Square signal level > 20ms not looking good - Siglent SDS800X HD - Page 1
Forum section: electronics / beginners

author: eTobey
author_rank: Super Contributor
timestamp: May 27, 2024, 09:58:54 pm
content: I have compensated my probe, but when looking at longer signals, they dont look that good. The probe is directly plugged into the BNC of the signal gen (SDG1032) with a proper adapter for it.Is this a bad probe or what do i do wrong?Edit:Found some good information:https://blog.teledynelecroy.com/2018/10/putting-probes-in-perspective.htmlThe following articles/post can be found on the dates list on the right.

author: tggzzz
author_rank: Super Contributor
timestamp: May 27, 2024, 10:24:29 pm
content: Quote from: eTobey on May 27, 2024, 09:58:54 pmDefine the signal: voltage, frequency, source impedance.Define the probe: type.Define the "proper adaptor".Yes, I'll probably regret responding to this.But maybe, just maybe, the OP will learn how to ask a questionthat can be answered.https://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/More likely: I am too optimistic.

author: Manul
author_rank: Super Contributor
timestamp: May 27, 2024, 11:27:43 pm
content: At such low frequencies, probe compensation does not really matter (it's for high frequencies). I would say that what you see is an actual signal, in other words, it really happens. Does not look like probe's or scope's frequency response issue. We don't know enough about your setup.

author: thm_w
author_rank: Super Contributor
timestamp: May 27, 2024, 11:53:16 pm
content: Only perform the adjustment and calibration using the front cal signal as explained in the manual.Quote from: Manul on May 27, 2024, 11:27:43 pmThere are LF and HF probe compensation adjustments:https://www.picotech.com/library/application-note/how-to-tune-x10-oscilloscope-probesMost likely the probes OP is using only has LF adjustment.But I agree, assuming the probe is properly compensated its probably the actual signal.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 27, 2024, 11:59:33 pm
content: If you can borrow someone else's probe, you could verify whether the signal source is faulty.Another possibility is that the scope channel itself is faulty. Have you tried the test on a different channel?? I know this is a remote possibility, but it will take you less than a minute to perform this test.

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2024, 12:18:49 am
content: If the input signal starts out significantly off of the display, then you are likely overloading the oscilloscope. When the operating point of the signal amplifiers is changed, then thermal effects on the order of milliseconds will affect the signal.

author: MrAl
author_rank: Super Contributor
timestamp: May 28, 2024, 02:33:49 am
content: Quote from: eTobey on May 27, 2024, 09:58:54 pmHello there,Unfortunately, the only way to be absolutely sure if it is the scope, probe, or signal generator, the only way to know for sure is to get a different scope or different frequency generator and test using that. If a different (manufacturer) generator or different scope shows the same thing then it's the scope or probe. You can always try a new probe they are not that expensive.You may not have either of these extra pieces of test equipment though so the only thing you can do is think about the quality of the generator, or perhaps just get some low-cost electrical parts like a 555 timer and generate a pulse with that and see what comes of it.If you are using a homemade circuit already to generate the pulse, that could be the culprit, but without more information it's hard to say.

author: BillyO
author_rank: Super Contributor
timestamp: May 28, 2024, 03:03:35 am
content: Generally, it looks fine to me. +/- 50mV on the leading edge of a typical probe compensation under typical conditions does not set off any alarms. However, I don't have all the facts.Could you let those of us that do not follow every minute of your amazing life (this is not a realty TV show!) the following:1) The scope.2) Has it been hacked?3) The signal source (generator, frequency, risetime, etc..)4) The probe5) Any other information that might possibly be pertinent.

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 07:15:01 am
content: I have now compensated the probes again (see picture "compensated on scope 1" and "compensated on scope 2"). The CH1 is the original probe the scope came with, and the CH4 probe is a Testec "TT - LF 312".The signal does not look any better (picture "still not that good").I also made sure, that the connection of the other probe (CH4) is not affecting this issue too much. Picture "ugly" shows how it looks with that ugly connection, and picture "ugly fixed" shows it with a proper BNC adapter.The setup can also be seen at the picture "setup".The scope is a Siglent SDS800X HD which is not hacked, but "opened" via licence key. The generator is a Siglent SDG 1032X. I made sure, that the inputs are not overdriven (its not visible on the pictures).

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 08:03:31 am
content: Now try to switch voltage ranges on the scope, to the point where the relay will click (and adjust the signal level if necessary), and you'll very likely see yet another change in the vicinity of the rising/falling edge.I've never been able to compensate a probe to get an ideal waveform display in every condition. Yes it works fine if the waveform is zoomed out sufficiently: when 5-10 periods fit on a single screen, then it's a perfect square wave. Things change when you start to zoom in.That's why I stop at "good enough" for the probes. It works for practical purposes, considering all the other limitations of passive probes, but it still does not explain the fine effects of it. I am pretty sure there are good articles on what probe compensation actually does, and they can be used to get a better understanding.To answer the "is it the probe or is it the signal?" question* you need to compare what you see with the probe with what you see without it: connect the signal generator, assuming it has 50Ω output impedance, using a coax cable with a 50Ω feed-through terminator at the scope input, that will allow you to see the signal with as little distortion as possible. Of course, internal 50Ω termination would be much better, but your scope doesn't have it. External terminator should be fine except very close (few ns) to the edges.*but not the "is it the scope?" question: that is yet another variable.p.s. You do understand that a compensated probe is only compensated relative not only to a given scope, but also to a given input of that scope (and as I mentioned above, also, it seems, to a given vertical scale range), don't you? The capacitors and wiring on each input can differ sligthly, so they will require a different amount of probe compensation.

author: CosteC
author_rank: Regular Contributor
timestamp: May 28, 2024, 09:05:00 am
content: I would try to observe signal from generator directly, without probe but with 50 Ohm termination at the oscilloscope and with 50 Ohm cable.This will tell if signal from generator is as rectangular as we expect it. It may not.Then I would check if generator risetime is better than oscilloscope calibration output. Generator again, I would use with 50 Ohm termination rather than Hi-Z mode for probe termination.Finally, what accuracy is guaranteed by Siglent for this oscilloscope? Maybe it is fine "as is"?

