Measuring Distortions with the Scope:What you see is not what you really have.. - Page 1
Forum section: electronics / testgear

author: Martin72
author_rank: Super Contributor
timestamp: December 22, 2022, 11:29:19 pm
content: Hi folks,I got a sdg1062X(formerly 1032X...) and I´m quite happy with it.But the day will come I´ll need more than 60Mhz from time to time, so I decided to buy a SDG2042X and (try to) doing the upgrade to the 120Mhz.With a little bit luck, the generator will arrive on saturday - my personal christmas gift..My sdg1062X I´ll sell it because two gens I don´t need.But I got an idea, as long the both are here, I could make some comparisions between the two.Last year I had to decide to take the 1000X or 2000X - And I´m sure I was and am not the only one..Both models are popular among hobbyists.So my idea is, tell me what I should compare between the two and I´ll post it here.I could set both to say 1khz sinewave and measure with my HD scope the distortion for example, or, or, or...This is my idea.Will post here again when the other gen has arrived.Martin

author: BillyO
author_rank: Super Contributor
timestamp: December 22, 2022, 11:32:44 pm
content: When you generate keys for a SDG1032X it gives you a license key for 120MHz. It would be interesting to see if that would work and compare the results to an SDG2000 "improved" to 120MHz.

author: Martin72
author_rank: Super Contributor
timestamp: December 23, 2022, 12:05:31 pm
content: Hi,not sure if I want to do this on the 1032X, must think about it.Meanwhile, a "few" hours later, the SDG2042X has arrived...I let it acclimate a while..

author: rf-loop
author_rank: Super Contributor
timestamp: December 23, 2022, 12:48:27 pm
content: Quote from: Martin72 on December 23, 2022, 12:05:31 pmJust don't. (these simple license generators don't really understand anything about these devices. If they generate 120MHz license for SDG1000X ... well, mad(software) can do what mad(software) do.)

author: Martin72
author_rank: Super Contributor
timestamp: December 23, 2022, 07:25:54 pm
content: I got twins....nearly.Building quality is absolute the same, which is good to know for the ones, who wants to buy the 200 bucks cheaper 1032X.System menu got one feature more I didn´t notice on the 1032X so far, "Multi Device Sync", must rtfm to understand what this means.The SDG2042X got a touchscreen which is in my opnion not really necessary - And it´s a little bit annoying when trying to touch it in the upper/lower areas of the screen - Maybe my fingers are too fat, I better use the knobs...So...next thing I´ll try is to hack the 2042X to 120 Mhz, must read the threads for it.To be continued.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2022, 07:36:19 pm
content: Quote from: Martin72 on December 23, 2022, 07:25:54 pmNot quite as there are some crucial differences and in some corner cases the 1000X is better.However it cannot beat the 16 bit and longer Arb waveform depth of the 2000X however if we want a faster edge a SDS1000X has that.Even with a much shorter Arb waveform depth capability if it fits within memory it can be made to loop for xN cycles or continuously.

author: Martin72
author_rank: Super Contributor
timestamp: December 23, 2022, 08:18:18 pm
content: Hi Rob,With twins the outside was meant...

author: pope
author_rank: Frequent Contributor
timestamp: December 23, 2022, 08:41:38 pm
content: Interesting thread. Thanks!

author: electr_peter
author_rank: Unknown
timestamp: December 23, 2022, 09:55:03 pm
content: 1) Basic test would be to see if there is any second/third/... edge jitter when frequency is not "nice". There should be none.2) Try looking at jitter of CH1/CH2 vs SYNC. I think SDG2000X does not have jitter, but that would be interesting to know for sure.3) Also check FM modulation drift w.r.t. SYNC. SDG2000X definitely drifts in FM mode even with the newest firmware (many other AWGs drift the same for some reason).4) Max frequency of SYNC on SDG2000X is only 10MHz. How does it compare with SDG1000X?5) SDG2000X has newFW releasewhich seem to improve rotary encoder performance. It would be a good comparison with SDG1000X variant.

