Oscilloscopes - What happened to equivalent-time sampling? - Page 1
Forum section: electronics / projects

author: gcewing
author_rank: Regular Contributor
timestamp: March 25, 2018, 12:13:21 am
content: I'm looking for a low-cost scope with reasonable specs. There are a couple of offerings at the moment in New Zealand that look interesting, a Rigol DS1102E and a Siglent SDS1202X-E, for about the same price.The Siglent appears to be the better one by far in all ways except one -- it doesn't have an equivalent-time sampling mode like the Rigol. And as far as I can tell, neither do any of the other modern scopes in a similar price range.Does anyone know why? Is ETS considered obsolete technology that's been superseded by something else? Will I be missing anything important by not having ETS?

author: ataradov
author_rank: Super Contributor
timestamp: March 25, 2018, 12:24:42 am
content: What are you using it for now?I thought ETS was always a marketing gimmick. Am I missing something?

author: Marco
author_rank: Super Contributor
timestamp: March 25, 2018, 12:27:50 am
content: Equivalent time sampling doesn't really have a point unless the signal path and capture and hold have sufficient bandwidth to make use of it, which for the Rigol isn't the case. It was mostly useless marketing wank.

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2018, 12:32:14 am
content: Quote from: gcewing on March 25, 2018, 12:13:21 amWelcome to the forum.Check my profile for a link to my website and email link.I can offer better direct pricing than listed on Trademe.

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2018, 03:28:42 am
content: Quote from: ataradov on March 25, 2018, 12:24:42 amNo you didn't miss anything.If we look at 2 Siglent models, a recent one and an older series:SDS1202X-E, absolutely no mention of ETS in the website blurb.https://www.siglentamerica.com/digital-oscilloscopes/sds1000x-e-series-super-phosphor-oscilloscopes/SDS1000CML+, straight away trying to make something look better than it is.https://www.siglentamerica.com/digital-oscilloscopes/sds1000cml-series-digital-storage-oscilloscopes/I've sold both and I know very well which I'd prefer.

author: Bud
author_rank: Super Contributor
timestamp: March 25, 2018, 04:21:10 am
content: Quote from: Marco on March 25, 2018, 12:27:50 amIn garbage chinese scopes it was useless. It was useful in Tek scopes.

author: tggzzz
author_rank: Super Contributor
timestamp: March 25, 2018, 08:41:41 am
content: Quote from: ataradov on March 25, 2018, 12:24:42 amYes, you are missing something. It is what allows you to see a 50ps risetime with an oscilloscope that runs at <<100kS/s, optionally on a thermal chart recorder - with neither a computer nor an ADC anywhere in sight.For a more modern example, the first google hit leads tohttps://www.picotech.com/library/oscilloscopes/equivalent-time-sampling

author: ebastler
author_rank: Super Contributor
timestamp: March 25, 2018, 09:15:55 am
content: Quote from: Marco on March 25, 2018, 12:27:50 amIn my understanding, ETS dates back to a time when fast A/D converters were really expensive, and the cost for a fast analog front end was small in comparison. So it made sense to derive some work-around to sample the fast signals your front end could handle.With the cost of A/D conversion having come down as it has, the cost of the front end is no longer negligible in comparison. Modern low-cost scope designs have to control cost on the front end too, so don't provide the generous headroom there -- and equivalent sampling no longer makes sense.Quote from: Bud on March 25, 2018, 04:21:10 amThat's a less technical way to say it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2018, 09:51:20 am
content: Quote from: ebastler on March 25, 2018, 09:15:55 amit still is, its just the BW is increased. this is relative term...rigol get by at cheap cost because they interleaved several (2 or 4?) lower BW ADC sampling to reach 1GSps, somewhat similar technique to ETS was. since 1GSps is only worth 100MHz in nyquist term, so they match their front end to that BW only, to lower development cost as well, so we get an affordable 100MHz scope like today. otoh if anyone is working in repetitive GHz signal, this will call for higher BW ADC, say if you want to do with 4 X 1GSps ADC chip, to get effective 4GSps or more rate, let alone trying to do development for the matching AFE which need to be tested with a house and a ferrari. go at digikey and look at how much a 1GSps ADCer worth? a single chip cost a DS1054Z already. and then the question of practicality of it, people are now working on intermittent bluetooth or wifi signal making ETS is a useless feature, in the end people who can afford 4 x 1G sampler might as well just go for a house and ferrari making ETS market shrinked like a peanut.so... ADCs are still expensive real quick, once you go outside the hobbiest market. my 2cnts.

author: chris_leyson
author_rank: Super Contributor
timestamp: March 25, 2018, 10:12:01 am
content: Tektronix used ETS in the 7854, 10-bit ADC ran at a few 100kHz but the sampler before the ADC was high speed 400MHz bandwidth. HP54610B has 750MHz BW (measured) and 20MHz sample rate. Equivalent time sampling or under sampling is only good for repetative signals so that's why you don't see it in scopes anymore.