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 09:25:45 am
content: Quote from: shapirus on May 28, 2024, 08:03:31 amToo bad google has become useless. Thousands of beginner tutorials, that do not go deeper. And then there is a lot of AI shit generated out there.Do you remember those good old times, where there only were links on sites, and no google? Good thing was, it mostly was selected (good) ones.Quote from: shapirus on May 28, 2024, 08:03:31 amSee pictures.Quote from: shapirus on May 28, 2024, 08:03:31 amYes i do, but for every scale? Have to test this...

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 11:20:30 am
content: Some more tests:BNC cable direct into scope with 50 term (yellow), and probe with coax tip adapter into into a y-adapter on the generator.I also took a 12V battery, to "create" some signal with less variables (i guess). Not having a stable reading on it for 20ms does not feel so great to me?! How would, if there is, that specification of such a thing be called?

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 12:07:23 pm
content: Try dot mode display.

author: CosteC
author_rank: Regular Contributor
timestamp: May 28, 2024, 12:53:59 pm
content: Quote from: eTobey on May 28, 2024, 09:25:45 amWell..This is very good question.Answer is: Depends on construction of front end. If input changes impedance depending of attenuation, which is entirely possible, then YES - probe compensation will be correct on ONE gain setting or for example two. Depends.WHY?For example at 1, 2, 5 mV/DIV input may have 10 pF capacitance. Then relay clicks to other divider ratio for 10/20/50 mV/DIV and at this setting input has 12 pF... changes between 1/2/5 can be done in software. This is only example of course.

author: bdunham7
author_rank: Super Contributor
timestamp: May 28, 2024, 01:14:43 pm
content: This issue has been noticed and discussed regarding the older SDS1000X-E scopes and this looks like the same thing. Sharp transitions at low repetition rates (like a 100Hz square wave) will result in these distortions on what should be the flat section of the waveform. It is not strictly an overload recovery problem as it can be demonstrated with signals that are entirely on-screen. IDK about the input capacitance changing with vertical scale changes, but that would be another source of distortion if it is happening.

author: pdenisowski
author_rank: Super Contributor
timestamp: May 28, 2024, 01:17:13 pm
content: Quote from: eTobey on May 28, 2024, 09:25:45 amAs someone who has made a probe compensation video: how much deeper / what additional topics would be helpful?It's a serious question: I would be very happy to do a deeper dive, I'm just not sure what I would add.Edit: I should mention that towards the end of the video there is a slide on troubleshooting compensation issues

author: pdenisowski
author_rank: Super Contributor
timestamp: May 28, 2024, 01:18:34 pm
content: Quote from: eTobey on May 28, 2024, 07:15:01 amSorry if I missed it somewhere about, but is there a reason why you're not using the probe compensation outputs on the front of the scope?

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 02:13:31 pm
content: Quote from: pdenisowski on May 28, 2024, 01:18:34 pmThis brings up another question: is there any difference between using the scope's built-in probe compensation output vs an external source? Sometimes what's produced by the scope's one may be not the squarest wave possible.Another interesting question: does the probe compensation adjustment waveform have to necessarily be a 1 kHz signal?

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 02:32:10 pm
content: Quote from: pdenisowski on May 28, 2024, 01:17:13 pmGood vid. One small (if it's small) issue is that it doesn't cover the difference (in terms of schematics) of having the variable cap before or after the coax.A good follow-up video would include a howto guide of building an equivalent circuit and running a SPICE simulation of a 10x (and 1x as well!) scope probe. A simulation is helpful in understanding how the probe loads the DUT and how it distorts the original signal before it is displayed on the scope screen, and also what difference can be expected between a probe and a 50Ω-terminated (internal or external) coax.We already know where the resistors are, where the capacitors are, now we can build a netlist having them, and add an LTRA element between the probe tip and the scope input (with the variable cap being on one or the other side of the LTRA). The difficult part is that we need to know the actual R, L, and C values that typical probes have to run a meaningful simulation.

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2024, 04:37:20 pm
content: Quote from: shapirus on May 28, 2024, 02:13:31 pmAn external source will work fine for probe compensation. I sometimes use an external source because the BNC output can be connected coaxially to the probe tip reducing noise. Some older oscilloscopes had a BNC output for their compensation output which is especially useful.QuoteNo, it can be a different frequency. 1 kHz is commonly used because the probe calibration output is also a frequency and voltage reference.Quote from: CosteC on May 28, 2024, 12:53:59 pmI do not know how newer oscilloscope get consistent input impedance without trimming. Older oscilloscopes have a pair of capacitive trimmer for each high impedance attenuator. One adjusts the compensation, and the other adjusts the input capacitance, so all attenuators on one channel can have the same input impedance, and channels on one oscilloscope can have the same input impedance, and all channels on multiple oscilloscopes can have the same input impedance. This allows probes to be moved between channels and instruments without adjusting compensation.Quote from: eTobey on May 28, 2024, 11:20:30 amI am not sure what you are asking here, but a "reference level pulse generator" like a Tektronix PG506 is used for testing settling time and transient response. The design is a little different from a common pulse generator and produces a minimum of aberration even at high speeds. The probe compensation output is good enough for adjusting probe compensation, but may or may not be better than this.

