Siglent SDG1032 Amplitude not constant when sweeping - Page 1
Forum section: electronics / testgear

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 10:42:48 am
content: Is there a way to get a constant amplitude, or just much less?Sweep goes from 1KHz to 30Mhz.

author: _Wim_
author_rank: Super Contributor
timestamp: May 18, 2024, 10:58:34 am
content: Some remarks:- I see the scope in sampling at 50MS/s, this is too slow for capturing 30MHz (minimum 60MS/s required)- I presume you are using the 1M input on the scope? Can you give it a try on 50 Ohm input (with a 50 ohm adapter is ok)

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 11:21:57 am
content: Quote from: _Wim_ on May 18, 2024, 10:58:34 amYes, with 50MS/s, the waveform looks rather bad, but its not the form that matters. I have checked with 2Gsa/s, which confirm this issue.I do not have 50 Ohm, but i set the load option of the gen to HighZ. Should that help?

author: 2N3055
author_rank: Super Contributor
timestamp: May 18, 2024, 11:28:55 am
content: How many topics will you open?Will you open individual topic for EVERY SINGLE thing you don't understand and then blame equipment for your lack of knowledge.Are you going to open a topic per buton per piece equipment you own.3 topics on same generator so far?

author: rf-loop
author_rank: Super Contributor
timestamp: May 18, 2024, 11:43:48 am
content: Quote from: eTobey on May 18, 2024, 11:21:57 amCould you really stop fooling around and learn even the basics.

author: rf-loop
author_rank: Super Contributor
timestamp: May 18, 2024, 12:39:33 pm
content: Here SDG1000X sweep from 1Hz to 30MHzSignal set in SDG, Sine, 0dBm and sweep time 10msIn scope (SDS3104X HD, image zoom window is top of signal. (10x vertical magnification)Cursors in zoom window.. if I drop signal level around 0.5dB thensignal topis just as cursor Y2 level.And naturally this include also oscilloscope level imperfection (nonsense in this case).Signal from SDG to oscilloscope 50 ohm input directly using 1m Suhner RG223/UWhere is this problem?I don't see any other problem here than your fooling around with total lack of enough knowledge and experience.

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 01:47:44 pm
content: Quote from: rf-loop on May 18, 2024, 12:39:33 pmDamn, i thought i could get through with my things that i want to do, with that level of knowledge i have. Seems i cant do so. Thank you investing your time to show that to me.I thought that this problem would only be problematic with fast rising signals.Should it work with sticking a 50R resitor in a y-adapter?

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2024, 01:53:22 pm
content: Quote from: eTobey on May 18, 2024, 01:47:44 pmYes, a 50 Ohm resistor would do just fine for 30MHz. But I think the problem you are seeing might be due to overdrive recovery as the sweep you are showing shouldn't be so attenuated (with peak-detect enabled as you seem the have done already) even when using an 1MOhm input especially at the lower frequencies (assuming your sweep starts at the low frequency and ends at 30MHz). When you do the measurement again, make sure the entire amplitude of the signal fits vertically on the screen of the oscilloscope to be sure no effect from the analog front-end of the oscilloscope distorts the signal. If you still get a similar attenuation, zoom in at the start of the sweep further to get an idea what the waveform looks like.

author: _Wim_
author_rank: Super Contributor
timestamp: May 18, 2024, 04:36:53 pm
content: Quote from: eTobey on May 18, 2024, 11:21:57 amNo, this will not help.HighZ-mode and 50 ohm-mode on the siggen just change the signal output level. Your signal generator has a 50 ohm resistor in its output path. When outputting a signal to a 50 ohm input receiver, the receiver would only "see" half the signal (due to the voltage divider created by the two 50-ohm resistors in "series"). By configuring the siggen in 50ohm-mode, the siggen doubles its output voltage to ensure the receiver sees the signal level you entered on the siggen. This is the ONLY difference between these two modes, so it will not help...

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 05:34:20 pm
content: Quote from: nctnico on May 18, 2024, 01:53:22 pmHere are the results. They dont look much better. If i turn to 1X, it lookes like without termination. The wave fits into the vertical space. (see the cursors)I guess i should go and learn basics then... that time is probably better spent there.

