Mixing signals for single channel oscilloscope triggering - Page 1
Forum section: electronics / testgear

author: mmm22
author_rank: Regular Contributor
timestamp: May 31, 2020, 03:37:25 pm
content: I need to trigger the scope with an external trigger since the signal itself is quite unstable. My scope has 1GS/s rate for a single channel, decreases by half if both channels are used. Therefore I’d like to trigger with the signal channel, by mixing in the trigger pulse (which is significantly higher so the trigger level can be set higher than the signal itsels).The signals is about 10ns wide (pic attached). I am mostly interested in the integral of the signal, so the mixing circuit can change the shape as far as the integral stays proportional to the original pulse.Is this viable? What would the mixing circuit be?

author: tggzzz
author_rank: Super Contributor
timestamp: May 31, 2020, 05:33:31 pm
content: What is your scope? Someone with experience of it might be able to point to techniques you haven't considered (e.g. use an external trigger input).Apart from that, to answer the specific point, you could consider a three resistor circuit:Rt from the trigger signal to node 1Rs from the signal to node 1Ro from node 1 to groundoutput taken from node 1 to groundCan't give suitable values for the resistors without knowing levels, trigger/source output impedance. It might be advantageous to put a small capacitor in parallel with Rt, to emphasise the trigger edges.Most importantly, at these frequencies you will have to be very careful about layout and construction. Don't even think of using solderless breadboard, because the parasitic inductance and capacitance will be too high.You might also be able to use an RF (power) combiner/splitter, which is little more than the above with 50ohm impedances.

author: mmm22
author_rank: Regular Contributor
timestamp: May 31, 2020, 06:26:51 pm
content: Thanks for the advice.The scope is PicoScope 2207B, it doesn't have an external trigger.The trigger is a 5V TTL, the signal is around 100mV. I am not exactly sure about the signal and trigger output impedances, but I guess they should be only a couple Ohm.Do you think the commercial TV splitters/combiners are good enough for this?

author: nctnico
author_rank: Super Contributor
timestamp: May 31, 2020, 06:59:43 pm
content: A regular 50 Ohm splitter should do to combine the signals but the original signal will also be loaded by the trigger signal as well. It is not a very nice solution. Can't you disable the channel and still trigger on it? Or buy/borrow a better oscilloscope?

author: tggzzz
author_rank: Super Contributor
timestamp: May 31, 2020, 08:09:08 pm
content: The scopes sample rate is irrelevant; it is a 100MHz scope.The 50* higher trigger level means you could, in theory, have a 10* higher resistor in that side of the splitter which would reduce the loading on the signal side (assuming the signal can drive 50ohms).Another possibility, depending on levels, would be to isolate the trigger with a diode that is reverse biassed when the trigger is off.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 07:31:44 am
content: Quote from: nctnico on May 31, 2020, 06:59:43 pmDo such scopes exist?

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2020, 07:35:12 am
content: Quote from: mmm22 on June 01, 2020, 07:31:44 amHiding a channel yet still triggering on it is not uncommon.

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 07:54:20 am
content: Hello,can you show a picture of your signal (single trigger) with one channel and with two channel on.Best regardsegonottoPS:Can you use Equivalent-time sampling?

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 08:25:06 am
content: Quote from: tautech on June 01, 2020, 07:35:12 amDoes this hiding result in keeping the digitization rate same as if only one channel is used? Any examples of the scope models?Edit: only for the scopes that reduce digitization rate if additional channel is used.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 08:28:19 am
content: Quote from: egonotto on June 01, 2020, 07:54:20 amThere is just twice less points per time unit if 2 channels are used. What else is to see there?I cannot use ETS since my signal is not repeatable.

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2020, 08:40:09 am
content: Quote from: mmm22 on June 01, 2020, 08:25:06 amNo, it's only hidden from the display so still is being used for whatever scope function is selected therefore still impacts on sampling rate and shared memory.However DSO's with dual ADC's are not affected in the same way if 1 channel is assigned to each ADC.QuoteAll the later Siglent X and X-E models.

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 08:48:15 am
content: Hello,mmm22 wrote "There is just twice less points per time unit if 2 channels are used. What else is to see there?"Why you show us the picture not?Your scope has only 70MHz. It is possible that 1GS/s or 500MS/s makes not much difference.The pictures are from a 200MHz scope and the difference are small.Best regardsegonotto

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 08:51:04 am
content: Quote from: mmm22 on June 01, 2020, 08:28:19 amWhether or not it affects the results you see!If, for example, your signal only contains energy below, say, 1MHz, then the reduced sampling rate would be unimportant.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 08:55:42 am
content: Quote from: egonotto on June 01, 2020, 08:48:15 amThe difference is quite large for my application since I need quantitative measurements of the pulse.