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 06:57:01 pm
content: Quote from: pdenisowski on May 28, 2024, 01:17:13 pmThats a very good video. I have one question on this: Why is a variable capacitor on the prope tip unpractical?It would be interesting to know (if the probe is the cause), why a signal takes 20ms to become stable. To me it seems it comes from the probe, because touching a battery (with additional cap) looks the same. Also on my testec, i can not get a perfect square, since there always will be a short overshoot at the beginning, or a tiny and longer flat drop after it.Quote from: pdenisowski on May 28, 2024, 01:18:34 pmWhy would you think, that i did not use the comp outputs of the scope?

author: G0HZU
author_rank: Super Contributor
timestamp: May 28, 2024, 08:00:26 pm
content: If it helps, see below for a screenshot from my old Infiniium 54825A scope (500MHz) with an Agilent 10073C x10 scope probe (compensated) fitted to channel 2.A 2.9V pulse from an HP pulse generator is being measured with the scope and I've set the scope to 100mV/div and channel 2 is set up for a x10 scope probe.You can see the pulse is still quite flat even when zoomed in at 100mV/div.The pulse generator is an HP 8002A (made in 1972).

author: pdenisowski
author_rank: Super Contributor
timestamp: May 28, 2024, 08:11:30 pm
content: Quote from: eTobey on May 28, 2024, 06:57:01 pmThanks! It's due to size and also because it's a good idea to keep a reactance away from the measurement point.Quote from: eTobey on May 28, 2024, 06:57:01 pmSorry, maybe I misunderstood you when you said:Quote from: eTobey on May 27, 2024, 09:58:54 pmI don't have an SDS800X HD, but from the pictures I've seen it looks like it has the "typical" probe compensation signal and ground connectors on the front (see attached). These are the points I always use for probe compensation. Again, my apologies if I mis-read your post or if I'm just asking a stupid question

author: G0HZU
author_rank: Super Contributor
timestamp: May 28, 2024, 08:15:59 pm
content: Here's a similar test using a Tek MSO4104 scope and another Agilent 10073C x10 probe. In this case, the scope didn't make it easy for me to show the pulse top on 100mV/div.So I set the pulse to about 4.5V and set the scope to 500mV/div.It still looks quite flat. I rarely use this scope as the UI is horrible so I'm probably not getting the best from it.I have compensated it for the Agilent 10073C x10 probe.

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 08:21:14 pm
content: Quote from: eTobey on May 28, 2024, 06:57:01 pmYou also got pretty much the same result using a coaxial cable with a 50Ω terminator, didn't you? So the probe can be ruled out.And since you got the same result with a 12V battery... Do you have any other scope to compare with the one you've been using? :)Well, strictly speaking, you haven't fully ruled out the probeandthe source of the signal (the one that's not the battery). To do that, you need to use the battery with something that is not a 10x probe.Since the battery is not a proper 50Ω source and simply adding a 50Ω resistor in series with it is not really an option because of the power dissipation ((12V)²/100Ω = 1.44W), you can use a higher value resistor to create what's known as aresistive probeto test that. It's basically a piece of 50Ω coaxial cable terminated with a 50Ω load (a feed-through terminator in your case) that has a resistor in series with the center conductor on the DUT side. A 2400Ω resistor, for example, will create a 49:1 attenuation ratio and such a probe will draw only about 50 mW from the battery, so there will be no overheating issues.Alternatively, use a smaller resistor on the battery side and use a lower voltage battery.If even in this case you will be seeing the slow settling time... well, then it's the scope. At least in the scenario when you are overloading the input to zee the zoomed in portion of the waveform. You will need to repeat the same tests that you did in post #8 to make sure it's not a consequence of the input overload.

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 09:02:59 pm
content: Speaking of range switching, here's a demonstration.Same 10x probe, same scope, same signal source (50Ω output impedance), same input.100 Hz 3V p-p square wave.1. 1 V/div (actually 100 mV/div). This is the waveform used to adjust the probe compensation.2. 500 mV/div (actually 50 mV/div). An input relay clicks when the vertical scale is changed into this position. Same probe, same compensation setting!It's quite clear that the input capacitance is quite different in these two cases (and it sucks!)3. Coaxial cable, feed-through terminator, 300 mV p-p across the terminator. 100 mV/div, same input path as in case 1. A tiny bit of overshoot/undershoot (as in an overcompensated probe) can be barely recognized.4. Same, but 50 mV/div, same input path as in case 2. A little bit of what would be undercompensation in case of a 10x probe can be barely seen.Someone on this forum has a signature about lies, big lies and what you see on the scope screen. This is a perfect illustration. One always has to take this into account and understand that there is always some uncertainty at fast transitions. Better built scopes will likely be better at this.p.s. The above is reproducible very consistently across all the 4 channels, so it doesn't look like a simple deviation of component values within a tolerance range. It must have something to do with the actual implementation of the analog frontend.p.p.s. Easy: as the vertical scale is switched and the relay clicks, the actual measured input capacitance changes from ~15.8 pF to ~16.3 pF. My bet is on a PCB parasitics difference coming from different lengths of the traces. PCB designer's failure!p.p.p.s. Now I've got an itch to add something for an extra 0.5 pF on the lower-capacitance input paths in each channel. I will try to resist it and simply remember to re-adjust the probe compensation for each of the two possible input signal paths whenever it may matter.

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 09:34:24 pm
content: Quote from: G0HZU on May 28, 2024, 08:00:26 pmWhat a nice signal. Would it look the same, if you put the probe on a battery (maybe with a cap)?

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 09:50:05 pm
content: I had some intuition, and changed the duty to 99.9%, and there you go, flat as Australia. Except a little spike on the probe (green), but that looks the same as before. The coax (blue) looks perfect. Its a 40us gap, but the coax looks pretty good until about a gap of 300us.I had this intuition, but cant really tell whats up with this. My guess is, that scope has some issues?Quote from: shapirus on May 28, 2024, 09:02:59 pmMaybe a blob of hot snot on the traces would add 0.5pF?

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 09:58:30 pm
content: Quote from: eTobey on May 28, 2024, 09:50:05 pmThat's an interesting thought. I wonder if there are any solderless ways of slightly increasing capacitance of a trace. Need to add just 0.5 pF, it's not that much.A piece of copper tape connected to ground and put over the trace? Hmm...

author: thm_w
author_rank: Super Contributor
timestamp: May 28, 2024, 10:04:09 pm
content: Quote from: eTobey on May 28, 2024, 09:50:05 pmYou also changed the voltage from 10V to 5V, if that matters.

