My Siglent SDS1202X-E shows wrong Pk-Pk voltage - Page 1
Forum section: electronics / testgear

author: Ryssen
author_rank: Newbie
timestamp: April 19, 2022, 10:21:37 pm
content: Yes its off ,shows a little to much,fex If I run my generator in 2 volt (FY6900) the scope sows 2 .08v.I have messaured the output with my DMM and it shows 0.700v so that should be 2v Pk-Pk.Is there a pot inside the scope to trim it?Any one have picture of it to see the inside?

author: tautech
author_rank: Super Contributor
timestamp: April 19, 2022, 10:31:24 pm
content: Welcome to the forum.What on earth makes you suspect the FY6900 is accurate ?Most AWG's expect a 50 Ohm load, install one and check your measurement again.No, there are no physical adjustments inside modern scopes and their Cal is saved onto a lookup table.Have you run the Self Cal in the scope after some 20 minutes run time so it's up to operating temp ?

author: Ryssen
author_rank: Newbie
timestamp: April 20, 2022, 02:13:05 pm
content: No I cant be shure that the 6900 is outputting the right voltage.I Will do a self test

author: Fgrir
author_rank: Regular Contributor
timestamp: April 20, 2022, 03:01:27 pm
content: What is the Pk-Pk measurement when you set the function generator to 0V output? The scope is probably including noise in its measurement.

author: Fungus
author_rank: Super Contributor
timestamp: April 20, 2022, 03:25:23 pm
content: Quote from: Ryssen on April 19, 2022, 10:21:37 pm4% error? I don't expect 'scopes to be any more accurate than that.

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2022, 03:52:05 pm
content: There's no trim pot and for all the reasons mentioned above--added noise, tolerances of the scope and the generator, etc, what you are seeing is 100% normal and within expectations.

author: metrologist
author_rank: Super Contributor
timestamp: April 20, 2022, 04:08:09 pm
content: I could not find product specifications. The MFG buried them behind a link that resolves to 404.Another thing to think of is the bit depth of the measurement works with the vertical scale setting. These scopes are not going to have that much resolution and are not intended for that level of accuracy.

author: Fgrir
author_rank: Regular Contributor
timestamp: April 20, 2022, 04:18:33 pm
content: https://siglentna.com/wp-content/uploads/dlm_uploads/2020/02/SDS1000X-E_DataSheet_DS0101E-E04C.pdf3% DC Gain accuracy

author: rf-loop
author_rank: Super Contributor
timestamp: April 20, 2022, 05:58:53 pm
content: @Ryssen, it would be a good idea to sacrifice some time and become familiar with the properties of oscilloscope measurements in a little more detail, also taking into account the quantization noise and analog signal noise involved. It is also good to note that when measuring with an oscilloscope, it is quite different from measuring with a very slow DVM. In this trap, the oscilloscope measures even nanosecond stuff that it does not see at all in the DVM.Also it is good to realize all erros sum. If you use probes also probes have many kins of inaccuracies. Osciolloscope accuracy limits in data sheet is from BNC input.Then need remember ADC is 8bit. Full range is 256 levels. If FS isexampletheoretical 10.24V (as is 1V/div setting with this scope one ADC step is 40mV and with 500mV/div as is used in my example image, 20mV one ADC step) and in real life it do not work as ideal theoretical ADC. It also have some amount of noise and other things.Now then... it need carefully think what is oscilloscope Peak-Peak measurement.I do not start here oscilooscope measurements ABC lesson number one.But here is one really tiny example. Just think why there is two different values Vp-p and Ampl and then also look Top and Base.Difference is not big but you need understand why there is difference. Why Vp-p is too high and also wrong way to measure amplitude, exept if you want peak to peak... and here is difference to DVM... DVM do not see these small peaks what oscilloscope see because it take every single ADC sample and look highest individual samples even if there is just one sample bit higher than all others. DVM is totally blind for these.

author: Ryssen
author_rank: Newbie
timestamp: April 20, 2022, 09:03:12 pm
content: Quote from: Fgrir on April 20, 2022, 03:01:27 pmWhen I turn the voltage down or even Turn the 6900 of it shows 40-60mV.Edit: I did a self calibration,did no difference.I guess I have to live with this.

author: StillTrying
author_rank: Super Contributor
timestamp: April 20, 2022, 10:26:00 pm
content: "When I turn the voltage down or even Turn the 6900 of it shows 40-60mV."The scope's pk-pk noise will be less than 2mV.How many divisions high have you got the waveform, unless you get the height of the waveform a few divisions high. pk-pk measurements don't mean much because of the low vertical resolution.If you can get the waveform 4 or 5 divisions high it will improve a lot, if the signal's less than 1 MHz turn the 20 MHz limit on. if it's repetitive a small amount of averaging will help a lot.