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 08:57:54 am
content: Quote from: mmm22 on June 01, 2020, 07:31:44 amYes. But it is not something you find in the datasheet. Also expect to still have a reduced samplerate on some oscilloscopes because they do triggering in the digital domain. However if the Picoscope does analogue triggering you could use the second channel as a trigger input.But I get the feeling you are pushing the Picoscope too far anyway. A 10ns pulse has a fundamental frequency of 100MHz. I'd do this measurement with an oscilloscope which has at least 200MHz of bandwidth in order to get a somewhat accurate measurement result. The limit of your Picoscope isn't the samplerate but the bandwidth.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 09:00:34 am
content: Quote from: tggzzz on June 01, 2020, 08:51:04 amMy signal is only 10ns wide, how it can contain only energy below 1MHz?

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 09:06:16 am
content: Quote from: nctnico on June 01, 2020, 08:57:54 amThat is true, but I am mostly interested in the integral of the pulse. As far as I understand, the bandwidth slightly below the fundamental frequency shouldn't affect it.

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 09:08:12 am
content: Quote from: mmm22 on June 01, 2020, 09:00:34 amIf that is the signal, as opposed to the trigger, then that is correct.If you are interested in the area under the curve, have you considered:that your scope's bandwidth is a limiting factor, not the sampling rateusing an RC filter to compute the integral, and measuring the result with a multimetermeasuring the integral directly with a ballistic galvanometer

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 09:11:43 am
content: Quote from: mmm22 on June 01, 2020, 09:00:34 amI think tggzzz just took 1MHz as a random number to clarify the point that an excessive samplerate doesn't give extra information. My estimation is that 500Ms/s and a bandwidth of 200MHz is good enough for your purpose.

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 09:49:43 am
content: Hello,again, why shows mmm22 not the picture? Than we would see the difference between one and two channels.Now i use 50 Ohm and use a 70 MHz low pass filter (green) and one picture with 1GHz.The difference after the 70 MHz low pass filter is very small.Best regardsegonotto

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 11:24:23 am
content: Quote from: egonotto on June 01, 2020, 09:49:43 amThe pictures might look alike, but as I said I am interested in quantitative measurements that cannot be properly estimated with the pictures.Also, try not to use the interpolation and leave only the actual points on the screen; the effect of sample rate will be more visible.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 11:27:06 am
content: Quote from: nctnico on June 01, 2020, 09:11:43 amHow have you come up with this estimation? The usual rule is that bandwidth should be 1/10 of sample rate.

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 11:32:56 am
content: Quote from: mmm22 on June 01, 2020, 11:24:23 amQuoteTwo questions with a related answer:Interpolation is the wrong word here. Reconstruction is a better term since the reconstructed signal on a DSO always includes the actual sample points using sin x/x. If you look at Nyquist theory you'll see that all the information of a signal can be captured up to half the sampling frequency. In reality you need some margin to allow room for the low-pass filter. Signal reconstruction is mostly a visual aid to make a signal look like a signal instead of a bunch of dots. You are still looking at what has been captured but with a curve fitted onto the dots. The '1/10 rule' is not valid for modern digital oscilloscopes which have a sharp roll-off in the anti-aliasing filter. Nowadays all common DSOs need a samplerate of 2.5 times the bandwidth. So for 200MHz 500Ms/s is enough to capture the signal.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 11:41:19 am
content: Quote from: tggzzz on June 01, 2020, 09:08:12 amThe bandwidth shouldn't affect the integral, as far as I understand. Also, this advice seems to be in contradiction with the following one.To use a multimeter I would need to low pass the frequency at least by 10 times; that results in lowering the amplitude by 10 times too, which would be 10mV. For precise measurements the multimeter needed would cost not less that the scope.Are there any commercial models of ballistic galvanometers?

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 11:47:12 am
content: Quote from: nctnico on June 01, 2020, 11:32:56 amDo you think I still need the bandwidth even if I am only interested in the integral of the pulse?

author: 2N3055
author_rank: Super Contributor
timestamp: June 01, 2020, 11:47:53 am
content: Quote from: mmm22 on June 01, 2020, 11:24:23 am

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 12:03:22 pm
content: Quote from: mmm22 on June 01, 2020, 11:27:06 amThere is no such rule. You might be thinking of what turns out to be convenient for a manufacturer with a specific implementation architecture.

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 12:13:13 pm
content: Quote from: mmm22 on June 01, 2020, 11:41:19 amW.r.t. time/amplitude: that is true for all mechanisms including a scope, so why not exploit it.Have you understood the amplitude specifications for your scope, particularly linearity and resolution? DMMs with good low voltage resolution are cheap nowadays.QuoteI haven't looked recently. They were used to measure charge in a short current pulse (Q=It), so that might be a starting point.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 12:25:48 pm
content: Quote from: tggzzz on June 01, 2020, 12:13:13 pmThe DMM should have not only good low voltage resolution, but also be triggerable in some way since the pulse with 10mV amplitude would still be only 100ns wide.