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 10:05:43 pm
content: Quote from: shapirus on May 28, 2024, 09:58:30 pmYou may not even need to connect it to GND, because of displacement currents?

author: eTobey
author_rank: Super Contributor
timestamp: May 28, 2024, 10:06:58 pm
content: I have now made a testsignal with 400us rising edge. Still looks ugly.

author: G0HZU
author_rank: Super Contributor
timestamp: May 28, 2024, 10:10:39 pm
content: Quote from: eTobey on May 28, 2024, 09:34:24 pmIt looks the same on a battery although it took quite a few attempts to avoid bounce effects when connecting it. A cap would help here.I also tried changing the V/div on the Infiniium scope when set to 1ms/div (to match shapirus' settings) and the x10 probe compensation remained flat on every setting.This 500MHz Infiniium scope only has about 7.5pF input capacitance and the input attenuator is designed on a film PCB. See the image below. This is actually an image of someone else's scope attenuator (damaged?) but I think mine is the same design. You can see the designer used quite complex printed shapes to try and maintain good performance for each attenuator.A lot of modern scopes (even from HPAK and Tek) save on cost by attempting to do this stuff with ordinary SMD and ordinary PCB technology and they do it all on the main PCB. These scopes generally have about 13-18pF input capacitance and I guess the performance will be less consistent across all V/div settings.

author: shapirus
author_rank: Super Contributor
timestamp: May 28, 2024, 10:19:04 pm
content: Quote from: G0HZU on May 28, 2024, 10:10:39 pmDoes your scope have a relay that clicks when the vertical scale is changed from a certain value to another? If the response remains flat in both ranges, then the scope has a well matched input capacitance for both paths, or maybe it compensates for the difference in software (not sure if it's even possible though). Mine unfortunately doesn't.

author: G0HZU
author_rank: Super Contributor
timestamp: May 28, 2024, 10:32:02 pm
content: Quote from: shapirus on May 28, 2024, 10:19:04 pmYes, it has relays. If you look closely, you can see the PCB pads for the relays in the previous image. There are six relay pads for each attenuator section. I think that having relay switched attenuators also helps with the noise performance.I think that some later Infiniium scope models (as used at my place of work) don't use relays. These scopes (eg 600MHz 8064A) are much noisier than my old 54825A and they have about double the input capacitance.

author: eTobey
author_rank: Super Contributor
timestamp: May 29, 2024, 05:08:23 am
content: Quote from: eTobey on May 28, 2024, 10:05:43 pm@shapirus: What do you think about this? I should note, that there has to be GND, that the copper tape overlaps.

author: shapirus
author_rank: Super Contributor
timestamp: May 29, 2024, 07:13:10 am
content: Quote from: eTobey on May 29, 2024, 05:08:23 amI don't know. Might work. This has to be tested, before that there's little point in speculating. I'm not too enthusiastic about taking the scope apart now. Maybe some day later.p.s. but the idea itself may be easily verified on a piece of a random PCB.

author: eTobey
author_rank: Super Contributor
timestamp: May 29, 2024, 07:56:46 am
content: I have made more measurements:The change of the rising edge does not seem the alter the shape of the trace at all, just cutting it off.It looks, like the magnitude of the fluctuation depends on how the signal wasbeforethe rising edge! That would match with the observation, that with only a gap in between, there are no fluctuations.

author: eTobey
author_rank: Super Contributor
timestamp: May 29, 2024, 08:03:25 am
content: Measurments on a shorter gap:

author: eTobey
author_rank: Super Contributor
timestamp: May 29, 2024, 08:17:08 am
content: Measurment on a different level rise (0-5V):

author: shapirus
author_rank: Super Contributor
timestamp: May 29, 2024, 07:08:48 pm
content: Quote from: shapirus on May 29, 2024, 07:13:10 amThe effect is surprisingly strong!I did a quick test with a piece of PCB.1. First I measured the capacitance between the two marked points and got ~12 pF:2. I applied a patch of some copper tape (~11 mm x 15 mm) that overlaps the trace and the ground plane, and now it measured 19 pF (!):So it should not be difficult at all to tune the required track to have an extra 0.5 pF capacitance, as long as it's not routed between two other non-ground tracks. That'll be an interesting experiment, if I ever have enough inspiration to do it.

author: eTobey
author_rank: Super Contributor
timestamp: May 29, 2024, 09:58:15 pm
content: Have you got it still stuck on the PCB? Rub it with some hard round object to make it really flush. How much does it read then?BTW: Its a bit offtopic, and i think its worth a dedicated topic for it.

author: EEVblog
author_rank: Unknown
timestamp: May 30, 2024, 06:52:45 am
content: FYI, some probes have both low frequency and high frequency compensation adjustments.

author: BillyO
author_rank: Super Contributor
timestamp: May 30, 2024, 07:00:31 am
content: Quote from: EEVblog on May 30, 2024, 06:52:45 amNot these $20 70MHz probes. Which, in my testing, do quite well.The OP is not using any kind of controlled procedure here. Everything is in doubt on this one.

author: shapirus
author_rank: Super Contributor
timestamp: May 30, 2024, 07:46:26 am
content: Quote from: eTobey on May 29, 2024, 09:58:15 pm27 pF!It'll need to be a really tiny piece (or far enough from the nearest ground) to add just 0.5 pF :).Quote from: eTobey on May 29, 2024, 09:58:15 pmNot a big deal at this point, I think. It'll be a separate topic if/when I get to tuning my scope's input capacitance using this method.

author: eTobey
author_rank: Super Contributor
timestamp: May 30, 2024, 08:23:02 am
content: Quote from: BillyO on May 30, 2024, 07:00:31 amI would love to know how you tested it.It doesnt look much different when using a probe or a coax with a resistor as 50Ohm terminator. But i guess a terminator would make no difference at all, when the rising edge is 4ms?