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2022, 10:37:32 pm
content: Quote from: StillTrying on April 20, 2022, 10:26:00 pmNot at the 1V/div setting he is likely using for a 2Vp-psignal. A quick test with mine gives 80mVp-pon the 1V/div setting where each bit appears to equal 40mV or so. It's going to be hard to improve on that with an 8-bit scope. At the 500uV/div setting, the noise is ~500uVp-p. All full 200MHz BW.

author: 2N3055
author_rank: Super Contributor
timestamp: April 21, 2022, 04:49:18 am
content: Quote from: Ryssen on April 20, 2022, 09:03:12 pmLive with what? Lack of information ?What did you do to make sure signal generator is dead on 2V P-PHow do you know your meter is dead on? What meter is it and what is BW?What is the frequency?How is exactly scope setup?How much background noise you have in your measurement?How did you connect gen and scope?Do you have generator terminated with 50 ohm?What is the waveform?What are distortions of the generator?

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2022, 04:59:13 am
content: Quote from: Ryssen on April 20, 2022, 09:03:12 pmNot just you. Every SDS1202X-E owner in the entire world.Welcome to reality.

author: Ryssen
author_rank: Newbie
timestamp: April 21, 2022, 08:47:14 am
content: Welcome to reality.Yes.....

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2022, 09:36:10 am
content: (Also other brands/models... there's not many 'scopes in this price range that don't use 8-bit ADCs)

author: masterx81
author_rank: Frequent Contributor
timestamp: April 21, 2022, 09:48:35 am
content: In any case the FY6900 isn't that perfect piece of equipment. I have one, and the factory calibration is quite bad, and also calibrating it with internal pots, the calibration is lost quite soon. Isn't intended to be a calibration standard.And, for sure, the scope isn't the right object to measure voltage with accuracy. Any scope that i had (in the medium/low range) was quite inaccurate in voltage readings. To be fair, before the 1104x-e i had an Owon SDS7102 and was way more inaccurate.

author: tunk
author_rank: Super Contributor
timestamp: April 21, 2022, 10:44:41 am
content: I would assume the 3% accuracy is full scale. If using 0.5V/div and there's 10 divs, full scale is 5V and 3% is 150mv.

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2022, 11:16:21 am
content: Quote from: tunk on April 21, 2022, 10:44:41 amYes. That's with the waveform occupying the maximum height on screen.If you make it half a screen high it will be 6%.Quarter of a screen? 12%And if you dial the wave down to half a division it will be alotmore than that.

author: Ryssen
author_rank: Newbie
timestamp: April 21, 2022, 06:08:06 pm
content: The waveform is sinus.I have connected the generator with a 50 ohm bnc cable to the scope.The 50 ohm terminator isnt that only for higher signals in the MHz range?

author: LazyJack
author_rank: Frequent Contributor
timestamp: April 23, 2022, 06:26:34 am
content: Quote from: Ryssen on April 21, 2022, 06:08:06 pmThe short answer is no. For the long answer you should really do some research and get familiar with this stuff before trying to fix your not broken instruments.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2022, 09:29:20 am
content: Quote from: Ryssen on April 21, 2022, 06:08:06 pmYes, but only for short cables.Slightly longer answer: It's to do with the ratio of wavelength to cable length.

author: pope
author_rank: Frequent Contributor
timestamp: April 23, 2022, 09:47:05 am
content: Quote from: Fungus on April 23, 2022, 09:29:20 amCan you elaborate a bit?Is there a rule of thumb? For example, do we need a 50 Ohm terminator with 1m cable for frequencies up to 200kHz ?