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 12:31:25 pm
content: Quote from: mmm22 on June 01, 2020, 12:25:48 pmIf you are taking the integral, the 100ns is completely irrelevant. If you don't like using an RC filter, use an opamp as an integrator.

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 12:45:31 pm
content: Quote from: mmm22 on June 01, 2020, 11:47:12 amYes because you don't want the pulse to be attenuated too much. Still you should expect an error in the ballpark of 10%.

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 01:04:36 pm
content: Hello,it seems that 70MHz band width and 500MHz sampling rate are no problem.I integrate with full band width and 20 MHz band width and with 1GS/s and 500MS/s.The integral is roughly the same.Best regardsegonotto

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 01:38:47 pm
content: Quote from: tggzzz on June 01, 2020, 12:31:25 pmIt is not completely irrelevant since with a long gate it would integrate also the noise besides the pulse.An op-amp alone would amplify the noise as well.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 01:42:16 pm
content: Quote from: nctnico on June 01, 2020, 12:45:31 pmIf attenuation is the only problem then it's fine since the signal is still in 100mV range.Where the 10% comes from?

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 01:51:26 pm
content: Quote from: mmm22 on June 01, 2020, 01:42:16 pmOn a 200MHz oscilloscope the frequency response won't be flat up to 100MHz. 1dB is around 10% already. Added to that there is the inherent DC accuracy which typically is between 1% and 2%. If you are going to measure the pulse beyond the specified bandwidth you get over 3dB attenuation and a much larger error (in absolute sense).

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 02:08:56 pm
content: Quote from: egonotto on June 01, 2020, 01:04:36 pmOne thing I can see immediately is the difference between 1GS/s and 500MS/s at full BW. What is the reason that the integral doesn't stay constant?

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 02:57:36 pm
content: Hello,in the connection between waveform generator and USB-scope I got a mains hum. Perhaps this is the reason as sometimes one get negativ voltage.Best regardsegonotto

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 03:07:30 pm
content: Quote from: egonotto on June 01, 2020, 02:57:36 pmBut why it only has effect at 500MS/s?

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 03:13:36 pm
content: Quote from: nctnico on June 01, 2020, 01:51:26 pmSince the pulse shape stays the same and only the amplitude changes, I don't see why the BW unflatness should affect the measured integral. Especially since most of the energy is within a quite narrow frequency band.

author: egonotto
author_rank: Super Contributor
timestamp: June 01, 2020, 03:28:37 pm
content: Quote from: mmm22 on June 01, 2020, 03:07:30 pmHello,this is random. If in time of the measure is the negative half wave of hum than one have the effect.Best regardsegonotto

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 03:33:09 pm
content: Quote from: mmm22 on June 01, 2020, 01:38:47 pmIn both cases the integral of white noise will be zero.Any decent voltmeter will integrate over an integral number of mains cycles.If you have some other form of noise present then do let us know.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 03:42:12 pm
content: Quote from: egonotto on June 01, 2020, 03:28:37 pmThe mains hum is in the 50Hz range, and the signal in the 100MHz range. Are you sure it is from the mains?

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 03:48:32 pm
content: Quote from: tggzzz on June 01, 2020, 03:33:09 pmEven the integral of white noise is 0 only when integrated over infinite time. And the real noise is never really white. So I do need to gate the measurement (preferable to within 100ns).Also, I can't afford to integrate over multiple mains cycles since the pulses come at kHz rate.

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2020, 03:56:55 pm
content: Quote from: mmm22 on June 01, 2020, 03:48:32 pmSigh. I suppose you think you can only know a frequency if you measure for infinite time.Either do the maths or do a simulation.QuoteAnd yet you wrote "...my signal is not repeatable" in a slightly different context.Quite frankly I'm becoming tired of playing "20 questions" to elicit your signal and your problem.

author: mmm22
author_rank: Regular Contributor
timestamp: June 01, 2020, 04:07:27 pm
content: Quote from: tggzzz on June 01, 2020, 03:56:55 pmThat's right, the difference is in the context. For ETS use, the signal is not repeatable since the amplitude fluctuates.