author: Dubbie
author_rank: Unknown
timestamp: March 25, 2018, 10:24:29 am
content: Quote from: Mechatrommer on March 25, 2018, 09:51:20 amWhich textbooks are you reading? You’d have to have a pretty crap input filter to be Nyquist limited to 100MHz with 1GS/s sample rate. At least 200MHz should be possible even with a very poor filter.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2018, 11:47:59 am
content: Quote from: Dubbie on March 25, 2018, 10:24:29 amno textbook but practical application. nyquist theoritical limit is 2X sampling. many high end high BW brand scope sample at 2.5X, 10X is a comfortable margin. with proper AFE, DS1054Z (or any brand 1GSps rate for this matter) can be made a 400MHz scope in practice, but in reality some people made a test and at 200MHz, the DS1054Z already show significant lost, we still can eyeball some 400MHz signal though from it.

author: jpb
author_rank: Super Contributor
timestamp: March 25, 2018, 01:14:23 pm
content: I really like the ETS on my LeCroy WaveJet, it is certainly not a gimmick.It only works on repetitive signals of course, and the scope is still limited to its 350MHz bandwidth but this is equivalent to 1 nanosec rise time and under normal sampling the maximum sample rate is 2 GS/s which gives just a couple of points on a rising edge. ETS is equivalent to 100GS/s which give a nice smooth rising edge.I think the reason ETS is no longer a feature is not because it is superseded but because modern scopes use digital interpolation even for triggering rather than analogue so there is nothing to be gained (that is they always sample at the same integral time points and then interpolate whilst analogue triggers happen at in-between time points if my understanding is correct).Here is a photo of the Leo's pulser to show what I mean:

author: nctnico
author_rank: Super Contributor
timestamp: March 25, 2018, 02:58:41 pm
content: Quote from: gcewing on March 25, 2018, 12:13:21 amETS is the ability to use a lower samplerate to capture high frequency signals. Nowadays most DSOs have a high enough samplerate for their bandwidth so ETS is no longer needed. After all ETS only works on repetitive signals so it is of limited use anyway.

author: jmelson
author_rank: Super Contributor
timestamp: March 25, 2018, 03:21:23 pm
content: Quote from: ataradov on March 25, 2018, 12:24:42 amETS was a way to see fine time details of absolutely repetitive signals. If the signal had a little bit of time jitter or voltage noise, then the trace turned into fuzz.But, in the 1960's, sampling scopes were the ONLY way to get high bandwidth. In the '70s and 80s, high bandwidth real time scopes cost a LOT, but ETS made it possible to get high bandwidth (but with the above caveat) quite a bit cheaper. There really was a niche where ETS could allow you to see what you needed at an affordable price, but you needed to be AWARE of the limitations and especially aliasing effects. Some ETS scopes put in a jitter so that aliasing would produce muddy traces rather than make beautifully sharp but totally false pictures.Jon

author: ataradov
author_rank: Super Contributor
timestamp: March 25, 2018, 06:49:02 pm
content: Quote from: jmelson on March 25, 2018, 03:21:23 pmYes, I know what ETS is. I've used some highly specialized gear that was basically purpose built for ETS-only use.But why would anyone want it in a consumer grade DSO? Because that's what OP is asking about.

author: tggzzz
author_rank: Super Contributor
timestamp: March 25, 2018, 06:54:46 pm
content: Quote from: ataradov on March 25, 2018, 06:49:02 pmIn the context you shipped, it can be seen that people are answering *your* question / assertion - not the OP's.

author: ataradov
author_rank: Super Contributor
timestamp: March 25, 2018, 06:56:53 pm
content: Quote from: tggzzz on March 25, 2018, 06:54:46 pmI guess my original question was not specific enough. It should have read "I thought ETS was always a marketing gimmick in low-end DSO."

author: tggzzz
author_rank: Super Contributor
timestamp: March 25, 2018, 07:16:50 pm
content: Quote from: ataradov on March 25, 2018, 06:56:53 pmFar less contentious

author: Bassman59
author_rank: Super Contributor
timestamp: March 25, 2018, 09:18:04 pm
content: Quote from: ataradov on March 25, 2018, 06:56:53 pmI think a better way to phrase it would be to write, "I thought that in a low-end DSO, ETS was always a marketing gimmick."

author: gcewing
author_rank: Regular Contributor
timestamp: March 25, 2018, 10:18:27 pm
content: Quote from: ataradov on March 25, 2018, 06:49:02 pmActually, I was assuming it had some benefit on the DS1102E, and asking whether the newer scopes have something that provides an equal or better benefit.It seems some people are claiming that it was never any use on the DS1102E anyway, because it already had enough samples for its bandwidth. Can anyone with experience of using a scope from that era confirm this? Seems to me it can never hurt to have a few more samples, particularly since I gather these scopes don't have a particularly sharp cutoff above their rated bandwidth, leaving room for aliasing. Provided it actually works as advertised.Another explanation put forward is that the newer scopes can't do it because they do all their triggering in software. In that case, it's a tradeoff to keep cost down, but it still leaves the question of how much functionality is being traded away.I'm wondering whether the digital-phosphor mode gives a similar kind of effect? Or does the software-triggering thing kill that idea?