author: BillyO
author_rank: Super Contributor
timestamp: December 23, 2022, 10:21:56 pm
content: Quote from: Martin72 on December 23, 2022, 12:05:31 pmI just got back home and tried the 120M license key on my SDG1032X (improved to SDG1062X) and it simply did not accept it.Oh well. Never ventured, never gained.

author: Martin72
author_rank: Super Contributor
timestamp: December 23, 2022, 10:26:26 pm
content: Hi,QuoteThat was pretty fast:https://www.eevblog.com/forum/testgear/siglent-sdg2042x-hack-door-closed/msg4599439/#msg4599439Now I got a SDG2122X...Nice.

author: Martin72
author_rank: Super Contributor
timestamp: December 23, 2022, 10:29:09 pm
content: Quote from: electr_peter on December 23, 2022, 09:55:03 pmAha....And indeed I wondered about why the encoder is so...slowish and "stuttering"...Thanks for the hint!

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2022, 10:58:24 pm
content: Quote from: electr_peter on December 23, 2022, 09:55:03 pmAre datasheets too hard to read ?(Square wave) Jitter (rms), Cycle to cycleSGD1000X 300 ps + 0.05 ppm of periodSDG2000X 150 psSDG6000X 100 psSDG7000A 20 ps cycle to cycle rms, >10 kHz,1Vpp, 50 Ω load

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 01:50:24 pm
content: FFT with 1khz sinewave, 2.85vpp...Separate, then both together using both mathchannels, unfortunately the top of the tables are "cutting off" by the screenshot..

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 02:08:23 pm
content: Short video of the setup...https://youtu.be/ekRBBR1Xj0k

author: Roger Need
author_rank: Regular Contributor
timestamp: December 24, 2022, 06:09:13 pm
content: Attached is a pdf document which compares the SDG1000x to SDG2000x. One feature I like in the 2000x is that you can trim the internal 10 MHz. reference from the service menu using a software DAC control. This gives much better synthesizer output frequency accuracy.Roger

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2022, 07:13:05 pm
content: Quote from: Martin72 on December 24, 2022, 01:50:24 pmSave/Recall/Save PNG/Keep menus visible.

author: electr_peter
author_rank: Unknown
timestamp: December 24, 2022, 07:30:24 pm
content: Quote from: tautech on December 23, 2022, 10:58:24 pmThere is no harm in testing, isn't it? SDG1000X shows some significant cycle to cycle jitter on page 13 in PDF comparison linked above. Devil is in the details, AWGs have many modes, so testing helps to clarify things.Cycle to cycle refers to CH1/CH2 output. Is it the same spec for SYNC?

author: 2N3055
author_rank: Super Contributor
timestamp: December 24, 2022, 08:14:02 pm
content: Quote from: electr_peter on December 24, 2022, 07:30:24 pmAcronym of the day here is NCO.. Inherent in their operating principle is fractional relation between generated frequency and clock. At certain frequencies CH1/CH2 will have 0 phase jitter to SYNC. On other frequencies it will have rounding errors in phase comparison. There is also a fact that at certain settings, a slight phase dithering will produce cleaner spectrum of the output signal (instead of few large peaks, harmonics gets dithered into wideband noise and into noise floor).AWGs are great because they are universal. That comes with certain tradeoffs.They will not have same pulse specifications as specialized pulse generators, but will be good enough for many uses.They will not have distortion as low as audio analyser but will have very low one, better than many old "legendary" audio sources (my SDG6000X has 0.02% THD at 1kHZ, -10dBm. SDG2000X is actually slightly better because of smaller BW. Those are very respectable numbers),SDG6000X has CCJ of 2.9ps (that's picoseconds) at 10MHz.. That includes built in TCXO and NCO engine.These are absolutely fantastic numbers for a simple AWG.