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2020, 06:14:29 pm
content: Quote from: mmm22 on June 01, 2020, 03:13:36 pmIf the frequency content of the signal is the same every time and you are after a relative measurement then flatness is not important but it does put an extra constraint on the shape (frequency content) of the signal. Personally I'm not a fan of measuring above the bandwidth of an oscilloscope because the filter may roll-off steeply so a change in frequency content can have large effect on the result.

author: mmm22
author_rank: Regular Contributor
timestamp: June 04, 2020, 07:04:49 am
content: Quote from: nctnico on June 01, 2020, 11:32:56 amSo there is no way to see the real ADC values, without interpolation/reconstruction?Also, most of the current scope manufacturers significantly exceed the "2.5x" rule you suggest. Is there any reason for that?

author: 2N3055
author_rank: Super Contributor
timestamp: June 04, 2020, 07:36:29 am
content: Quote from: mmm22 on June 04, 2020, 07:04:49 am"Real ADC values" don't exist in a sense you are suggesting. What you have are discrete samples of voltage at time time intervals. You need to reconstruct signal from it. That is the whole point of math behind it. And it is correct, but as any math, you need to understand it well to make sense out of it and make it intuitive.Some scopes will let you put scope in "dot mode" that will show only sampled points on screen. How useful that is depends...And no, "most of the current scope manufacturers" certainly don't significantly exceed the "2.5" rule. Most of the 1 GHz scopes in mid range are sampling at 2.5 ratio. Even when you sample at 10 GS/s, you still get only 10 points per period of 1 GHz signal. Without reconstruction not much to see.. Keysight used to recommend 5x ratio for scopes with classic Gaussian roll-off on input filters. Scopes with sharper (brick-wall filters) sample at 2.5x without problem.Fact is, that, same 1 GHz scope is being sold as a 100 MHz artificially locked down version for marketing purposes. But they do it by using same board and software, just changing few components or sometimes completely in software. So you end up with a 100 MHz scope that samples at 5 GS/s and also a 1 GHz scope that samples at same 5GS/s..Old RIS scopes sampled repetitively at random intervals, assembling image from thousands of pixels on screen from thousands of triggers.Same as with ETS. They did that because technology didn't allow them to reconstruct signal mathematically.

author: mmm22
author_rank: Regular Contributor
timestamp: June 04, 2020, 07:50:41 am
content: Quote from: 2N3055 on June 04, 2020, 07:36:29 amWhy the discrete samples of voltage at time intervals aren't the real signal? Does the 'dot mode' still show the reconstructed values?What is the marketing purpose of selling a better product (and with higher cost) for lower price?

author: tggzzz
author_rank: Super Contributor
timestamp: June 04, 2020, 07:51:43 am
content: Quote from: 2N3055 on June 04, 2020, 07:36:29 amThe sampling rate wasn't fast enough. Post processing wasn't the issue.

author: tggzzz
author_rank: Super Contributor
timestamp: June 04, 2020, 07:57:35 am
content: Quote from: mmm22 on June 04, 2020, 07:50:41 amBecause they are samples of the continuous signal! You need to learn some introductory sampling and maths theory.Quote"Reconstructed values" is an improper concept in this context. It indicates that your mental model of what is happening needs to be improved."Dot mode" shows the discrete samples.

author: 2N3055
author_rank: Super Contributor
timestamp: June 04, 2020, 07:59:44 am
content: Quote from: tggzzz on June 04, 2020, 07:51:43 amYou misunderstood. I'm talking about old analog one RIS that had storage tube. They had to sample thousands of points to assemble whole screen worth of waveform. Every single pixel was acquired by sampling. When scopes started using RIS/ETS together with slow A/D converters, they could sample smaller number of points and would/could also interpolate between the samples...

author: 2N3055
author_rank: Super Contributor
timestamp: June 04, 2020, 08:11:20 am
content: Quote from: mmm22 on June 04, 2020, 07:50:41 amLookup what "discrete sample" means. It's a mathematical concept.Dot mode shows samples from A/D converter (or at least it should) without interpolated data in between.Why they have that marketing strategy? They probably believe it makes them money....

author: tggzzz
author_rank: Super Contributor
timestamp: June 04, 2020, 08:57:41 am
content: Quote from: 2N3055 on June 04, 2020, 07:59:44 amAccepted.But there's no need to invoke storage scopesThe twinkling display on an analogue sampling scope is hypnotically beautiful to watch. I tried taking a video of mine, but the strobing between the camera frame rate and the display was too distracting, so you'll have to make do with a static picture (pulse caused by a 72mm length of FR4 shorted at the far end):That photo is of several traces, hence the appearance of apparent "holes in the width" of the trace.

author: tautech
author_rank: Super Contributor
timestamp: June 04, 2020, 09:12:03 am
content: Quote from: mmm22 on June 04, 2020, 07:50:41 amThey are, but related to the sampling rate, memory depth setting and/or timebase setting.QuoteNo, dots are the sampled points on which the signal is reconstructed with interpolation.With a free running dot mode trace and a repetitive waveform and a sufficient WFPS rate, the displayed waveform can appear unreconstructed until Stop is pressed when the last sweep is shown as just a collection of dots.

