Wuerstchenhund on October 09, 2014, 05:40:17 amAre you sure that it was not 12-bit? It contained the ADC601JG.The datasheet can be found here.http://pdf.datasheetcatalog.com/datasheet/BurrBrown/mXyrvyx.pdfThe ADC601JG can do a fast conversion in 900ns.Was the maximum sample rate limited by the processing power of the Motorola 68000 processor?This scope featured already a 12-bit AD converter in 1989.Most of the mainstream scopes in 2016 still have a 8-bit AD converter.We are talking more than 25 years ago! What happened?Do the manufacturers want to keep us stupid?Are we facing the same evolution as with the Maya civilization?

author: wraper
author_rank: Unknown
timestamp: May 17, 2016, 05:15:36 pm
content: Quote from: pascal_sweden on May 17, 2016, 05:03:43 pmYou talk about 1.1 MSPS ADC and then bash modern scopes for no improvement.

author: pascal_sweden
author_rank: Super Contributor
timestamp: May 17, 2016, 08:05:38 pm
content: You have a point about the sample rate, but it would have been nice if 12-bit would have become mainstream as well, next to faster sample rates.

author: borjam
author_rank: Unknown
timestamp: May 18, 2016, 08:51:54 am
content: Quote from: marmad on October 10, 2014, 07:03:42 pmI have observed this on my "enhanced" DS1074Z actually.I was looking at some Wordclock and S/PDIF signals. When using a single channel I saw a nice square signal. When enabling another channel, I noticed some "ringing" on the right half of the high part of the signal. I think it was aliasing causing a real misbehavior of the sin(x)/x interpolation.At first I thought that I had some odd problem with the converter, but I think it was aliasing. It didn't help that I was looking at a clock signal expecting a 75 ohms termination using a 10x oscilloscope probe (LeCroy PP005) with the BNC adaptor and no proper 75 ohm termination.I can try to reproduce it and show some captures.

author: rf-loop
author_rank: Super Contributor
timestamp: May 18, 2016, 10:00:31 am
content: Quote from: borjam on May 18, 2016, 08:51:54 amIf not reason in just this special case but...Rigol DS1000Z Sin(x)/x is just bad joke.About DS1000Z it is explained here from start to message #7https://www.eevblog.com/forum/testgear/rigol-ds1074z-weird-signal-level-problem/msg649723/#msg649723

author: borjam
author_rank: Unknown
timestamp: May 18, 2016, 10:09:33 am
content: Quote from: rf-loop on May 18, 2016, 10:00:31 amNot exactly, mine was certainly not aliasing on the fundamental frequency, as I was displaying several periods on screen, which means that necessarily my sampling frequency was higher than double the fundamental frequency. I think that some harmonics were causing aliasing due to not good enough filtering. The interpolation (if I remember well) was just going nuts on itAnyway I will try to do some captures this week. At first it was quite puzzling.

author: macboy
author_rank: Super Contributor
timestamp: May 18, 2016, 03:05:41 pm
content: Quote from: pascal_sweden on October 08, 2014, 10:29:49 pmQuote from: Wuerstchenhund on October 09, 2014, 04:50:51 pmAs someone already pointed out, the PM3340 was 2 GHz, not 2 GSps. I have the PM3320A, little cousin to 3340. The PM3320A is only 250 MHz but did up to 10 GSps effective-time sampling. At 5 ns/div the register is 512 samples deep, which affords 50 samples per division (slightly more than 10 divisions are shown on screen). Then 50 samples / 5 ns is 10 samples per ns, or 10 GHz. The PM3320A service manual goes into much more detail about the sampling system than the user manual. Do you have the PM3340 service manual? The following is for the PM3320A but the PM3340 is so similar this will be useful to understand the PM3340 too. The PM3320A has three sampling modes: Real time, effective time, and random.The PM3320A had two different real-time sampling modes. First, using the ADC directly. This was good up to 200 kSps, so 2 ms/div (4096 samples/ ~10 div screen or exactly 400 samples per div. At 2 ms/div, that works out to 5 microseconds/sample or 200 kSps). The other way is using the CCD, a charge-coupled device, a.k.a. bucket-brigade device. Analog samples are clocked into the CCD at a high rate, then clocked out later at a lower rate (50 kHz or so I think) for digitization. The CCD was only 512 samples deep (actually there are two 512 sample CCDs, but every other sample on each was the ground reference, so still 512 effective samples), so for true real time, the resolution is limited to 512 samples/screen (1/8 of the maximum resolution), with interpolation between those for the display. If you use the "Max Res" function, then it combines 8 captures of 512 samples each, shifted slightly in time, to give a 4096 sample deep capture, but that's not real time any longer. The maximum real-time sample rate is 250 MSps at 200 ns/div (2 Gsps effective time at 200 ns/div). If using "Max Res" at the same time/div setting, the effective time sample rate is then 2 GSps (8x as high).Beyond the 200 ns/div or 250 MSps limit, the PM3320A did random sampling. The CCD would still be filled at 250 MSps, but when reading/digitizing the samples, only 1 in N were kept and stored in the register (sample memory). At 5 ns/div, only 1 in 40 are kept, meaning that only 6 samples are captured and stored for each trigger. The displayed waveform is build up over many triggers, so obviously the input waveform must be repetitive and stable. It could take a few seconds to get a complete waveform on the display, and if you were using averaging to reduce noise, then you are in for a good wait while enough of each sample slot are captured. Luckily you can see the waveform building on the display. It's called random sampling because the phase/time difference between the 250 MHz sampling clock and the trigger is effectively random, so the first captured sample is randomly positioned on the incoming waveform. This contributes to the rather long capture time, luck determines when you get the full set of 512 samples from those 6 randomly placed samples per trigger event.The antiquated sampling system of the PM3320/PM3340 is offset by a couple of features that set this apart from modern competition. First, true 10 bit resolution. I don't need to say more. Second, the screen is 4k. Yes that's right, 4k. The displayed waveform on the CRT is 4000 pixel wide by 1024 tall. What's the screen resolution of your Rigol? or your >$30k Keysight?

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2016, 07:13:57 am
content: I did not know that Phillips (or anybody else) made CCD based digital storage oscilloscopes. That places the PM3320 in the same class as the Tektronix 2440 series of DSOs and neatly explains how they achieved 10 bits of resolution at 250 Msamples/second at that time. The service manual says that the cycle time on the output of the CCD is 2.5 microseconds and the ADC is a Burr-Brown ADC803 which uses variable resolution successive approximation of 500ns 8 bits, 670ns 10 bits, and 1.5uS 12 bits. The lowest end of current Tektronix models still use CCD sampling.The PM3340 is a completely different beast; it is a *real* digital sampling oscilloscope with a 2 GHz bandwidth and 50 Gsample/second sequential equivalent time sample rate. Amazingly they included delay lines which is unusual in a sampling oscilloscope but they make it much more useful. I remember finding a reference saying that its real time sample rate is like 50 kSamples/second which is about right; that is actually fast enough to produce a real time display.Phillips was not the only one using high resolution vector CRTs. Many of the early Tektronix DSOs like the 7854 and 2232 use 1024x1024 on their 5:4 5 inch diagonal CRTs producing 260x325 dpi. Some of the early HP DSOs with raster CRTs had doubled horizontal resolution.I think LeCroy makes some stand alone real time DSOs with 12 bits of resolution but they are expensive.