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2022, 08:27:02 pm
content: @ 2N3055And why we have (2000 & 6000) and use them, they are that goodPeople bitch and complain about the UI (granted it's poor), but the "end product justifies the means"Best & Happy Holidays

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 11:19:04 pm
content: Quote from: tautech on December 24, 2022, 07:13:05 pmI know it but in the moment I´ve made the shots, it wasn´t set...Now it is.

author: tautech
author_rank: Super Contributor
timestamp: December 25, 2022, 01:13:16 am
content: Quote from: electr_peter on December 24, 2022, 07:30:24 pmOkay sorry but you need become more familiar with your instrument as you can tune phasing for better channel edge alignment however it only allows for precise alignment at thetunedfrequency and if you change frequency you will need to realign for precise phasing.First we lock phasing then apply phase+corrections.

author: electr_peter
author_rank: Unknown
timestamp: December 25, 2022, 10:01:08 am
content: @tautech, maybe CH1/CH2 in text was not clear enough, I meant single channel CCJ. As I understand, CCJ specs refers to a single channel. Relationship between CH1 and CH2 is a separate question (but both channels should have the same CCJ).What I am asking it to check jitter between CH1 and SYNC output, as it is not directly mentioned in specs. In my experience, SDG2000X does not exhibit any issues in this area, but on SDG6000X thread there were some questions about this (maybe wrong test setup, I am not sure).

author: rf-loop
author_rank: Super Contributor
timestamp: December 25, 2022, 10:06:50 am
content: Quote from: Roger Need on December 24, 2022, 06:09:13 pmThis comparison is obsolete and extremely poor -- today also partially false.Years ago example SDG1000X series have get TrueArb function.(2019-02-25 FW 1.01.01.33R1: Supported TrueArb: 2~16 kpts)True arb jitter SDG1000X 300ps, SDG2000X 150pshttps://www.siglenteu.com//wp-content/uploads/dlm_uploads/2022/06/SDG1000X_DataSheet_DS0201X_E01I.pdfThen also quite fun to see that square wave performance is not at all included in this ([www.batterfly.com] stamped) comparison what was "big" advantage in SDG1000X series after older SDG2000X series. If this is Siglent made comparison it looks like they somehow "compete with them selves". Sad there is not any datecode in this obsolete pdf.(of course SDG2000X is better in many other things).Btw, it is good question why it is so many times seen that some publications do not have any date and/or version with history (changelog).SDG2000X series max is 25MHz (all models) and rise/fall 9nsSDG1000X series max is 60MHz (32X 30MHz, 62X 60MHz) and rise/fall 4.2ns (3.8ns @1Vpp)These are of course out from box. (for compare, factory specs are suitable and recommended to use)Example my 1062X have 11.8ns pulse rise and fall time minimum. After Siglent it was 16,8ns and example max sweep time is now 100000s when it was 500s after factory (Because one day I needed slow 24 hour sweep so 100ks was nice number) and some other "tweaks".

author: Hexley
author_rank: Regular Contributor
timestamp: December 25, 2022, 11:55:27 pm
content: Quote from: rf-loop on December 25, 2022, 10:06:50 amAgreed.Which invites a comment on Siglent's own documentation -- I look forward to the day when Siglent steps up to the next level of documentation standards for their products, not only including change logs but also having an index at the back, displaying chapter numbers in the footer, updating the manuals to keep them consistent with firmware changes, etc.Maybe 2023 will be the year for that. I can't wait.

author: Martin72
author_rank: Super Contributor
timestamp: December 26, 2022, 12:00:13 am
content: For the SDS2000X+ I can say, they did.Currently the manual status is issue "D" from may 2022.

author: tautech
author_rank: Super Contributor
timestamp: December 26, 2022, 01:38:32 am
content: Quote from: Hexley on December 25, 2022, 11:55:27 pmMuch is already available about document release dates on the HQ website:See the dates for each doc in this list:https://int.siglent.com/products-document/sds2000x-e/#navs