author: mmm22
author_rank: Regular Contributor
timestamp: June 04, 2020, 05:53:32 pm
content: Quote from: 2N3055 on June 04, 2020, 08:11:20 amWhy don't they also reduce the sampling rate proportionally?

author: mmm22
author_rank: Regular Contributor
timestamp: June 04, 2020, 06:02:59 pm
content: Quote from: tautech on June 04, 2020, 09:12:03 amShouldn't the dot mode always appear unreconstructed, independently on the repetitiveness of the waveform, wfps rate, and Stop being pressed?

author: tautech
author_rank: Super Contributor
timestamp: June 04, 2020, 07:00:59 pm
content: Quote from: mmm22 on June 04, 2020, 06:02:59 pmNo.Consider any timebase setting as a small part of a second and that even entry level scopes that sample at 1 GSa/s so that at most timbase settings we have a string of dots so close together it looks like a continous waveform and also that the screen is updated much less frequently than the WFPS data buffer so only when we press Stop do we see a single sweep of dots or if we are to use a Single trigger setting if in dot mode.DSO's with good WFPS and sampling rates can almost exclusively be used in dot mode except in their fastest settings where Stop and Single captures might display too few dots to be truly useful.

author: mmm22
author_rank: Regular Contributor
timestamp: June 04, 2020, 07:17:05 pm
content: Quote from: tautech on June 04, 2020, 07:00:59 pmIs it because of equivalent time sampling?

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2020, 03:07:04 am
content: Quote from: mmm22 on June 04, 2020, 07:17:05 pmThe modern DSO's I use don't support ETS so it's RTS and update rates of sampled data points into the display buffer then onto the display where dots look like a continious line.Some screenshots as further explanation:Probe comp signal 1 KHz 3VFirst 2 are free running traces in dot mode only with different timebase settings.When Stop is used at a fast timebase we can clearly see the sample points from with the waveform is reconstructed in Vector display mode.

author: mmm22
author_rank: Regular Contributor
timestamp: June 06, 2020, 11:55:44 am
content: Quote from: tautech on June 06, 2020, 03:07:04 amOn the 2nd picture the time intervals formed with delayed dots are clearly seen. Isn't it the same as ETS mode?

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2020, 12:17:00 pm
content: Quote from: mmm22 on June 06, 2020, 11:55:44 amThe overlaid dots are the total of the probe compensation output jitter and timebase jitter and successive waveform updates overlaid on/near one another, nothing else.Just a few ps/s jitter is enough to lay dots in a line like that.OK so we might have say 1 GSa/s but knows to say at what timebase a successive acquisition will fall exactly on the last.

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 07:27:23 am
content: Quote from: tautech on June 06, 2020, 12:17:00 pmThen I don't understand how the sub-ns time resolution is achieved if the points within the ADC sampling period are due to jitter, i.e. the relative delays are not defined as is the case with ETS. The ADC with 1GS/s cannot resolve them.

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2020, 07:51:38 am
content: Quote from: mmm22 on June 07, 2020, 07:27:23 amIt is what it is from successive data point overlays into the memory buffer and the true sample set of data points @ 1 GSa/s is only revealed with a Stop.Count them if you must but the top right box says there's 280 at a 10ns timebase setting.

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 08:16:00 am
content: Quote from: tautech on June 07, 2020, 07:51:38 amThat is the question, what is the reason to show data points that are not true? It is not an analog phosphor scope where you don't have much control of what data shows.

author: gf
author_rank: Super Contributor
timestamp: June 07, 2020, 09:01:16 am
content: Quote from: mmm22 on June 01, 2020, 11:24:23 amAs long as the sampling theorem is fulfilled, sinc interpolation is supposed to reconstruct the original signalexactly, so it is perfectly valid. Thus it may be even useful to upsample (with sinc interpolation) before doing further calculations (like integral) when the sampling is so sparse.If the sampling theorem is not fulfilled, then you are likely lost anyway - except in special cases, it won't be possible to uniquely reconstruct the original signal then, from the samples.In your initial message you said that you are interested in the integral. The integral is basically proportional to the (average) DC component present in your signal. Therefore, if you send the signal through a low-pass filter, the waveform will change, but the integral won't change if you integrate over the full (now prolonged) pulse width. So if you are only after the intergral, then you can well limit the bandwidth with a low-pass filter in order to ensure that the sampling theorem is still fulfilled at 500MSPS, w/o affecting the integral.

author: nctnico
author_rank: Super Contributor
timestamp: June 07, 2020, 09:24:56 am
content: Besides that the math function will use the sampled points anyway and not the reconstructed signal. The reconstruction is only there to visualise a signal.