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2018, 10:37:44 pm
content: Sampling in most entry DSOs is generally just 1Gsa/s but there's quite a variation in memory depth between brands and models with more being much more useful than less. Today you're better to consider it just as important as sampling rate.From phone

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2018, 02:10:07 am
content: There are a couple of reasons:1. The economics of increasing integration eventually produced relatively inexpensive ADCs with a high enough sample rate to support the bandwidth without equivalent time sampling. This is just Moore's law at work.2. Support for equivalent time sampling requires dedicated non-integrated hardware to measure the trigger to sample time. If the ADC sampling rate is high enough, then this hardware is not necessary because triggering can be done digitally and again, this processing eventually became cheap enough due to increasing integration and Moore's law. At that point, digital triggering becomes less expensive than including the hardware to measure the trigger to sample time. In other words, it is *cheaper* to not support ETS.With that said, I still prefer to use ETS where possible because:1. Low sample rates are sufficient for almost all control loops while the higher samples rates provided by ETS are an advantage for repetitive and non-repetitive signal integrity analysis.2. The increased sample rate prevents measurement artifacts likeGibbs phenomenon. Try measuring a fast edge using a Rigol DS1054Z with 4 channels enabled to see an example. That preshoot does not exist and the same corruption is present immediately after the edge.Quote from: chris_leyson on March 25, 2018, 10:12:01 amThe Tektronix 7854 probably is not the best example because it does ETS in a way very few other digital storage oscilloscopes do; the only comparable example I know of is the equally odd Tektronix 2252. These instruments simultaneously sample both a vertical signal and the analog sweep to produce a YT display. In the case of the 7854, this results in sample rate equivalent to 200GS/s at 500ps/div.Contemporary to the 7854 (1981) are DSOs like the 468 (1981), 7D20 (1983), and 2230 (1986) which have the hardware although I think only the 2230 takes advantage of it to implement ETS while the others use it for jitter reduction of a single acquisition. I do not remember when HP started using ETS but maybe one of the HP gurus can say.

author: rhb
author_rank: Super Contributor
timestamp: March 26, 2018, 01:51:29 pm
content: Here are screen dumps from a DS1102E of the output from Leo Bodnar's 40 pS rise time pulser with a 50 ohm thru termination. The first is ETS, the second is RTS.If you're going to buy a scope because it has ETS, I strongly recommend buying one of Leo's pulsers. You'll be able to tell in seconds whether it's of any use. It's actually a great tool for a quick eval of the front end of any scope.

author: free_electron
author_rank: Super Contributor
timestamp: March 26, 2018, 04:01:20 pm
content: Equivalent tie sampling is only good to study repetitive signals. for anything else : useless.Sample faster,wider deeper is the motto these days. ram is cheap.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2018, 06:53:44 pm
content: Quote from: rhb on March 26, 2018, 01:51:29 pmIsn't this a good example of Rigols fake Sinc ?First mentioned in this forum in 2010https://www.eevblog.com/forum/chat/rigol-ds1000e-series-possible-errorfail-in-sin(x)x-interpolation/msg2526/#msg2526

author: nctnico
author_rank: Super Contributor
timestamp: March 26, 2018, 08:26:20 pm
content: Quote from: David Hess on March 26, 2018, 02:10:07 amThat is because of aliasing artefacts and not the result of the Gibbs phenomenon.

author: rhb
author_rank: Super Contributor
timestamp: March 26, 2018, 08:46:30 pm
content: Quote from: tautech on March 26, 2018, 06:53:44 pmActually, no. It's a fake screen capture :-(It was a trace of dots *until* i saved the screen. Then it turned into what i posted.

author: Andie
author_rank: Contributor
timestamp: March 26, 2018, 08:47:05 pm
content: Quote from: tautech on March 26, 2018, 06:53:44 pmIs there really sinx/x-interpolation involved in this screen dump? To me it looks like that the waveform is represented by only two (or so) samples per (x-)division. The reason why rhb compared RTS and ETS modes. The latter allows higher temporal resolution.

author: rhb
author_rank: Super Contributor
timestamp: March 26, 2018, 08:53:35 pm
content: There was no discernible difference between the actual scope displays when I toggled from ETS to RTS. So there is no benefit to ETS at all on the DS1102E. If there were, Leo's pulser would have shown it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 27, 2018, 12:10:31 am
content: Quote from: rhb on March 26, 2018, 08:53:35 pmwe can ask rigol to make very fine 200GSps clock interleaving in the fpga implementation but still i suspect we'll get the same plot like above,except more curvy like the 2nd picture (sinx interpolation), this is BW limited analog front end or slower adc comes to play, not anything to do with samplerate..2cnts.