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 06:14:27 pm
content: I have found a really helpful video:I now put the probe directly into the AWG. But the result is only somewhat better. (see picture)

author: _Wim_
author_rank: Super Contributor
timestamp: May 18, 2024, 06:29:35 pm
content: Quote from: eTobey on May 18, 2024, 06:14:27 pmAre you using a scope probe instead of a BNC cable? A scope probe uses resistive coax and cannot be used for this purpose. Just do "siggen => bnc cable => 50-ohm adaptor => Scope"

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 06:45:31 pm
content: Quote from: _Wim_ on May 18, 2024, 06:29:35 pmBut what do i do, if i want to measure that signal? It confuses me, that i cannot use a probe for measuring a signal, as you say.

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2024, 07:20:41 pm
content: Quote from: eTobey on May 18, 2024, 06:45:31 pmFollow this rule: Either use the probe as-is (no 50 Ohm termination at the oscilloscope side) OR use a BNC cable with 50 Ohm terminator at the oscilloscope side.Did you adjust the probe correctly? See probe compensation procedure.But this still doesn't explain why the amplitude is near zero at low frequencies. First use a BNC cable between the generator and the oscilloscope and do the measurement again. Leave the probe out.

author: eTobey
author_rank: Super Contributor
timestamp: May 18, 2024, 09:05:49 pm
content: It made "click"The solution i did: i put a 50 Ohm termination on the point, where i measured with the probe. It doesnt look perfect, but good enough for me. (Its 51Ohm but never mind)I tried to put up a little breadboard with a trimmer, to get to exactly 50Ohm, but then it looked like a cross section of a mountain range.So i guess, everything, that is behind the 50 Ohm termination, can still reflect waves. Or is it because of the rather bad connections of the breadboard?

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2024, 10:01:45 pm
content: This sweep is shorter compared to the previous one. For repeatability, don't change the signal settings you are using. Unless you are using an extremely long coax cable, reflections shouldn't be an issue at all. Either way, you should zoom in on the areas which show low amplitude. A potential problem could be the decimation (sample skipping) in the DSO to create the screen causes the signal envelope shown to be lower than it actually is.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 05:37:48 am
content: Quote from: nctnico on May 18, 2024, 10:01:45 pmThe sweep has the same length. It did not start at 0ms. I had no trigger there.From the video of dave, i think reflection DO play a role? Your statement conflicts with my understanding of what i learned from the video. Did i got something wrong?I could not reproduce the waveform on that other picture, but a different one. I have testet it from 10ksa/s to 500Msa/s: The amplitudes stayed the same on the zoomed out view.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 09:37:14 am
content: This is now the best result i could achieve on the sweep.Could it be improved by some more measures?

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2024, 09:40:35 am
content: Can you please show 3 screendumps where you can see the actual waveforms at the start, middle and end of the sweep? And make sure that the signal fits on the screen of your oscilloscope vertically? Only then it is possible to understand what is the problem with your measurement.

author: ebastler
author_rank: Super Contributor
timestamp: May 19, 2024, 09:51:30 am
content: Quote from: eTobey on May 19, 2024, 09:37:14 amYou should really measure with the probe directly at the generator's BNC output. I am not quite sure from your photo, but it seem like you still have a length of two loose wires as part of your connection, which obviously don't have 50 Ohm impedance and may cause reflections.There are dedicated adapters for that, e.g.here, or they come included with some scope probes. (I got mine with the Testec probes I bought a few years ago.) But in a pinch you can improvise by plugging the bare probe tip into the BNC jack and making the ground connection with a piece of wire or foil.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 10:06:13 am
content: Quote from: ebastler on May 19, 2024, 09:51:30 amLook at the picture again. Its in plain sight ;-)I had that adapter, and i tried it, it looks better then, but i would like to use that signal of the gen, and not just measure it. I gues the right thing would then to build my own ouput stage, that would limit problematic voltages, and cope with the termination. But i want to finish my project and dont do new ones.Maybe there is something to buy like this?