author: eTobey
author_rank: Super Contributor
timestamp: May 30, 2024, 08:34:23 am
content: Quote from: EEVblog on May 30, 2024, 06:52:45 amI found herehttps://cdn-reichelt.de/documents/datenblatt/D100/TT_HF_612.pdf, that compensation of the low frequency is done with a 1kHz square. That has 0.5ms of high level, but the maximum of the fluctuation is somewhere around 5ms. That compensation would not really help here, would it?

author: BillyO
author_rank: Super Contributor
timestamp: May 30, 2024, 01:55:33 pm
content: Quote from: eTobey on May 30, 2024, 08:23:02 amI used a 600MHz scope and a frequency sweep. The probes themselves were -3 dB @ 214MHz. More than 3 times the rated bandwidth. So I imagine the problem you are having has nothing to do with the probes. As to the scope, the bandwidth was "improved" using a certain python script to generate license keys and was tested using a Bodnar fast risetime oscillator and found to be approximately 255MHz.I did a short video.The conditions you are looking at with a "microscope" seem rather pointless and out of bounds of any realistic use case. I'm just wondering what it is you are looking to find?

author: eTobey
author_rank: Super Contributor
timestamp: May 30, 2024, 02:14:26 pm
content: Quote from: BillyO on May 30, 2024, 01:55:33 pmAt the moment, i would like to know, why there is a fluctuation of that signal, that is quite long. Also why it is depending on how the signal was milliseconds before. One interesting thing is, that with different risingtimes, the curve does not change at all, apart that it is "cutoff" or somehow "hidden" (https://www.eevblog.com/forum/beginners/probe-compensated-but-level-of-square-signal-gt-20ms-not-looking-good/msg5521063/#msg5521063).I do not care so much about how high the deviation is (except that spike in the 100's mV), but rather how much later it happens, and why that happens. It would be also good to know if there are other things that can make it worse or better.

author: bdunham7
author_rank: Super Contributor
timestamp: May 30, 2024, 03:04:32 pm
content: Quote from: eTobey on May 30, 2024, 02:14:26 pmThis appears to be because the input amplifier has dual paths, one for DC/LF and one for everything else. It is the DC/LF path that is causing the issue, it has nothing to do with probes or input capacitance. I noticed this issue on my SDS1104X-E a while back and posted about it here:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-anomaly-on-what-should-be-a-simple-task/msg3453152/#msg3453152https://www.eevblog.com/forum/testgear/dmmcheck-plus-multimeter-calibration-reference-experiences/msg4955995/#msg4955995Originally I thought is was an overload recovery issue, but the second link showing a 100Hz square wave clearly illustrates that you don't need an overload to see it. They never addressed this issue in the SDS1000X-E series and now it appears that Siglent has gone ahead and released another scope with the same issue. More expensive Siglent models don't seen to have the same issue. I'm not sure why this shows up in the lower-priced designs, but it does appear to be consistent.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2024, 05:13:15 pm
content: Quote from: bdunham7 on May 30, 2024, 03:04:32 pmTektronix used DC coupled dual path input amplifiers going back to at least 1982 once JFET operational amplifiers became available. The advantage is no requirement for a matched pair of JFETs because the operational amplifier can control the DC offset.In the old Tektronix design, there is a medium frequency compensation control to make sure that the gain of the fast path matches the gain of the slow path. Modern designs still have this requirement, but somehow try to get away with no adjustment.

author: thm_w
author_rank: Super Contributor
timestamp: May 30, 2024, 09:59:17 pm
content: Quote from: bdunham7 on May 30, 2024, 03:04:32 pmDave did point out in the teardown that the front end was different between 1000 and 2000. SDS800 and SDS1000 HD appear to have the same front end. Maybe not a direct copy of the 1000x but its similar with the two trim pots.Would be disappointing to see on a $1400 scope if this is the case, not as bad when its a $400 unit.youtu.be/Pf1TSnMRh7E?t=777youtu.be/XLC1CPWznTE?t=660

author: eTobey
author_rank: Super Contributor
timestamp: May 31, 2024, 06:00:32 am
content: Is there any video, that puts some light on this issue? At least it would be good to know, that you have a tolerance of +-25mV on the signal in this case.

author: eTobey
author_rank: Super Contributor
timestamp: May 31, 2024, 09:18:41 am
content: More measurements about this:The fluctuation depends on a few circumstances i have figured out so far:- the vertical scale- the position of the level (signal) in the vertical range (without extending out of screen)- size of the step of the signal- probe att. / coax (less important)Too much things, that one could account for roughly. The total deviation is about +- 40mV, without that initial spike. (picture "t1" -"t3")Not something i would have expected from such a scope. If it has 12 bits, and a zoom function, then it should show the signal more accurately (a bit more at least).Some more pictures that show what happens when overdriving the signal a added too. I named the pictures so that one could make a slideshow with them, and see how it behaves.

author: eTobey
author_rank: Super Contributor
timestamp: May 31, 2024, 12:12:19 pm
content: I was curious, how my cheap, old and tiny DSO would perform. Never thought, that they would look so much better. Notice, that the DSO Nano V2 has a 10mV scale.Its a 6.7V squarewave with the same frequence as above.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 31, 2024, 12:56:27 pm
content: First noticed the long ground lead, unterminated output, then wondered if you were saturating the input and there was a recovery time. Not that these aren't contributing but then followed bdunham7's links, looks like some of the low end Siglent scopes have a problem.https://www.eevblog.com/forum/beginners/probe-compensated-but-level-of-square-signal-gt-20ms-not-looking-good/msg5523046/#msg5523046This one is just wow!https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-anomaly-on-what-should-be-a-simple-task/msg3451402/#msg3451402

author: tautech
author_rank: Super Contributor
timestamp: May 31, 2024, 10:56:28 pm
content: The OP one day after gaining additional experience will discover the meaning of Step Response and how that might or not, impact onrealmeasurements.Until that day this can be summed as, chasing ghosts.Anyone that has repaired and calibrated scopes should know about step response and now adjustment for best isalwaysa tradeoff.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2024, 11:40:44 pm
content: If I had an oscilloscope with that transient response, I would consider it broken. The response below is from the fast rise output of my PG506, which may be considered perfect, into my Tektronix 2232 which is well within its specifications. The digital storage mode trace is practically identical. There is nothing to see past the time shown.

author: thm_w
author_rank: Super Contributor
timestamp: May 31, 2024, 11:51:05 pm
content: Quote from: tautech on May 31, 2024, 10:56:28 pmNo one expects a poor step response at such low frequencies (100Hz).Why not admit its a cost saving measure?