author: rf-loop
author_rank: Super Contributor
timestamp: December 26, 2022, 02:04:29 pm
content: Quote from: tautech on December 26, 2022, 01:38:32 amYes of course some times Sherloc Holmes work and guessing is ok but...But if we talk good documenting practices... Only right place for datecode is in document itself. Period. And it need be in every single published document. Period.Siglent need understand this and do it. Period. It is mandatory.And it need be started over 10 years ago but still better later than never. It need do asap and including whole product portfolio. There need be "house standard" for documentation and not so that different design groups do different way. Some better and some not.We can see there has already been a slight improvement in this matter over the years. That's good, but not at all enough yet.I don't think I'm the only one who has tried to move a mountain in this matter.Lot of development is still needed.In terms of documentation, Siglent is still not at the appropriate level. The hardware can be done quite well, but there are major flaws in creating documents.And it's also pointless to claim (if some people want to do so) that someone else doesn't make good documents either and that's why they aren't needed - maybe no one even reads them (as is often seen unfortunately).Such thinking would be a mistake. Let's hope it doesn't happen.If that happens, you have to ask who you are trying to ski after. Usually the skier behind is not the winner.

author: Hexley
author_rank: Regular Contributor
timestamp: December 26, 2022, 04:59:23 pm
content: Quote from: rf-loop on December 26, 2022, 02:04:29 pmTo further illustrate the point about the standards of Siglent documentation:Consider this post from May, 2020https://www.eevblog.com/forum/testgear/siglent-sdg1000x-waveform-generators/msg3051348/#msg3051348that describes issues in the SDG Series Programming Manual. That post gives details on how the description of one of the essential commands for the unit, the modulated wave command, MDWV, is simply incorrect.Though that was posted more than 2 1/2 years ago, with full details, the latest version of the programming manual (version PG02-E05A) contains the same errors. Verbatim.I found that disappointing.Of course, the Programming Manual does have a version label printed on its title page, so there's that.

author: Martin72
author_rank: Super Contributor
timestamp: December 26, 2022, 08:22:32 pm
content: Back to topic, comparison...FFT of SDG1062X and 2122X, sinewave 1khz, appx 0dBm @50Ohm:SDG2122X:SDG1062X:

author: 2N3055
author_rank: Super Contributor
timestamp: December 26, 2022, 09:33:57 pm
content: Martin,Try using Markers, On harmonics, and enable relative scale... Start FFT on horizontal from 500Hz to get rid of DC peak..When using 0dBm or 0 dBV you have direct readout, but this way you can use it easily on any level..Try varying AWG output level a bit. You will see there are sweet spots where AWG uses full DAC scale and output amplifier drive has low distortion. Play with input sensitivity of scope too... Like most SA, best distortion is not when driven full scale... There is a fine balance between a noise floor and amplifier spurs..SDG2000 will generally have lower distortion, especially in sweet spots.Best,

author: Martin72
author_rank: Super Contributor
timestamp: December 26, 2022, 09:55:50 pm
content: Hi my friend,Can I test tomorrow(vacation), although it looks like it's not too far from what the spec sheet states.(OK, the 2nd harmonic is a little "high" (appx 0.09%))

author: markone
author_rank: Frequent Contributor
timestamp: December 27, 2022, 01:22:52 am
content: Quote from: Martin72 on December 26, 2022, 09:55:50 pmI would take with grain of salt averaged FFT spectrums, especially for THD and similar math, the wonderful dynamic range is only apparent, my trust ends at best at "SNR = 6.02N + 1.76dB"Anyhow, here is my SDG2042X @ 1KHz - 0dBm as seen by HDO1000 :

author: rf-loop
author_rank: Super Contributor
timestamp: December 27, 2022, 09:01:53 am
content: Quote from: markone on December 27, 2022, 01:22:52 amYou can take what ever salt or sugar...Btw, this averaging doesn't affect the signal levels, but it does affect the noise floor, so we can peek "underneath" the noise level...(without true need here) but there we can ONLY see these signals that are in sync(!) what is sometimes very important (depending what we are doing). As long as this is true etc...... you know. Of course it works and this way we can communicate in a very simple way with radio signals that are much below the noise level, which cannot be detected at all by a normal "modern/high-end" radio. And no bystander knows that communication is taking place there (until they find how to sync and how to open messages, but how they do it if they do not know what to try find and where.https://www.analog.com/media/en/training-seminars/tutorials/MT-001.pdfBtw, in Rigol image there is RBW: 90.90mWhere is specified what it really mean.In your image can read that FFT samplerate is 1MSa/s and you have there 20kHz span.With 1MSa/s normal FFT full span is 500kHz and you have then selected 0-20kHz span. If there is 1Mpts and full span is 500kHz then Delta f is 0.5Hz. But what is this RBW "90.90m"?What is FFT window used in this image.DHO1000 user manual or data sheet do not tell anything but "resolution".Also in DHO1000 (HDO1000) manual: "FFT resolution is the quotient of the sample rate and the number of FFT points. If the number of FFT points is a fixed value (65,535at most),......."