author: ebastler
author_rank: Super Contributor
timestamp: May 19, 2024, 10:38:38 am
content: Quote from: eTobey on May 19, 2024, 10:06:13 amSorry, we may be misunderstanding each other. What is in "plain sight" for me in the photo is- a long red/black wire which I assume comes from the generator,- a resistor at the end of that wire,- a pice of cardboard (?) in a crocodile clip,- a probe lying next to it, apparently unconnected to anything.So I am not sure at all how you had your probe connected to the generator.Edit: Oh, and if you need to deliver that 1..30 MHz sweep to your device under test, you need to provide a proper input for it on the DUT. (BNC, 50 Ohm terminated.) Then measure with the high-impedance probe tip right where the signal enters your DUT. I was under the impression that the sweep was just for performance/stability testing of the generator, not related to a current project.Edit²: Or did your "in plain sight" refer to the scope probe, which does look like a Testec probe? So you have one of those BNC adapters? If so, I would love to see a frequency sweep measured directly at the generator's BNC output, with nothing else connected. (And the same for the spurious voltages you see when powering off the generator, discussed in the other thread.) It's the cleanest connection you can make with the equipment at hand.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 11:15:24 am
content: Quote from: ebastler on May 19, 2024, 10:38:38 amThat resistor is the termination, i thought i mentioned it. Should have mentioned it again.Didnt notice, that the cardboard did actually hide the pin header (where the probe would go) on that angle. It would not stick in there on its own.

author: Aldo22
author_rank: Super Contributor
timestamp: May 19, 2024, 11:24:08 am
content: Quote from: ebastler on May 19, 2024, 10:38:38 amOr SMA with adapter, which is handier.I always try to keep the distances as short as possible so that there is less noise etc.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 11:28:23 am
content: Quote from: Aldo22 on May 19, 2024, 11:24:08 amAre you supporting the cable? If not, you put a lot of stress on the right side SMA adapter.What is this board??

author: Aldo22
author_rank: Super Contributor
timestamp: May 19, 2024, 11:33:26 am
content: Quote from: eTobey on May 19, 2024, 11:28:23 amThat doesn't matter. I just wanted to show that SMA connectors are quite common for such devices.You can get them cheaply:https://de.aliexpress.com/item/1005003311808070.htmlThe DUT in the example is this:https://de.aliexpress.com/item/1005005704247436.html

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2024, 12:55:01 pm
content: Quote from: Aldo22 on May 19, 2024, 11:24:08 amBut do not get overworked about the frequency. Keep in mind that the wavelength of a 30MHz sine wave is around 10 meters. With a cable length of up to 2 meters, reflections aren't an issue. If I needed to bet money, I'd put it on a display artefact of the oscilloscope. The differences in amplitude as shown by screenshots are a tell-tale sign. This is why it is so important to get some images which show the waveforms at the frequencies where the amplitude is highest and lowest.

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 01:47:38 pm
content: I now zoomed it further. I deliberately picked a place where there was an artefact. It was a break in the sweep without a signals, but it is rather narrow, and it gets overlayed by the signals on the side. i could measure 3 break in about 13.4 ms interval after each other.It is only visible in normal aqusition, and not in peak acq. Changing the acqs. mode does not change the sweep amplitude that is seen on the scope.

author: Aldo22
author_rank: Super Contributor
timestamp: May 19, 2024, 02:39:36 pm
content: @eTobey: Somehow I'm lost.Could you repeat the question?Is it about Siglent SDG1032 sweeping, coax/wiring, oscilloscope settings, frequency response of your DUT or what exactly?Maybe it would also be useful if you could show your DUT?

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 03:06:11 pm
content: Quote from: Aldo22 on May 19, 2024, 02:39:36 pmIt was a bit of everthing, except scope settings. I just added that screendump, because someone wanted it (kind of).But i am satisfied with the information here and my findings:https://www.eevblog.com/forum/testgear/siglent-sdg1032-amplitude-not-constant-when-sweeping/msg5506162/#msg5506162

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2024, 03:09:36 pm
content: Quote from: eTobey on May 19, 2024, 01:47:38 pmTo me this looks like a display artefact. You have to keep in mind that a digital oscilloscope (in general, not just Siglent) can't use the entire memory length to produce the image for a trace because that would be way too slow. So what the oscilloscope does is called sub-sampling. It takes the value for every Nth sample (1 in 100 for example) and use that to produce a trace envelope outline. Typically this works but for frequency sweeps, you can get intermodulation products which then start to show as valeys which don't make sense.