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2024, 12:02:29 am
content: Quote from: thm_w on May 31, 2024, 11:51:05 pmMaybe it is but I have personally seen sub-par step response adjustment from the production line.Also in several old CRO's I've repaired.In real use cases it not need be absolutely perfect and in most cases adjustment is a tradeoff.Then we can introduce different probes, cable assembliesandterminations to deliver all manner of results.Which is correct to the uninitiated ?How can they know they might be doing it wrong, something that is not industry standard ?The rabbit hole is deep for those with limited experience.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2024, 02:57:08 am
content: Quote from: thm_w on May 31, 2024, 11:51:05 pmPoor lower frequency settling time is now a more difficult problem in the sense that it less understood now than in the past. I expect it in modern designs, but not old ones. Those old discrete transistor designs used lots of circuit tricks to achieve it, which are now lost.Operational amplifier settling time is a good example. In the short term, it only depends on slew rate and bandwidth, and the math is straightforward, however over a longer period of time, it depends on thermal or electrostatic effects, which is why if you want fast precision settling time, factors other than slew rate and bandwidth are important. In datasheets you just have to rely on the 0.1% and 0.01% settling time specifications, and notice that they have very little to do with slew rate and bandwidth.I keep laughing at the newer 12-bit DSOs, because they never settle to 12-bits, and do not present 12-bit noise performance. Such performance might be possible, maybe, but requires at least a heroic effort involving hybrid construction and lots of discrete parts.

author: eTobey
author_rank: Super Contributor
timestamp: June 01, 2024, 05:03:12 am
content: I could have given some real use case measures, but the Siglent SDG1032 is not a device i would trust. It sometimes does things that would ruin your day, as it is buggy and sends a few negative volts down the line when going 1mV below 0V with the knob.And from experience, i cant tell, that there will be more bugs. This is why i never connected it to my circuit.Is a 4ms rising edge still considered a step response? It does perform as bad in this case. That is quite dissapointing.

author: bdunham7
author_rank: Super Contributor
timestamp: June 01, 2024, 03:37:29 pm
content: Quote from: David Hess on June 01, 2024, 02:57:08 amIt's a matter of degree. Nobody actually expects a 12-bit DSO to have 12 bits worth of noise (LF or HF) or DC gain performance. It's been discussed elsewhere, but an ENOB of 10 would be spectacular.However, even an 8-bit DSO can do much better than what we're looking at here. This is an SDS2354X+, an 8-bit scope with a "10 bit mode" (using an averaging algorithm) that seems pretty much perfect at first glance:If you use the vertical zoom to look at the top you see a very slight dip in 10 bit mode, but the noise in 8-bit mode masks it entirely. The 10-bit mode does wonders for HF noise and resolution but really can't improve the performance of the front end and 8-bit ADC when it comes to LF noise issues.The previous examples were all "on-screen", but if you increase the vertical sensitivity so that only the top is on the screen--but not overloading the input amp beyond the stated DC offset--you start to see more of the same LF dip. So this model is much better, probably acceptable to most, but still not perfect.

author: eTobey
author_rank: Super Contributor
timestamp: June 01, 2024, 04:09:00 pm
content: Quote from: bdunham7 on June 01, 2024, 03:37:29 pmWhat alternative 8-bit scope would there be for the SDS800X HD? Lets say to 800$? I dont mind it having fewer features.That Siglent SDS800X is like having a Ferrari, that has cheap tires. It looks good, sounds good, and accelerates fast, but you cant go fast on the curves.

author: eTobey
author_rank: Super Contributor
timestamp: June 01, 2024, 08:35:03 pm
content: I have now made the use case tests with the SDG1032, and the scope. For voltage levels and timing (1.65V to 3V; 34kHz; 10%-50% duty). Coax is of course terminated, but with cheap termination)The fluctuations are now "hidden", but i guess they will confuse me one day, and lead me to look for problems in my circuit that are not there.The green trace is a 10x probe, and the red is a coax (50Ohm term).This fluctuation alone would add an error of +- 0.9% in any of my calculations.I wonder if a 350MHz probe would look better here (Green trace at "Use case levels an timings.png")

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 08:53:04 pm
content: Quote from: eTobey on June 01, 2024, 08:35:03 pmWell that sure doesn't look good. So is it the same with other signal sources, have you finally ruled out the signal generator?OTOH it's "only" about a 1.5% deviation in the 10Hz signal fed via coax... but still not good. More in the 34 kHz signal.It'd be interesting to know how it would compare to the Rigol DHO800/900 in the same test. Can't really check because it can't zoom in like that.What's especially weird is how different the transitions look for the 10x probe. Did it have the variable compensation cap in the same position in both tests?

author: tautech
author_rank: Super Contributor
timestamp: June 01, 2024, 09:16:14 pm
content: Clint Eastwood:A man needs to know hislimitationsequipment.2 screenshots from a perfectly compensated 10x probe.And they are perfect when the LED downlights above are set to OFF.

author: eTobey
author_rank: Super Contributor
timestamp: June 02, 2024, 05:07:44 am
content: Quote from: shapirus on June 01, 2024, 08:53:04 pmBut you can use math channels and scale these?What both tests?Quote from: tautech on June 01, 2024, 09:16:14 pmI would like to know that equipment you are using. Cant be an SDS800, because it has a time without LAN connection.I guess its "perfect" at 15 Hz too?