author: rhb
author_rank: Super Contributor
timestamp: March 27, 2018, 02:52:12 am
content: The days of ETS are over. That was something that made sense when you could build a 10 GHz front end, but not a 20+ GHz ADC. Now the cost limiting factor is the front end.Gibbs phenomena has nothing to do with sample rate. It's a mathematical consequence of summing Fourier series. Overshoot, which looks similar, is a consequence of the transfer function of the vertical section. Many, but not all, scope makers have a hard edge on the BW of the scope so they can claim a higher BW than if they used a Gaussian taper edge. This is easily investigated using Octave or MATLAB.The sad part is that with an FPGA, it's easy to implement a set of digital filters that lets the user choose the desired transfer function for a step.

author: mbrandt
author_rank: Newbie
timestamp: August 28, 2018, 09:41:25 pm
content: Interesting: Equivalent Time Sampling days are over.....??Well, perhaps. But some experiments made me wonder what enables my Rigol DS1054Z as well as my Siglent SDS1204X to measure time differences smaller than the 1 nanosecond interval of the maximum achievable 1GSa/S.In fact I found the Rigol capable of detecting time differences as small as 100 picoseconds while sampling at 500Msa/S and the Siglent managed to detect even 10 picoseconds and seems to have a resolution of even 5 picoseconds and albeit a few picoseconds are "drowning" in jitter, 10 picoseconds definitely is detectable on the screen of the Siglent.Since I did not understand this at first I did some research on the internet and I couldn't find a better explanation than either Equivalent Time Sampling or Random Time Sampling has been implemented in both scopes, albeit the functionality is limited to measuring time differences between waveforms on different channels.I made a video of it and though I did my comments in dutch I think it's not very difficult to understand what I'm pointing at.I simply set my wavegen up to generate a square wave with a period time of 360 nanoseconds on both channels and by doing so every degree of phase shift between both channels equals 1 nanosecond.https://youtu.be/pGv_jxUr7PQSo if there's no ETS of RTS involved I'm very curious about how this is made possible.Does anyone have an explanation for this?

author: coppice
author_rank: Super Contributor
timestamp: August 29, 2018, 12:12:03 am
content: Quote from: mbrandt on August 28, 2018, 09:41:25 pmYour video shows a square wave with rise and fall times which are several samples long. The scope is sampling the signal with 8 bit resolution, and filtering it properly. Why would you not expect this to have much finer time resolution than the sample rate of the scope?

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2018, 01:06:18 am
content: Quote from: mbrandt on August 28, 2018, 09:41:25 pmThing is, with channels 1 and 3 active only the Siglent offers 1GSa/s as it has two 1GSa/s ADC's whereas the Rigol has only one.Other than that ^ I didn't see anything in you video that wasn't normal behavior......but I can't understand dutch so maybe I missed something.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2018, 04:02:57 am
content: It does it the same way that any oscilloscope with sin(x)/x interpolation does. In the case of the Rigol, sin(x)/x interpolation upsamples the digitized waveform to the display resolution and then the measurement is made.The actual process is to zero fill extra samples and then low pass filter the result through a sin(x)/x low pass filter which also annoyingly produces preshoot and overshoot if aliasing is present. An oscilloscope using equivalent time sampling has a much higher sample rate so this either does not need to be done or there is no aliasing and no resulting preshoot and overshoot which is one way to distinguish ETS and non-ETS oscilloscopes. This becomes very apparent on ETS oscilloscopes which support sin(x)/x interpolation when ETS is disabled.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 29, 2018, 01:30:09 pm
content: I don't think it's dead. Still currently available in some scopes and AFAIK, the only way to acquire signals at very high frequencies (such as tens of GHz) and "reasonable" cost provided that you don't need single shot acquisitions. Often called "sampling oscilloscopes". For lower frequencies, it's also a means of using high-res ADCs which have lower sampling rates than their low-res counterparts.https://www.tek.com/oscilloscope/dsa8300-sampling-oscilloscopehttps://www.picotech.com/oscilloscope/9300/picoscope-9300-sampling-oscilloscopes

author: Wolfgang
author_rank: Super Contributor
timestamp: August 29, 2018, 02:47:08 pm
content: Hi,Sampling scopes can resolve (periodic) waveforms with a time resolution per buck that is still about an order of magnitude higher than real-time scopes.Extreme example: a real time 110GHz scope from keysight will set you back easily for a million, a sampler for the same frequency will only cost a small fraction of this.For avalanche stuff, i still use a sampling PicoScope and I like it (for the money I spent):https://electronicprojectsforfun.wordpress.com/pulse-generators/avalanche-transistor-pulsers/A 12GHz real time scope is an awful lot of money. For about 15k€ you can get a 20GHz sampler.