author: markone
author_rank: Frequent Contributor
timestamp: December 27, 2022, 10:13:31 am
content: Quote from: rf-loop on December 27, 2022, 09:01:53 amI can confirm, among many other, there is a bug regarding acquisition parameters display during FFT operation.Right yesterday, when i was doing the meas that i attached in my post, i found out that rotating horizontal scale FFT acquisition sample rate, sample interval and samples number were not changing in accordanceAnyhow, the produced spectrum graph is "aligned" with an equivalent meas made with my SSA3021X in sweep mode, so i'm confident about that image, RBW apart for obvious reasons.

author: Performa01
author_rank: Super Contributor
timestamp: December 27, 2022, 03:37:18 pm
content: Quote from: Martin72 on December 26, 2022, 09:55:50 pmSo the third harmonic is about the same on both generators, but the 2nd is about 10 dB stronger on the SDG2000X, which is quite unexpected.Do you happen to have some decent voltmeter that can measure down to 100 µV with sensible accuracy?If so, please check the offset voltage on both generators and report the results here...

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 04:35:22 pm
content: QuoteIt is 6dB lower on SDG1062X, the table (peaks) is a little bit irritating."Best" multimeter here is a calibrated brymen 869s...Could try with it.Martin

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 04:53:46 pm
content: Checked DC-offset, on both generators the same (appx 400µVdc (Meter shows 0.41mVdc), output 640mVpp@50Ohm terminator).

author: Performa01
author_rank: Super Contributor
timestamp: December 27, 2022, 05:32:04 pm
content: Quote from: Martin72 on December 27, 2022, 04:35:22 pmOh yes ... maybe it would be more intuitive if you sort by frequency instead of amplitude?

author: Performa01
author_rank: Super Contributor
timestamp: December 27, 2022, 05:43:10 pm
content: Quote from: Martin72 on December 27, 2022, 04:53:46 pmI'm too lazy to do a detailed calculation, but a quick estimate tells me that in this case the 2nd harmonic would be worse than -60 dBc. In other words: this DC-offset measurement is just a streetnumber (no wonder: check the specifications and calculate the error margins from that). What do you get if you measure a short? Does it show precisely 0 µV then?Anyway, my theory was that even harmonics, especially the second, would occor especially with unsymmetric waveforms as we could get them with non-zero DC offsets. You could still try to tweak the DC-offset and see how it affects the even harmonics.

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 06:44:51 pm
content: Hi,QuoteIt shows 000.00mV when shorted or the 50 Ohm termination is connected - Open the last digit change between -/+ 5.I agree that could be only streetnumber.If you set a dedicated offset voltage on the generator, it will also be measured accurately by the meter.I could test how "low" I can get with this...With the next test I´ll sort the table.