author: tautech
author_rank: Super Contributor
timestamp: June 02, 2024, 08:25:33 am
content: Quote from: eTobey on June 02, 2024, 05:07:44 amThis question explains a whole lot, the man just cannot read or take note of any fine detail.

author: shapirus
author_rank: Super Contributor
timestamp: June 02, 2024, 08:48:11 am
content: Quote from: eTobey on June 02, 2024, 05:07:44 amYes. I believe noise will make it useless, though, but I'll still try it some time later.Quote from: eTobey on June 02, 2024, 05:07:44 amBoth screenshots.

author: eTobey
author_rank: Super Contributor
timestamp: June 02, 2024, 09:38:14 am
content: Quote from: tautech on June 02, 2024, 08:25:33 amThat man is surely not alone in this regard.Quote from: shapirus on June 02, 2024, 08:48:11 amYou can use averaging?What both screenshots? I posted a lot ;-)

author: shapirus
author_rank: Super Contributor
timestamp: June 02, 2024, 10:03:29 am
content: Quote from: eTobey on June 02, 2024, 09:38:14 amI was referring to a specific post that I quoted. It had exactly two screenshots.

author: eTobey
author_rank: Super Contributor
timestamp: June 02, 2024, 03:38:50 pm
content: Quote from: shapirus on June 02, 2024, 10:03:29 amThese were from the same setup without any changes, only the freq/duty was changed.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 03, 2024, 12:31:16 pm
content: Quote from: tautech on June 02, 2024, 08:25:33 amI couldn't find what you used as well in your four posts.

author: tautech
author_rank: Super Contributor
timestamp: June 03, 2024, 09:10:05 pm
content: Quote from: joeqsmith on June 03, 2024, 12:31:16 pmNote the attachment filenames.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 03, 2024, 11:01:51 pm
content: Quote from: tautech on June 03, 2024, 09:10:05 pmSo that's using the probes that came with the scope and internal calibrator?

author: eTobey
author_rank: Super Contributor
timestamp: December 16, 2024, 10:26:15 pm
content: I just came across this issue again, while measuring my BLDC driver:I looks exact like the deviation in that post:https://www.eevblog.com/forum/beginners/probe-compensated-but-level-of-square-signal-gt-20ms-not-looking-good/msg5524375/#msg5524375Its the same shape, and it also levels out after 25ms. (compare with the timebase)picture 73 to 75 are actually the same capture, but the scope decides to mess up the averaging trace after changing the timebase.I was again confused about my circuit, but its actually the scope.Edit:I predicted it:Quote from: eTobey on June 01, 2024, 08:35:03 pm

author: eTobey
author_rank: Super Contributor
timestamp: December 17, 2024, 10:41:27 am
content: Quote from: tautech on June 01, 2024, 09:16:14 pmCompares a 20ms timebase measurement with a 50us timebase measurement and even a 50ms signal with a 500us...Does not answer my question about this wrong comparison...That is ... (cant find any words for this)Now i am looking at scopes from the last century, that do better in this regard.(Yes i know, this topic is old, but that limitation of this scope came up again.)

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2024, 12:50:37 pm
content: Let me repeat a question that, as far as I recall, you never answered: what if you try the dot mode display? I believe your scope can do it. It will reveal if that's an interpolation artifact or something actually present on the ADC input.

author: eTobey
author_rank: Super Contributor
timestamp: December 17, 2024, 04:25:39 pm
content: Quote from: shapirus on December 17, 2024, 12:50:37 pmThere is no such question in this topic...But i probably ignored that "suggestion", because it seemed not plausible at all to me. As you say "display": its just another way of displaying. And there are no repeating waveforms, that there would be any artefacts right?

author: eTobey
author_rank: Super Contributor
timestamp: December 17, 2024, 05:22:32 pm
content: Here is my current use case:I am measuring the current of the lowside mosfet when switching. Using the 10x probe (red trace) is yielding quite some deviations. With 1x probe (green) it looks much better.The traces are the low side mosfet drain to source. There is a current flowing of about 77A before A. The mosfet:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjKrdHLqK-KAxWCgv0HHfnAKD8QFnoECBYQAQ&url=https%3A%2F%2Fwww.vishay.com%2Fdocs%2F76233%2Fsir180dp.pdf&usg=AOvVaw17UiD23QCaDR2MAh1WU18F&opi=89978449A: The highside mosfet turned off.B: The lowside mosfet starts conductingC: The lowside mosfet is fully conductingJust before B is the forward drop of the diode in the mosfet.

author: shapirus
author_rank: Super Contributor
timestamp: December 17, 2024, 06:15:02 pm
content: Quote from: eTobey on December 17, 2024, 04:25:39 pmIt's in post #13. Not a question, techically, but come on, you understand what I mean.Dot display is meant to show raw data points coming from the ADC before any interpolation takes place. This will allow to rule out one possible culprit, namely, the interpolation algorithm. That's how you troubleshoot things, generally: you rule out each factor one by one, thus narrowing down the set of possible reasons causing the issue.

author: eTobey
author_rank: Super Contributor
timestamp: December 18, 2024, 01:31:53 pm
content: I have now made many more tests, and i came to the following results:- The probes seem to have little to no effect on this matter. Except switching 1x to 10X (each input has its own "fingerprint")- The fluctuations seems to be somewhat proportional to the voltage change (see images)- 7.5V: Having the signal vertically fill out the screen has less noise, but is generally a bit to high (see yellow trace of "7V square.png"- 2V/DIV seems to be the best setting for measuring after big voltage drops (if it was a substantial time high).The signal shown, is right after it went from xV to 0V.It was 70ms high(!). The measurements were averaged. The stock probe (1x) has been used with a croc clips at the wavegen (UTG2025A).20M BW limit.measurements.xml is the setup file.