author: mbrandt
author_rank: Newbie
timestamp: August 29, 2018, 04:50:00 pm
content: Thanks for all the reply's!From what I understand from them, the phenomenon I experienced could be explained by sin(x)/x interpolation and filtering.I do understand that interpolation will enable you to estimate time/voltage values between the actual sample points. That's is just what interpolation does and Sin(x)/x interpolation as a mathematical concept, as far as I understand, will theoretically give you infinitely high time resolution, so it's not the acquisition rate that determines the smallest time interval the scope will present you with, its been the firmware programmer's choice to determine the time resolution of the interpolated points between the acquired sample points.However I don't understand how this explains the capability of the scopes to measure time differences smaller than the interval time of the acquisition rate between two channels. And that is what puzzles me, if there's no Equivalent Time Sampling involved that is.Perhaps I need to simplify what I'm doing in my experiment. I'm using an AWG to create two square waves that are kept in sync within the AWG. The AWG can delay one of the square wave relative to the other in steps as small as 10 picoseconds though it's rise/falltime is about 8 nanoseconds.In my video I'm testing how small a time delay the scopes can measure.Lets say I only want to measure a 100 picosecond time delay to keep it simple.To further simplify the situation, I'll just use one signal, lets say a 1 kHz square wave with a rise/fall time of 10 nanoseconds.We'll use a BNC T-splitter on the generator and we'll connect channel 1 of the scope with a coax cable of 1 meter and a second channel of the scope with a coax cable of 1 meter and two centimeters. Now if I did my calculations correctly, the result will be that the edges of the square wave will arrive about 100 picoseconds later on the second channel of the scope than on the first channel of the scope.Actually, I tried this. And the scopes will show you the 100 picosecond delay.Now how can that be explained by sin(X)/x interpolation and/or filtering?I don't understand that, since that would imply that interpolation and/or filtering are somehow integrating the samples of two channels.And of course I could do another experiment using just dots mode, as far as I understand, when using dot's mode, there's no interpolation involved.In fact @16.50 in my video I'm switching to dot-mode, but I haven't tried time delay steps smaller than 1 nanosecond in dots mode....Could try that another time of course.

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2018, 07:28:28 pm
content: Quote from: mbrandt on August 29, 2018, 04:50:00 pmCorrect, dots are the acquisition sample points and if you worked with just enough to form a continuous trace you could accurately compare results against vector traces and sin(x)/x interpolation.

author: coppice
author_rank: Super Contributor
timestamp: August 29, 2018, 08:33:49 pm
content: You are looking at this like you are using the logic level thresholded inputs of a logic analyser. You are actually using inputs with 8 bit amplitude resolution. Your rise time is so slow it takes several (about 5, it appears) sample times to rise. Lets say you don't use sinc filtering, and just display the raw samples as dots. Each of the 5 dots from the undelayed signal will be right above the corresponding dot from the delayed signal. They will form 2 diagonal lines with 100ps of x-axis displacement between them.I don't know how much the displayed rise time is controlled by the output of your generator, and how much comes from the bandwidth limits of the scope's front end. Either way, it isn't behaving like a logic analyser input.

author: mbrandt
author_rank: Newbie
timestamp: August 29, 2018, 08:43:59 pm
content: And so I did just that: I did the same test in dots mode with the siglent sds1204x-e.There's no significant difference wether I'm using sin(x)/x interpolation, just straight x interpolation or dots mode, except when using dots mode you can clearly see it's in dots mode. There certainly are enough dots plotted on the screen to get a clear view of the waveform though.So, even without interpolation, the siglent can still detect time delays as small as 10 picoseconds between two channels.There's no difference whether I'm using channel 1 and 2 or 1 and 3 as a pair, except of course when using channel 1 and two, the acquistion rate drops down to 500 Msa/s.When I say you can detect a 10 picosecond time delay difference between two channels, its no more than that: You can just see the waveform moving. I think it's safe to say that any serious measurement is limited to steps of about 100 picoseconds with an uncertanty of about 50 picoseconds.Another interesting fact, it also works in single mode; so just one trigger event is enough.It's clear there's something built into these scopes that enables them to measure time intervals that are only a fraction of the acquisition time interval of 1 or 2 nanoseconds, provided you're measuring a delay between two channels.So could that mechanism be (related to) equivalent time sampling and if not, how could this be done.....?

author: mbrandt
author_rank: Newbie
timestamp: August 29, 2018, 08:51:14 pm
content: Quote from: coppice on August 29, 2018, 08:33:49 pmI think I'm beginning to see what you're pointing at, I'll give it some thought, thanks!

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2018, 08:56:27 pm
content: Quote from: mbrandt on August 29, 2018, 08:43:59 pmThe primary use of Deskew in the channel input menu is to allow nulling of input propagation delays due to different cable lengths of say passive and current probes and so then allow more accurate measurement of signals in the time domain.