author: Someone
author_rank: Super Contributor
timestamp: April 23, 2022, 09:57:58 am
content: Quote from: Ryssen on April 21, 2022, 06:08:06 pmA 50 Ohm terminator might be used for damping/matching high frequency signals but that is only one possible purpose for it.Start with the basics:https://www.tek.com/en/video/how-to/50-ohms-output-impedance-on-function-generatorshttps://edadocs.software.keysight.com/kkbopen/why-your-function-generator-outputs-twice-the-programmed-voltage-589745012.html

author: Someone
author_rank: Super Contributor
timestamp: April 23, 2022, 10:00:43 am
content: Quote from: pope on April 23, 2022, 09:47:05 amIs the cable 50 ohm impedance? is the source 50 ohm impedance? The frequency is one of the considerations, but not the only one. Rules of thumb are best left for when you understand the underlying issues.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2022, 10:26:03 am
content: Quote from: pope on April 23, 2022, 09:47:05 amNo.Termination is needed atallfrequencies. It's more needed at high frequencies because you can create resonances and interference patterns inside the cable but you have to go a DC signal before you can say "termination wouldn't improve this".You should always terminate when you're talking about test gear.https://www.ti.com/lit/an/snla034b/snla034b.pdf

author: pope
author_rank: Frequent Contributor
timestamp: April 23, 2022, 10:56:51 am
content: Quote from: Someone on April 23, 2022, 10:00:43 amWhen talking about audio gear the answer to your questions is usually "no". So? Does this mean you can't measure audio gear?

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2022, 12:23:00 pm
content: Quote from: pope on April 23, 2022, 10:56:51 amIt means you have to understand the underlying issues.

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2022, 02:37:37 pm
content: Quote from: Fungus on April 23, 2022, 10:26:03 amWithout understanding the underlying issues?If the source impedance matches the cable characteristic impedance, you are probably good to go no matter what the sink impedance is, presuming the sink is a scope, DMM or similar measuring instrument. The reflected signal will be damped by the termination at the source and the superimposition principle will keep our direct and reflected waves travelling smoothly even in a single lane of coax.And at 200kHz and a 1 meter cable, termination of such a system will halve your amplitude and that's pretty much it. I have 'test gear' with input bandwidth specifications of 1 and 2 MHz that use binding posts for the inputs. And scraggly looking ordinary wires of random and unequal lengths going from there to the circuit board.Terminating a signal generator at the source and then connecting a probe does nothing except provide a 25R source impedance and perhaps give the correct indicated output amplitude if the source is expecting the 50R load.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2022, 04:10:16 pm
content: Quote from: bdunham7 on April 23, 2022, 02:37:37 pmEvery rule has exceptions, obviously.

author: Someone
author_rank: Super Contributor
timestamp: April 24, 2022, 12:04:53 am
content: Quote from: bdunham7 on April 23, 2022, 02:37:37 pmDepends what the output impedance is, just because its audio frequency doesn't mean is so low in impedance you can ignore it, plenty of mundane things have a high enough source impedance that you do need to consider it when probing/measuring. Halving the amplitude because of impedance is as stupid a "rule of thumb" as frequency vs length.

author: Someone
author_rank: Super Contributor
timestamp: April 24, 2022, 12:11:27 am
content: Quote from: pope on April 23, 2022, 10:56:51 amTo return the question back to you: if the audio system is not 50 ohm source impedance, why are you asking about a 50 ohm termination?If one part of the system cant be changed, then you need to understand how the things you can change will affect it. No measurements are perfect and have zero effect on what you are observing (except trivial counting number of objects on a table).

author: bdunham7
author_rank: Super Contributor
timestamp: April 24, 2022, 12:24:29 am
content: Quote from: Someone on April 24, 2022, 12:04:53 amNo, it is a mathematically correct statement provided you have a system worthy of termination--as in the source and cable characteristic impedance are the same. So 50R source, 50R cable, if you terminate that at the other end with 50R, your observed amplitude is half. If the source and cable don't match or you terminate it with something else, then the discussion isn't really worthhalvinghaving.You can connect your meter/scope/analyzer to an audio system with 50R 1M coax, whether it is the very low impedance speaker outputs or something like a phono cartridge, and it will just act as a pair of wires. Or you can just use a pair of wires. Obviously you are correct that in this case adding a 50R terminator to the 50R coax would decrease the amplitude by some factor other than half, as well as being totally pointless.