author: eTobey
author_rank: Super Contributor
timestamp: December 18, 2024, 04:13:17 pm
content: Another interesting measurement:Tried 2 different probes with different settings:- g = green probe (stock) / m = probe with metal BNC- 10x and 1x attenuation- probe g changed compensation (comp0 done by standart compensation procedure / comp2 tried to match other probe)The probes are compensated quite good (see pictures - comp2 of green probe - Y1 shows level after settling). With 1x att, the probes have practically identical proberties here (dark red and red overlay each other).Its very good to see, where the compensation actually comes into effect!I wonder, if there is a scope, that could show traces on a logarithmical scale??

author: eTobey
author_rank: Super Contributor
timestamp: December 18, 2024, 05:29:47 pm
content: I have now compensated my probe with the 10V square (70ms high) at the point of 1us, which looks much better than my compensation in the conventional way.Also i have switched to another channel. You can clearly see, that there are fluctuations, that depend on the channel, and that are rather big. Here i also compensated on 1us, but that does not affect the "bad" part on the right.

author: eTobey
author_rank: Super Contributor
timestamp: December 18, 2024, 07:35:07 pm
content: After one day and many dozends of measurements...I have made the final shootout of measurements:The best channel of the scope, with the best probe i have, vs. my cheap tiny DSO quad. I am quite dissapointed of the SDS800. The Quad has settled after 20us, but the SDS800 only after a time that is 1000 times longer... orders of magnitude!These measurements were done via a probe coax adapter and coax cable directly to the signal gen. The waveform of the DSO Quad was done in 2 steps, because it has limited memory.

author: iMo
author_rank: Super Contributor
timestamp: December 18, 2024, 08:04:54 pm
content: So your "settling time" (let us call it that way) is around 20ms? That is too long duration for having something with probe's compensation, sure. Even when considering the resistance of the probe's inner wire (the central wire in the coax) be typically around 200ohm.That is a rather long settling time or transient coming either from the generator's output circuits (ie. its power source response to the sharp rising/falling edge sent into the probe), or in the scope's input circuitry - like a low frequency feedback somewhere in the analog front end (as people described here in past), or a kind of "AGC response" there in the input circuitry (like attack/release time), if any..

author: tautech
author_rank: Super Contributor
timestamp: December 18, 2024, 08:23:30 pm
content: A probe is not the correct tool for precise step response measurements.

author: iMo
author_rank: Super Contributor
timestamp: December 18, 2024, 09:27:49 pm
content: TEK on scope probes (from 1969, 9MB pdf)https://w140.com/tekwiki/images/6/62/062-1146-00.pdfA sim..

author: eTobey
author_rank: Super Contributor
timestamp: December 18, 2024, 09:38:42 pm
content: I believe its coming from the analog front end. Because the part after 200us only changes, if i change the channel on the scope. I wonder if it could be calibrated bit better?Another question:If i get a probe, that can be adjusted in LF too, could i compensate for these fluctuations?https://youtu.be/XLC1CPWznTE?si=uyq5GsawfXIFtcpS&t=591

author: iMo
author_rank: Super Contributor
timestamp: December 18, 2024, 09:57:24 pm
content: You would need the front end schematics, or provide a reverse engineering as Dave did ~10y back with 1054Z afaik, or Siglent will do it for you.. If it is in the front end you must see your long "settling time" with direct coax (terminated by 50ohm) fed into the input as well (more less the same effect)..

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2024, 12:16:14 am
content: Quote from: eTobey on December 18, 2024, 09:38:42 pmThat also is my view.You have already demonstrated some channels are better than others.Should I get an adjustment procedure maybe I can share it .....IDK.It will require my own tests.....That's all I'll say on this....at this time.

author: eTobey
author_rank: Super Contributor
timestamp: December 19, 2024, 08:54:21 am
content: Quote from: tautech on December 19, 2024, 12:16:14 amThat would be great!I made some more measurements:Compared all the channels, and measured the effect of the level of the signal on the screen.I hope all the labels explain the measurement sufficient.I found, that i am best of using CH1 2/div for measureing levels at < 200us, and for levels at > 200us CH4 1/div.One could go further, and subtract the errors, but that would go a bit to far here.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2024, 09:17:56 am
content: eTobeyCould you please show screenshots ofeachchannel with vertical Zoom.Direct BNC connect, no probes !Source = your SDG set to 1M output for voltage levels to be correct.Signals:Square wave with amplitude of 1.2 Vpp and frequency of 10 kHz, and set the vertical scale of the scope channel to 200 mV/divChange the amplitude of the square wave to 12 Vpp and vertical scale of the scope channel to 2 V/divCompare zoomed and unzoomed waveforms for clean Step response.

author: eTobey
author_rank: Super Contributor
timestamp: December 19, 2024, 01:14:34 pm
content: Yeah nah, 10kHz is far away from my measurements.What would you expect to see here? I was already about to use direct BNC, but i am already a bit tired with this issue.I would expect the signal to look much better, but i would not change the signal to make it look better on the scope. But indeed, my testsignal is not exactly like my use case.

author: eTobey
author_rank: Super Contributor
timestamp: January 02, 2025, 07:20:26 pm
content: Made another test run:With a MCP1407, and gnd spring of course.7V/div and a 3V - 18V step.+-80mV of fluctuation on a 56V scale comes out as beeing as good (700 values), as a 10 bit scope , that does not have such fluctuations. But it still gets worse, as the step increases.

author: eTobey
author_rank: Super Contributor
timestamp: January 05, 2025, 01:36:04 pm
content: Forgot the picture of the setup to my last post.I have also now a measurement of my project: i am not sure much i can trust this signal (pink, coming from about 38V), with that accuracy mentioned above.

author: eTobey
author_rank: Super Contributor
timestamp: January 05, 2025, 08:05:22 pm
content: After a test with shorting 40V, that the probe was on, i came to the conclusion, that this signal is distorted quite heavily. The actual measurment and test was done with gnd spring and alike.The arrows show how i suspect the actual signal to be.