author: nctnico
author_rank: Super Contributor
timestamp: August 29, 2018, 08:57:08 pm
content: Quote from: mbrandt on August 29, 2018, 08:43:59 pmIt is not just 'these' scopes but every DSO works that way. The principle is simple: you have 2 sample points and you know the time and voltage difference between them. From there you can calculate the time and voltage level along a line plotted (interpolated) between these two points. Do this for two channels and determine at which time the signals are at exactly the same voltage level and you can calculate the delay between the two channels. The limit for the accuracy of this measurement is the stability of the time base and the trigger mechanism.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2018, 09:06:21 pm
content: Assume for the moment that no aliasing is present when the sin(x)/x filtering is used to upsample the waveform to an arbitrarily high sampling rate. With no aliasing present, there is only *one* waveform consistent with the sampled input. (1) If the sampled input is delayed by an amount which is much smaller than the sampling time such that no samples shift in time, it still alters the amplitude of the samples and the sin(x)/x interpolation produces a *different* waveform which reflects the small change in delay. In this case the different waveform should only be delayed in time and the same in all other respects.In practical applications, intermodulation from the nonlinearity in the digitizer results in aliasing even if the analog signal is bandwidth limited so the sin(x)/x interpolation produces multiple solutions resulting in "wobbulation" if persistence is not used and smearing of the waveform if it is used like on the Rigol DS1000Z series. (2) This can be remedied by using averaging but I would argue that if you are going to use averaging of multiple acquisitions, then you might as well have used equivalent time sampling to start with and avoided interpolation and its problems altogether. Still, a lot of high performance DSOs are intended to work this way and it is less expensive than implementing equivalent time sampling.Analog oscilloscopes have no trouble distinguishing changes in delay which are an order of magnitude shorter than their transition time. A 100 MHz analog oscilloscope can not faithfully reproduce a 3.5 nanosecond edge but it can easily resolve a 1 nanosecond shift in that edge.(1) The sin(x)/x reconstruction filter used by Rigol is apparently broken as it results in an interpolated output which does *not* connect the original samples like it should but that is a different subject and it might not matter for a delay measurement. I suspect truncation during the computation causes this.Quote from: mbrandt on August 29, 2018, 08:43:59 pmDoes the Siglent manual discuss the algorithms used to make the automatic measurement?It would not surprise me if after doing the min/max or histogram calculations to extract the amplitude characteristics of the waveform including the mesial used for delay calculations, sin(x)/x interpolation is used to extract the timing information. The display interpolation mode really should not affect automatic measurements.Better DSOs actually produce a timing histogram so the delay measurement includes average, minimum, maximum, standard deviation, etc.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 29, 2018, 09:31:35 pm
content: Absolutely correct. Measuring time differences and slopes way beyond the sample rate of a digital scope is close to useless due to the many uncertainties involved. Get a sampling scope with a bandwidth a few times your fastest signal risetime and you can rely on results. Another big advantage of sampling scopes is a good vertical resolution (e.g., 16bits) while the ENOB numbers of DSOs approaching their bandwidth limit become really poor.

author: rhb
author_rank: Super Contributor
timestamp: August 29, 2018, 10:11:16 pm
content: There's a fundamental problem with DSOs measuring timing because of the trigger. A certain A list 1 GHz DSO I played with reported 26 pS jitter on a GPSDO and a Keysight 33622A. The 33622A is specified as less than a pS of jitter. The GPSDO is one of Leo Bodnar's dual output units. I don't recall a jitter spec, but obviously to be useful as a frequency reference it has to be small and with both the signals on screen there was negligible shifting between the two waveforms once I adjusted the 33622A to the same frequency as the GPSDO.Interestingly, the same DSO reported the jitter of one of Leo Bodnar's 40 pS 10 MHz square wave generators as about 13 pS even though it has more jitter. The frequency is much less stable than the other two signals.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2018, 04:33:11 am
content: If digital triggering is used which is common today in new DSOs which do *not* support equivalent time sampling, then it is achieved using the same sort of measurement as a delay measurement uses. The trigger level is known and interpolation finds the point on the waveform between samples which crosses it.LeCroy's marketing department was a big believer in digital triggering and used to show videos demonstrating how superior it was compared to analog triggering but if you watch them carefully when the signal source is a fast edge, and this applies to all DSOs which use digital triggering, you can see aliasing as I described above and noise shifting the edge around the trigger point producing results that are not inherently better than analog triggering.The really big advantages of digital triggering are that it is less expensive to implement and can inherently support high trigger rates. Time to digital converters used in frequency counters optimized for a high number of measurements per second may operate the same way with a data slicer feeding a digitizer followed by digital interpolation and comparison. The timing resolution may be calculated from the sample rate, ADC resolution, and noise. (1)(1)Centroid Timing or Transition Midpoint Timing TDC.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 05, 2018, 08:50:02 pm
content: ETS is quite useful if scope has less than 10x sampling rate of analog BW under some condition. Little practical example below. Given scope is rated at 100MHz analog. 1 ch active 1GSa/s, 4 ch active 250 MSa/s. However with ETS it can do 20GSa/s on all channels at the same time. Quite a difference in result. On the left is some falling edges observed with ETS, on the right with RTS. Both averaged (otherwise RTS would be even worse mess).So fairly handy if doing some activity related to tweaking physical RF signal paths etc. Essentially you are getting result like on analog scope and can be sure that you observe actual signal subject to physical limitations (frontend bw etc).Edit: As can see on setup x10 probes introduce residual ringing while signal at 50Ω termination is clean. This interesting detail is lost with lowish sampling rate despite valid Sin(x)/x post interpolation etc.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2018, 04:28:04 am
content: That is a great example. The probes should have a better response than that but maybe the extra length of the female-to-female BNC adapter explains it.

author: borjam
author_rank: Unknown
timestamp: September 06, 2018, 06:50:22 am
content: Quote from: tautech on August 29, 2018, 07:28:28 pmBeware, not in the Rigol DS1000Z series. Dots are, confusingly, post interpolation as well.