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2022, 07:26:44 pm
content: All this and the other somewhat related thread got me thinking about what's going on, so just a quick setup to check things out. Here's the related thread.https://www.eevblog.com/forum/testgear/siglent-sdg1032x-sine-distortion-at-1-khz/25/So, we got our Pico Scope 4262 (16 bit ADC) out and hooked up to a laptop. Then used the builtin AWG (only goes to 20KHz tho) and setup a 10KHz sine wave signal.We setup the scope for a 0-100KHz span and captured the spectrum shown below as Pico_AWG.Then connected the Pico AWG output to a SA as shown in PNG7.Then connected to our SDS2000X+ as shown in PNG_147.No attempt was made to "normalize" things and get all the displays setup up for a detailed comparison, just a quick look and see.It appears that the SDS2000X+ is introducing the harmonic distortion from within, as the dramatic difference in the spectrums show.Later we'll try and look at the SDG2000 & 6000 AWGs we have.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2022, 08:06:14 pm
content: See above, here's the added files for the SDG2000X and the SDG6000X set at 10KHz.Pico_AWG 2 is SDG2000XPico_AWG 3 is SDG6000XPNG 8 is SA SDG2000XPNG9 is SA SDG6000XPNG 148 is DSO SDS2000X+ for SDG2000XPNG 149 is DSO SDS2000X+ for SDG6000XThe AWGs seem much better than the SDS2000X+ can display regarding harmonic content.Best,

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 10:06:11 pm
content: Hi....Following one of Sinisas hints I´ve played with the vertical resolution between 100mV and 200mV/Div., same FFT settings as before.Two pics, one with 100mV/div. (as the measure before) and one with 130mV/div.Judge by yourself..

author: electr_peter
author_rank: Unknown
timestamp: December 27, 2022, 10:24:02 pm
content: Likely explanation of SDS FFT issue describedin other topic.

author: Martin72
author_rank: Super Contributor
timestamp: December 28, 2022, 12:03:16 am
content: One of the findings one could derive from this would be of a fatalistic nature, namely that one can forget the FFT function with oscilloscopes.Is this really the case, or do you have to try a little harder to get a more credible result?And if so, in which direction one would have to go for it....

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 07:06:18 am
content: Quote from: Martin72 on December 27, 2022, 10:06:11 pmYou can also do same but take all averaging off and watch screen more time than just one eyes blink because we do not have good min-max feature in this FFT application, and not much else either. After enough watching... take a piece of paper or nice cloth and make curtains to hide these very low level peaks and "problem" is "solved".:- //(= if the measurement instrument can't show enough truth then instrument must not display it)Btw, voltage band change between 100│102mV/div. (and 1.00│1.02V/div)Using this have minimal other effects than just band bottom and band top, so minimal change in signal level in ADC but different front end noise figure.

author: Performa01
author_rank: Super Contributor
timestamp: December 28, 2022, 09:00:33 am
content: Quote from: rf-loop on December 28, 2022, 07:06:18 am... because of vastly different PGA gain, which in turn could also lead to a different distortion level of the PGA (which probably is the main source of nonlinearity in a scope frontend).

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 10:39:14 am
content: Quote from: Performa01 on December 28, 2022, 09:00:33 amYes, this can apparently suspect.More research is needed.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 28, 2022, 11:14:08 am
content: Quote from: Martin72 on December 28, 2022, 12:03:16 amNot at all, in the same way that the frequency measurement function is not useless just because it does not have the same resolution and accuracy as a standalone frequency counter. An oscillscope is not a replacement for an audio analyzer (neither is a spectrum analyzer btw).Every instrument creates some distortion of its own (not only in the ADC but more importantly the front-end as well). It will depend on the frequency and amplitude of the input signal and the vertical setting as well. And unfortunately, it is usually poorly characterised in oscilloscopes (the SDS2000X HD datasheet only says SFDR >= 45dBc which is not very helpful). Once you get close to those distortion limits, the error bars increase. The distortion of the generator and the receiver do not necessarily add in a straightforward way either. In some cases they may cancel partially and a noisier input signal may show less distortion because of dithering effects.The SDG2000X has pretty clean output in the audio range (significantly better than the datasheet specs at most settings). Almost all oscilloscopes will struggle there, with some exceptions like the remarkable but specialized PicoScope 4262. Note that you are trying to characterize a 16 bit generator with a 12 bit oscillscope. Now that doesn't tell you anything definite about linearity at all but IMHO it should give you pause and make you proceed with careful consideration and healthy scepticism.What can you do to improve your measurements? First, know your instrument and its limitations. You can try to measure an oscillator that is known to have distortion levels significantly better than your oscilloscope as a reference. That also allows you to find the settings that give you the best results. A quick sanity check is to add some in-line attenuation (on a spectrum analyzer, change the input attenuator setting). If the relative levels of the harmonics change, you will know that the distortion is from the receiver.You can also add an external notch filter to attenuate the fundamental. This will enable you to measure down to very low distortion levels.