author: Someone
author_rank: Super Contributor
timestamp: April 24, 2022, 12:58:02 am
content: Quote from: bdunham7 on April 24, 2022, 12:24:29 amLets take what you said in full:Quote from: bdunham7 on April 23, 2022, 02:37:37 pmIts these sorts of half assed explanations that lead to the confusion these posters are having. You've had to come back and add a lot of extra detail to correct it, and its still not complete. There is no "rule of thumb" that is easily distilled down to a short statement like you made and I called it out. The actual explanation needs to be much longer, and fully understood, before the quick version makes sense. Leaving out the context is exactly why the posters are confused. And you're still only scratching the surface of their questions, impedance matching is beyond the idealised discrete models and leads into transmission line/waveguide effects.A 50 ohm coax 1m long connected to a high impedance source might cause significant changes to the signal, even if it is low frequency (<<20kHz ) and the sink is 1M or 10M. The non-ideal effects are ignored,until they get in the way. Which is why people need to understand why they are ignored before they choose dismiss the effects as insignificant in that specific situation.

author: mawyatt
author_rank: Super Contributor
timestamp: April 24, 2022, 01:19:58 am
content: Things about impedance matching and termination impedance usually relates to maximum power transfer, not maximum voltage or current. A simple analysis shows that when the load impedance is conjugately matched to the source impedance this is the condition for maximum power transfer from the source to the load.Unfortunately most folks don't bother to take this into account unless they are working in RF, MW or MMW where things are generally specified in power (dBm for example) rather than voltage or current.Best,

author: bdunham7
author_rank: Super Contributor
timestamp: April 24, 2022, 05:36:11 am
content: Quote from: Someone on April 24, 2022, 12:58:02 amI've not really added detail, just restated the context of this discussion which was connecting an AWG with a 50R source impedance to an oscilloscope using a 50R coax. Within that context, my three observations about 'halving' and so on are correct and readily proven so by simple experiment. These aren't generalized 'rules of thumb', they are specific assertions that I'll defend empirically, but only precisely as stated. I haven't attempted to produce a treatise on transmission lines and I'm mainly addressing comments which seem to indicate a desire to needlessly add unnecessary terminations that won't solve the OPs problem or any other.QuoteI suppose it might--it could pick up noise, it's capacitance might matter enough and so on. In fact the capacitance of a meter of RG-58 would probably be approaching the point of concern with something like a phono preamp input and it might even be a show-stopper in some really high impedance circuits. But in that regard, it isn't any different than any other pair of wires--or a 1X oscilloscope probe--which might have similar parasitics. Are you claiming that a 1M 50R coax connected to such a circuit would cause "significant changes to the signal" due to reflections or other transmission-line characteristics (other than parasitics like capacitance or leakage that can exist in any wires or components) and that termination would be appropriate to remedy that issue?

author: Fungus
author_rank: Super Contributor
timestamp: April 24, 2022, 05:44:11 am
content: Some questions simply can't be answered by a soundbite in a forum.

author: Someone
author_rank: Super Contributor
timestamp: April 24, 2022, 06:13:10 am
content: Quote from: bdunham7 on April 24, 2022, 05:36:11 amBut thats it, there is no halving. The OP starts off by asking why the measurement doesn't come closer than several % of the exact number, a few posters reply with possible causes for that. The OP comes back and asked about termination (without ever mentioning the specific frequency), clearly not understanding when/why termination is important.The thread stated with several % error, how accurate are 50 ohm sources/terminations? how accurate are 50 ohm cables? Tolerancing of those could easily explain a few % error from reflections, a cheap/poor BNC connector could be enough, even in your suggested open termination. Halving is about as relevant to engineering as Zero, only ever an approximation and vague reference. Thinking about voltages like assigning numbers to a variable in software gets people stuck like this all the time. Its an electronic circuit, resistances are inherent and non zero, currents are unavoidable and non zero, tolerances of the parts/equipment used are non-zero, there will be noise/interference that is non-zero. They should not be abstracted away with "rules of thumb" and ignored, when the original post is questioning a few %.