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 09:33:30 am
content: Quote from: gf on June 07, 2020, 09:01:16 amThat's true, but there are 2 reasons not to prolong the pulse: the amplitude attenuation (hence increased relative error) and gate width increase (which allows for more noise).So for my specific application the real question is whether 1GS/s is anyway excessive for 70MHz bandwidth, and 500MS/s is enough.

author: gf
author_rank: Super Contributor
timestamp: June 07, 2020, 09:58:07 am
content: Quote from: mmm22 on June 07, 2020, 09:33:30 amIf you are limited to 70MHz anyway, the question is, how steep is the roll-off beyond that frequency in order that >= 250 MHz are sufficiently suppressed. If it is not steep enough you could add an additional low-pass filter with a higher cut-off (say 150MHz). The prolonged pulse width (and reduced height) is dominated anyway by your 70MHz bw. It's also a matter of desired accuracy, since correct calculation of the integral from the samples eventuelly depends on how well the sampling theorem was honored.

author: tggzzz
author_rank: Super Contributor
timestamp: June 07, 2020, 10:41:58 am
content: Quote from: mmm22 on June 07, 2020, 09:33:30 amThere are some significant misconceptions in that statement.The energy (which is what ultimately determines the signal-to-noise ratio) contained in the full bandwidth pulse and averaged pulse is the same.The full bandwidth signal requires a wider front-end bandwidth and hence has more noise with the signal. That's why scopes have a 20MHz bandwidth setting: to make the trace look "less fuzzy".In this case, you need a longer gate width if you are integrating/averaging the waveform. Having a longer gate width enable you toreducethe noise, since you can average for longer. That is reflected in noise specifications having units of V/sqrt(Hz). It doesn't affect the signal-to-noise ratio.Note that in the limit the average value of noise is, by definition, zero.

author: 2N3055
author_rank: Super Contributor
timestamp: June 07, 2020, 11:28:12 am
content: If you only want to measure exact integral of the pulse you need triggered integrator with reset (S&H circuit) and measure DC with 6.5 digit meter...Reset, rinse and repeat..

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 11:33:36 am
content: Quote from: 2N3055 on June 07, 2020, 11:28:12 amAre there any commercial models within $500? I could find only scientific grade ones in multi k$ range.

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 11:40:57 am
content: Quote from: tggzzz on June 07, 2020, 10:41:58 amThat would be the case if both the signal noise and the internal scope noise were all white noise, which is not practically the case.Even if the noise were all white, the measurement time is limited.

author: tggzzz
author_rank: Super Contributor
timestamp: June 07, 2020, 12:03:34 pm
content: Quote from: 2N3055 on June 07, 2020, 11:28:12 amYup, as I outlined in reply #17. I don't know why that has been ruled out.It feels as if the OP has determined that he knows the only way forward.

author: tggzzz
author_rank: Super Contributor
timestamp: June 07, 2020, 12:16:18 pm
content: Quote from: mmm22 on June 07, 2020, 11:40:57 amIf the noise isn't white, what color is it?It sounds as if it is interference rather than noise. If you don't tell us that then you are wasting everybodys' time.As multiple people have pointed out, you don't need a scope to measure the integral.QuoteOf course it is! That merely limits how closely the integral approaches zero.If measurement time is unlimited, then the integral is zero. OTOH, with limited measurement time the noise value will be quoted in V/sqrt(Hz) between specified frequency limits, or more carefully defined with the Allen Deviation.Random example from an ADA4806-1 opamp datasheet: Input Voltage Noise f = 100 kHz: 5.2nV/√Hz, Input Voltage Noise 1/f Corner Frequency: 8Hz, 0.1 Hz to 10 Hz Voltage Noise: 44nV rmsAllen Deviation:https://www.eevblog.com/forum/metrology/allan-deviation-for-stability-measurements-on-voltage-references/

author: nctnico
author_rank: Super Contributor
timestamp: June 07, 2020, 12:22:49 pm
content: Quote from: tggzzz on June 07, 2020, 12:03:34 pmTo me this method seems more like a project in itself rather than a ready-to-go solution. There are several sources of error and getting an accurate measurement is going to take more than a trivial circuit. Especially since the pulse to be measured has a high frequency content.

author: tggzzz
author_rank: Super Contributor
timestamp: June 07, 2020, 12:41:06 pm
content: Quote from: nctnico on June 07, 2020, 12:22:49 pmAgreed. I principally (but not exclusively) included it as a pedagogical example to try to encourage the OP to think laterally. The OP is prepared to create a circuit for his signal.The OP has not been particularly forthcoming about the characteristics of the his signal nor the noise/interference, so it is difficult to come up with recommendations. Having said that, if a simple RC or RLC filter is appropriate then the high frequencies could probably be accomodated.