author: Martin72
author_rank: Super Contributor
timestamp: December 28, 2022, 12:31:01 pm
content: Hi,QuoteI realise that, but the scope should not show anything that is actually not there, and that is the current situation.And I don't think it's because you get a 16 bit signal with a 12 bit scope(the very most user got 8bit ones).Approaches have already been shown as to what the problem could be, and that worries me at least.Not that you generally have to doubt everything that the FFT function provides or that you first have to make an increased effort instead of just roughly adjusting it and that's it.A very interesting and important topic, it should be separated from this thread, otherwise you won't find it again so quickly.Regarding the original topic here:And then I wonder what else you can compare between the models that doesn't need to be doubted because you did it with a scope?

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 12:50:59 pm
content: Quote from: switchabl on December 28, 2022, 11:14:08 amWell said!Also example I have used several kind of spectrum analyzer over tens of years. Most (all) of them display "fake" signals. Non input related spurs and then input related. Look generator harmonics with SA and wonder what harmonics are generated or affected due to SA own generations... and so on. Simple: There is not ideal instruments on this Tellus, not in history, not now and never in future. Every instrument display sum of errors mixed with some kind of truth what we do not know.With FFT display human eye see these so... omg...so big false peaks. User need just understand and just know his instrument limits and lies and not believe everything.

author: Martin72
author_rank: Super Contributor
timestamp: December 28, 2022, 01:43:29 pm
content: OK, so we have the philosophical approach already.Do not trust what you see, use your imagination.Btw, this is what it should look like to me, a 1Khz square wave, steadily decreasing harmonics.

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 02:45:11 pm
content: Quote from: Martin72 on December 28, 2022, 01:43:29 pmYep, roughly so.You can now zoom in and look just 577th harmonic and check 576th and 578th... then measure these and tell "truth" and think why these levels are just right or how they are... (not seriously...)

author: 2N3055
author_rank: Super Contributor
timestamp: December 28, 2022, 03:03:00 pm
content: Quote from: Martin72 on December 28, 2022, 12:31:01 pmWell scope doesn't show anything that is not there in normal time domain mode.....FFT is math mode... In math mode there are no limits on result.You can make math channel where you multiply a signal by 10E6 and you will get weird stuff.Like others said, with SA you need to characterise for spurs to know if signal peaks are real.Whole thing stems from the fact that input noise is really low. On my Keysight 3104T noise floor is so high you cannot see spurs at -60dBm

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 03:37:58 pm
content: Quote from: 2N3055 on December 28, 2022, 03:03:00 pmIn this regard, I just previously suggested using paper or a piece of fabric to make curtains. Then just close a curtain in front that covers the things that shouldn't be taken so seriously. Of course, one could also make the curtains in another way... like Keysight, for example. This way you can conveniently cover it up and no one will make a fuss about "strange" signal spikes somewhere deep...Maybe someone should suggest that... you could easily implement a noise generator there to satisfy the users, because then the lies will be removed --- oops --- hidden.

author: Detlev
author_rank: Regular Contributor
timestamp: December 28, 2022, 09:12:37 pm
content: Hello everyone,for comparison I made an FFT of my SDG6022x with the RTB2004 (RTB2K-COM4). The values of the 6022 are very similar to those of the 2122XMany greetingsDetlev

author: Martin72
author_rank: Super Contributor
timestamp: December 28, 2022, 11:01:17 pm
content: Like the comment function of the RTB...By the way, I remembered that I still have something on the shelf with which I can measure THD.My Neutrik A1....With this puppy it should be possible to get "more trustful" values.