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2018, 07:43:46 am
content: Quote from: borjam on September 06, 2018, 06:50:22 amReally? That would be seriously annoying.I hate "helpful" instruments that prevent me observing what they are measuring. One example is the fuel gauge in a car.It used to be that the displayed value varied according to the angle the sender was at relative to the car's body. OK, that showed a different value when I was on a hill, but when I was on the flat it returned to the standard value. I could interpret it and know what it was (and wasn't) telling me.Now the displayed value is processesd by a computer, and some software implementor or marketeer decided that wasn't good enough. Now, when I park on a hill and return, about 1/8 of a tank of fuel has apparently evaporated - and it doesn't reappear when I'm back on a horizontal road. Itdoesreappear after I've been parked on the flat for more thanaboutan hour.I can interpret and understand reality. I can't double guess what a software engineer wants me to think reality is. even when, after I finally got around to adding a fuel gauge to one car, there was no smoothing so it wobbled violently between quarter and half full - and the meter was calibrated in roentgens/hour.

author: borjam
author_rank: Unknown
timestamp: September 06, 2018, 09:16:02 am
content: Quote from: tggzzz on September 06, 2018, 07:43:46 amReally. I though I had got insane when I tried Leo Bodnar's pulse generator.I own both a Siglent SDS1202X-E and a Rigol DS1074Z. The Siglent did the right thing. The Rigol still added some interpolation in dots mode. I can try again and post some screenshots.And yes, dots should be samples, pure and simple.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 06, 2018, 09:52:57 am
content: To me teasing a 100MHz scope with 3.5ns risetime with a pulse generator with 100ps or less is a *bit* of an academic exercise. Why do this ?

author: dzseki
author_rank: Frequent Contributor
timestamp: September 06, 2018, 10:06:26 am
content: Quote from: Wolfgang on September 06, 2018, 09:52:57 amI don't think so, back then everyone had a "different" understanding on scope bandwidth. You may know that some scopes were somewhat underspecified in terms of bandwidth, but this can't be generalized of course, so it is not a bad idea to check what you have.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 06, 2018, 11:03:40 am
content: Quote from: David Hess on September 06, 2018, 04:28:04 amChecked: Indeed probe is ok. Direct line probe - FF adapter - pulser is also pretty good. So it must be some effects due other components in setup, since even reconfiguring and connecting probes directly to T's did not change much.

author: borjam
author_rank: Unknown
timestamp: September 06, 2018, 11:17:07 am
content: Quote from: Wolfgang on September 06, 2018, 09:52:57 amMaybea buteor even a bruteNote that I don't own a similar pulse generator, so it was 100 ps or nothing.Nevertheless I learned something important about the DS1000Z, it interpolates samples in dot mode.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 06, 2018, 11:19:50 am
content: When you compare the gain in precision when you change from a 100MHz scope to a 200MHz to the extra information you gain by ETS its very clear that it *is* academic.Rule of thumb: When making risetime measurements, the scope risetime should best be an order of magnitude below the DUT risetime (or the bandwidth 10 times as much).Even this will get you just the first 4 harmonics of a square wave, you can image how square it would look then. Risetimes add in a root sum squares fashion, so instrument and probe risetimes and its uncertainties make measurements close to bandwidth limits very inaccurate. Why do this ?

author: borjam
author_rank: Unknown
timestamp: September 06, 2018, 11:29:03 am
content: Quote from: Wolfgang on September 06, 2018, 11:19:50 amThe pulse generator has a male BNC connector. It was connected directly to the scope input via a 50 ohm terminator. No probes.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 06, 2018, 11:32:15 am
content: Even without probes (I distrust them anyway for all high speed measurements) the risetime problem stays the same. I suggest that you at least verify your setup with a trusty scope with a large bandwidth (1GHz up), then you can believe whats on the screen.Of course, having fun when playing around is always OK