author: 2N3055
author_rank: Super Contributor
timestamp: June 07, 2020, 12:43:23 pm
content: Quote from: nctnico on June 07, 2020, 12:22:49 pmIn previous post on same topic I pointed out to OP what scope to buy for this purpose.https://www.eevblog.com/forum/testgear/oscilloscopes-averaging-speed/msg3027172/#msg3027172That one would have worked out of the box, with built in measurements. Even Siglent SDS2202X-E would still be 2GS/s, and more than 200 MHz bandwith... Could be hacked to 350MHz too (in practice more than 500MHz)..Also, with fast segmented mode it has, you could get 10s of thousands of captures without blind time and get that to PC and analyse it.It was always going to be a project, just have to choose if it was going to be more on the software or hardware side...

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 12:58:17 pm
content: Quote from: tggzzz on June 07, 2020, 12:03:34 pmHave you noticed my reply?https://www.eevblog.com/forum/testgear/mixing-signals-for-single-channel-oscilloscope-triggering/msg3090267/#msg3090267

author: mmm22
author_rank: Regular Contributor
timestamp: June 07, 2020, 01:15:57 pm
content: Quote from: 2N3055 on June 07, 2020, 12:43:23 pmI am still considering the scope you suggested, just by now I don't see enough advantages (for my application) compared to the Picoscope to justify its higher price.If it was the choice between software and hardware development, I would go for the former.

author: 2N3055
author_rank: Super Contributor
timestamp: June 07, 2020, 01:17:50 pm
content: Quote from: mmm22 on June 07, 2020, 12:58:17 pmSorry no, I didn't. To answer I don't know of anything commercial in that price range, unless you find something broken and repair it.

author: 2N3055
author_rank: Super Contributor
timestamp: June 07, 2020, 01:20:38 pm
content: Quote from: mmm22 on June 07, 2020, 01:15:57 pm350 MHz+ (measured to 500+ Mhz) is THE advantage that will enable you to get pulse with minimum error.

author: gf
author_rank: Super Contributor
timestamp: June 07, 2020, 03:58:27 pm
content: Quote from: tggzzz on June 07, 2020, 10:41:58 amI agree with mmm22 here. Including samples outside the pulse extent (where the useful signal component is zero) in the sum (or integral) is counter-productive. The noise of these samples still contributes to the standard deviation of the sum, but these samples don't contribute anything to the sum of the useful signal. Note that the standard deviation of the sum of N uncorrelated random variables with standard deviation s is s * sqrt(N). If the noise level is say 100uV RMS, then the sum of 100 samples has a standard deviation of 1mV (-> 100uV * sqrt(100)). Increasing the number of samples to 10000 (where 9900 of these samples have just a zero useful signal component) would lead to a 10 times higher standard deviation of 10mV (-> 100uV * sqrt(10000)), while the the summed/integrated useful signal is still the same and does not increase as well.

author: mmm22
author_rank: Regular Contributor
timestamp: June 08, 2020, 07:02:09 am
content: Quote from: gf on June 07, 2020, 03:58:27 pmRight. Just I think it is the relative error (not the standard deviation) that is directly proportional to sqrt(N).

author: mmm22
author_rank: Regular Contributor
timestamp: June 08, 2020, 07:24:03 am
content: Quote from: 2N3055 on June 07, 2020, 01:20:38 pmBut do I need the bandwidth if I am only interested in the integral of the pulse?

author: gf
author_rank: Super Contributor
timestamp: June 08, 2020, 03:35:44 pm
content: Quote from: mmm22 on June 08, 2020, 07:24:03 amDepends on the signal.For instanceWhat is actually your minimum/maximum puse width, and the rise/fall time of your pulses?What is the minimum/maximum amplitude of the pulses?What is the required accuracy for the integral? How many Volt seconds of error do you grant?How much noise is contained in the signal itself?Does the intrisic noise of the signal dominate, or rather the noise of the scope?As far as I understand the problem, the criterion to be optimized is the error of the integral. If you can achieve your error goal by integrating the low-pass filtererd signal, then you likely don't need the bandwidht. A higher sampling rate is certainly benefitial for your use case, but if the price for the higher sampling rate is a higher noise level, then you may still not benefit at the end of the day.

author: gf
author_rank: Super Contributor
timestamp: June 08, 2020, 06:51:57 pm
content: @mmm22, just another question: If I understand correctly, you already do have a 70MHz scope. What are your results so far? Were you able to reach your accuracy goals? If not, what exactly was the bottleneck, which needs to be addressed?