author: TurboTom
author_rank: Super Contributor
timestamp: May 19, 2024, 03:11:40 pm
content: Please deepen your knowledge in sampling theory! If you sample a signal swept to 30MHz at 10MSa/s, these aliasing patterns are exactly what to expect. Please make sure you have sampling memory on your scope set large enough to match the Nyquist criteria, i.e. Fsample > 2*Fsignal! This is digital scope basics!@nctnico beat me to itP.S: I've got the impression your DG812 wasn't that much "rubbish" after all...

author: eTobey
author_rank: Super Contributor
timestamp: May 19, 2024, 03:18:54 pm
content: Quote from: nctnico on May 19, 2024, 03:09:36 pmQuote from: TurboTom on May 19, 2024, 03:11:40 pmIndeed it is, but it does not really matter here, as those valleys are very very narrow, and zoomed out you can barely see them. Using peakmode, those narrow valleys even completely dissappear.

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2024, 03:39:21 pm
content: Quote from: eTobey on May 19, 2024, 03:18:54 pmThat is because peak-mode runs the ADC at full speed before doing the decimation so you won't get sampling related intermodulation products. The post-processing (to make the trace display) intermodulation will still be there though.

author: eTobey
author_rank: Super Contributor
timestamp: May 22, 2024, 09:29:39 am
content: I have now made a final measurement, using proper connection, and using the maxhold function.It shows clearly a asymmetry and the amplitude varation over the sweep in great detail. Too bad, that the math dont work with zoom mode (all of them?).

author: newbrain
author_rank: Super Contributor
timestamp: May 22, 2024, 11:39:10 am
content: Quote from: eTobey on May 22, 2024, 09:29:39 amDo I read it correctly that now, with proper cabling, the variation is about 50 mV from end to end of the sweep?And the signal level is about 2 Vp (no measure, and graticule very faint)?So ≈0.2 dB of change.That's not bad in itself, and also in spec, from the datasheet:Quote

author: eTobey
author_rank: Super Contributor
timestamp: May 22, 2024, 06:10:09 pm
content: Quote from: newbrain on May 22, 2024, 11:39:10 amIt is a 5.05 Vpp "amplitude", but this is at a limit, what i did not notice then. Have you read the details on the picture name? (I guess its a bad place for that?)Amplitude flattness is measured on a fixed frequency? If so, it could be better, but i think its probably a differend rail that is used for that (2.5V vs. 5V), and that could be different.

author: newbrain
author_rank: Super Contributor
timestamp: May 22, 2024, 07:45:04 pm
content: Quote from: eTobey on May 22, 2024, 06:10:09 pmOOPS! Totally missed the picture name.Then I stand corrected. It's a 50 mV variation from 1 to 30 MHz on a ~2.5 Vp signal.I.e, 20 x (log10(2.525 - 0.05) - log10(2.525) ~= -0.174 dB , a bit less than my estimation.Kudos to the output stage designers in Siglent (or FW designers for the calibration table, or both).But I think this is a good lesson for better probing, connections, and termination techniques.Don't worry, we all were wet behind the ears, once: just three days ago a dear and usually really smart friend of mine was probing a small RF power amplifier prototype.He sent me some screen capture - I could not understand what was happening - for me it had worked with no issue...Then he casually mentioned that he was not using the probe spring."Oh, definitely do not use the alligator for RF or fast edged signals!""No worries, I'm not using that either!"

author: eTobey
author_rank: Super Contributor
timestamp: May 22, 2024, 08:12:39 pm
content: Quote from: newbrain on May 22, 2024, 07:45:04 pmIndeed it is. Never thought that those kind of slow signal changes could have such a dramatic effect. Thought it would only be relevant for higher frequencies (> 60MHz) or digital signals. I also discovered today, that compensation is as important, while looking with 2 probes at a triangular wave and plotting the difference. If i remember right, it was even at just 1kHz.I do not worry, i do learn (while trying it out mostly) ;-) There are some good videos about the gnd spring on youtube. It seems to me, more than for proper termination.

author: thm_w
author_rank: Super Contributor
timestamp: May 22, 2024, 10:37:49 pm
content: I would say many of the issues were scope/gen settings and not probing technique.Remove the terminator and it should be flat up until ~20MHz.Compensation is important at low frequencies, yes, it is called LF compensation sometimeshttps://www.electronics-notes.com/articles/test-methods/oscilloscope/scope-probe-compensation.php