author: dzseki
author_rank: Frequent Contributor
timestamp: September 06, 2018, 11:39:57 am
content: Quote from: Wolfgang on September 06, 2018, 11:19:50 amBut beyond 100MHz source signal you'll have hard time to find an affordable oscilloscope to poke with... So you feel like you want to correct the seen rise time with the known constants (like the scope's self rise time, rise time of probing) while this is not elegant, but is often practical...Not that I'd use a Rigol DS1000 for high frequency measurements, but I don't think either it is overkill to characterize it's bandwidth. And for that the fastest pulse risetime is the most practical, as always.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 06, 2018, 12:00:02 pm
content: No need to buy a multi-GHz beast (see TEA thread), but just try it out on one you have access to.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 06, 2018, 12:21:51 pm
content: Quote from: dzseki on September 06, 2018, 10:06:26 amStill are. My 2048B has about 140MHz analog (~2.4ns RT) confirmed by various methods. All other properly designed 100MHz scopes should be about the same.On the subject of "pointlessness" of measurements at bw limit: Exact same setup (incl probes) connected to 500MHz scope. This time left window is 50GSa/s ETS all channels and right window 1.25GSa/s RTS all channels.Of course scope inputs itself may change picture a little, but what can observe? Somewhat more small details. But did 100MHz scope running in ETS mode miss important details - no! Just enough info to see ringing and seek the source in physical setup. On 4 channels at same time! Absolutely not possible at 250MSa/s.Also can notice that even on fairly fast scope running 4x1.25GSa/s averaging RTS trace may throw off timing. Both cursor and auto-measurments are spot on in ETS window (signal is exactly 20MHz) while tiny bit off in RTS one. Same can be said for signal traces. So overall - excessive sample rates rock. Excellent helper for shared ADC scopes.

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2018, 01:43:31 pm
content: Quote from: Wolfgang on September 06, 2018, 11:19:50 amWhether or not it is academic depends on the sampling rate.Example 1: the first digitising scopes I used professionally were two man lift boat anchors, with a 1GHz bandwidth and 25MS/s (or was if 40MS/s; I forget).Example 2: I have a sampling scope with a 50ps risetime source and a guaranteed 140ps sggregate risetime. The sampling rate is <<100kS/s.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2018, 02:31:32 pm
content: Quote from: Wolfgang on September 06, 2018, 09:52:57 amIt is routine for transient response testing and calibration of the oscilloscope.It can also be used to verify the measurement setup including the probes. I also regularly do this to test RF attenuators within the bandwidth of the oscilloscope since I lack a suitable VNA.Quote from: MrW0lf on September 06, 2018, 11:03:40 amI have used basically the same test setup before but with female BNC-T adapters so the x10 probe plugged into the 50 ohm transmission line without the extra length of the BNC gender changer and the probe response was practically identical to the oscilloscope's calibrated input response.For higher frequencies where the extra length of the probe tip adapter and BNC-T would cause problems, they used to make special in-line probe tip adapters which were like a BNC-T except the probe plugged directly into it. These were useful up to at least 1 GHz if an active probe was used.Quote from: Wolfgang on September 06, 2018, 11:19:50 amA higher bandwidth oscilloscope will of course provide a more accurate measurement at higher frequencies but that by itself is not sufficient if the sample rate is not high enough even with averaging. ETS provides an order of magnitude increase in sample rate where real time sampling is not required.Tektronix has been providing a lot of application notes talking about the advantages of real time sample rates with averaging for making accurate rise and fall time measurements but they leave out that this does not provide the accurate transient response measurements that ETS provides through much higher sample rates. I think this is just a case of marketing what you have and concealing its disadvantages.

author: rf-loop
author_rank: Super Contributor
timestamp: September 06, 2018, 04:02:44 pm
content: Quote from: borjam on September 06, 2018, 09:16:02 amHere is some findings from year 2014 about Rigol Sin(x)/x mess. DS1074Z.I have not anymore Rigol so I can not repeat tests using current FW.In this thread I show what kind of mistake Rigol do with with Sin(x)/x interpolation... oh and not only this alone.One main problem is that Rigol destroy real sample data what Siglent do not.there is enough test images / data what show this error so that this time I can say they do wrong, totally wrong. And still Rigol engineer do not mind or even understand what goes wrong with method they use.More fun that this nearly same debate was here in forum long long time ago when Rigol DS1000E ancient model was just new. I show this same error! Also at this time EEVblog Rigol fan club did not understand it at all. It feels nearly like "it must be Right because Rigol did it" Only rolling eyes and "so what".. who cares. Why repair errors if product sell well. (omg)This time I try contact Rigol Beijing. Not any single answer. Like talking to walls at this time.One most clear error is that they do not interpolate trace line via true sample points! THis is so bad error that I can ask what are they smoking in Rigol or do they have been never in any basic school for even know what is Sin(x)/x interpolation. They break-violate first rule. They do not respect true sample points and produce Sinc via true sample points. (exept if it is after years finally repaired in some new FW what I have not seen irl how it works.There is also displayed how it need right so that do not violate this one most important rule. Interpolate via true data points.Also here is Mechatrommer one comment from 2012I can not find very old thread where it was originally.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2018, 07:13:34 pm
content: Quote from: rf-loop on September 06, 2018, 04:02:44 pmI suspect the reason for this is linked with Rigol's use of an 8-bit display record which also results in averaging only producing an 8-bit result. If the display record is only 8-bits, then maybe all of the processing is also limited to 8 bits so their sin(x)/x interpolation algorithms suffers from excessive truncation errors.This design decision could be an optimization for greater speed at the expense of accuracy.