author: mmm22
author_rank: Regular Contributor
timestamp: June 09, 2020, 08:05:45 am
content: Quote from: gf on June 08, 2020, 06:51:57 pmNow I am using an old Tektronix (300MHz, 2.5GS/s), and it works fine. But I want to replace it with a cheaper and more modern alternative. Currently thinking about a Picoscope with 70MHz, 1GS/s.So the question is how it would influence the precision of measuring the integral of the pulse.The signal is a gaussian pulse with FWHM=10ns, the amplitude varies from 0 to 200mV, repetition rate 10kHz.

author: tautech
author_rank: Super Contributor
timestamp: June 09, 2020, 08:44:13 am
content: Quote from: mmm22 on June 09, 2020, 08:05:45 amWhy give up 2.5x the sampling rate and 4x the BW ?QuoteStep response of 70 MHz vs 300 MHz plus the much higher sampling rate will always give better measurement accuracy.As was offered before a SDS2202X-E might be a good fit for your needs and will certainly have better mem depth than your nice old Tek.As some idea of it's Math Integral mode, a screenshot from the little brother SDS1104X-E in the Integral menu attached.I've lent my good Arb to a mate so have no waveform source to give a real example.

author: 2N3055
author_rank: Super Contributor
timestamp: June 09, 2020, 09:41:06 am
content: Quote from: tautech on June 09, 2020, 08:44:13 amFor this, on SDS2000X+, you wouldn't use math integration but one of measurement Area functions.This is pointless, circular, discussion.This can be measured either with analog S/H circuit, or with a digital scope with sufficient bandwidth.Sufficient bandwith in this case is not defined by pulse width, but with pulse rise and fall time. Scope's rise time (actually system rise time, including scope and probes risetimes) has to be faster than measured pulse rise time.How much exactly? In attachment a whitepaper from Keysight...But, guestimate is that 70 MHz scope is not enough. It might be that even 300 MHz is not enough, if OP newer tried to look at the pulse with something faster than his 350 MHz old scope. Maybe he just thinks he knows how the pulse looks...Pulse width at base of pulse won't be much different, but height of pulse can be very different, and therefore integral of that pulse.Or you could do a project that consist for input pulse shaper/filter/integrator that wil convert superfast tall pulse to a slow, flat pulse with same area and than measure that with a slow scope..Explanation what is exactly being measured here might even lead to the revelation that maybe there is a standard solution for this....

author: gf
author_rank: Super Contributor
timestamp: June 09, 2020, 05:48:22 pm
content: Quote from: 2N3055 on June 09, 2020, 09:41:06 amI guess it is not a big project, though.Anylow-pass filter has this property. And a 70MHz frontend is yet another low-pass filter either.[ If the pulse were repeated at a fixed rate, one could theoretically even use a low-pass fillter with avery lowcut-off and measure the (average) DC voltage with a (milli/micro)voltmeter. ]EDIT:In order to get a rough feeling a did a few brief calcuations:A Gaussian pulse with FWHM=10ns1)has a 99.7% area2)base width of about 25ns. When I run this pulse through a 70MHz cut-off 2nd order low-pass filter, then then 99.7% area base width increases by about 50% to ~38ns, and the peak amplitue of the filtered pulse is still about 95% of the original gaussian pulse. So the pulse widening and amplitude drop are not that much.Regarding the sampling rate: At 250MHz, the spectrum of a Gaussian pulse with FWHM=10ns is already ~180dB below DC. So 500MSPS or even less should well suffice, in order to fulfill the Nyquist criterion sufficiently well. And for a 70MHz low-pass-filtered pulse all the more.1) I cannot veryfy the shape, so I can only trust the provided information2) Gaussian: +/- 3*sigma => 99.7% of the area, and FWHM=2.3548*sigmaEDIT:For demonstration, the Gausian pulse and four arbitrarily selected low-pass filter responses (1st and 2nd order, 70 MZh and 20 MHz Butterworth). X-axis in nanoseconds. As expected - all have the same area/integral of 10.455 nVs. Undershoot counts as negative area for the integration, of course.

author: gf
author_rank: Super Contributor
timestamp: June 10, 2020, 05:21:02 pm
content: Quote from: mmm22 on June 09, 2020, 08:05:45 amIf you want to get a feeling how lower bandwidth and/or lower sampling rate affect the integral, then I suggest that you simply turn on the 20MHz bandwidth limit of your Tektronix scope, select different sampling rates (say 2.5GSPS, 1GSPS, 500MSPS), and compare the results to your current result. I guess that it still has a chance to work, streching the pulse to say 100..150ns width at the base, and reducing the peak amplitide to 1/4...1/3, but the repeatability is likely worse (i.e. when you reapeat a measurement experiment for the same pulse N times, and calculate the standard deviation of the N measured integrals). Either you hit your accuracy target, or you don't. Just try and see... If it still happens to be acceptable with 20MHz, then likely 70MZh will suffice all the more.