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 12:50:13 am
content: Quote from: Detlev on December 28, 2022, 09:12:37 pmThanks for the plots!!If you check the earlier posts we provided, it's likely that the DSO is contributing more of the higher frequency harmonics than either AWG. This is because these AWGs provide pretty good signal fidelity from 16 bit DACs and thus more so than the typical general purpose bench top DSO can cleanly resolve.Best,

author: Performa01
author_rank: Super Contributor
timestamp: December 29, 2022, 09:50:57 am
content: My bad – In my initial comments I simply forgot that the linearity of the average scope frontend (including SDS2000X HD) is not up to the task of characterizing the harmonic distortion of an SDG1000X or 2000X.Quote from: Martin72 on December 28, 2022, 12:03:16 amIf you look up the datasheet of PGAs that can be used for a scope like this, such as LMH6518, you will find the harmonic distortions specified somewhere between -44 and -50 dBc. Considering this, the SDS2000X HD still does a fairly decent job.The FFT is still useful for many tasks, including distortion measurements within the usual range of interest, which hardly ever exceeds -60 dBc except for high end audio. There is a reason, why specialized audio analyzers exist.You just have to take into account that the SFDR of a general purpose scope frontend might be only about 60 dB in practice – many (if not most) of the more affordable old SA boat anchors haven't been much better btw.For instance, there is no restriction for single tone narrowband measurements, like phase noise or modulation spectra, where especially the SDS2000X HD will give you an exceptional dynamic range that clearly exceeds the ~72 dB that you could expect from the 12 bits.The THD of a general purpose oscilloscope frontend is usually not specified in the datasheet. On the other hand, the Picoscope 4262 has a guaranteed linearity of 16 bits – but that is certainly not a general purpose instrument, with its 16 bit converters and limited bandwidth of just 5 MHz. Yet this is the way to go if you need to measure down to -96 dBc and don't want to be restricted to the audio frequency range like with soundcards.

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 02:09:10 pm
content: Why we've often requested & performed the classic Two Tone IMD to evaluate DSO linearity. Also why we have the PicoScope 4262, altho don't care much for instruments that require a PC to function (and prefer Macs anyway), and because of this the 4262 rarely sees use!!Since these modern DSOs are venturing outside the old oscilloscope measurement realm, we are somewhat witnessing the effects of an instrument designed to display a waveform result of maybe 40~50dB on screen range. Whereas a proper Spectrum Analyzer is designed to display a 100~120dB range, big difference with intended usage and on screen Dynamic Range between these instruments.With this in mind we are seeing the modern DSO becoming a more versatile instrument than a Time Domain waveform display device (O-Scope), and the OEMS are addressing such by providing higher resolution core ADCs, Digital Logic Analyzer capabilities and such.As Performao1 indicated the DSO input PGA, along with the core ADC are the likely culprits limiting linearity, and for the intended usage (Time Domain Linear Display) provide very respectable performance, however when pressed into the Frequency Domain (Log Display) that begins to show limitations. We must remember that the DSO FFT displays are just a mathematical representation of the Time Domain captured raw data from the ADC and thus limited by the mentioned PGA and ADC channel linearity and noise.As we see better performing lower cost ADCs and PGAs ICs come available, I'm sure we'll see better FFT performance like the Pico Scope 4262 at much higher frequency capable stand alone bench DSOs, and at affordable prices. Maybe we'll see some new type ADC architectures emerging from the research channels, we know of one (Non-Uniform Sampling), but that's another topic as we've already diverted from the OP topic.Best,

author: Detlev
author_rank: Regular Contributor
timestamp: December 29, 2022, 04:02:23 pm
content: Hello everyone,I have here a comparison of three FFT analyses. The SGD6022 is always set identically. Once I measured without an attenuator and then one with 10dB and 20dB attenuator.I would expect the spectral distribution to be identical for all three FFT measurements since the AWG was not changed. However, we see three different FFTs.From my point of view, this underlines that a DSO is only conditionally suitable for these low levels of harmonics.Many greetingsDetlev

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 05:37:00 pm
content: Quote from: mawyatt on December 29, 2022, 02:09:10 pmHere's using the SDG2000X AWG with the Wave Combine feature producing the Two Tone IMD mentioned above. AWG Ch1 is set to 0dBm at 99.5KHz, Ch2 0dBm at 100.5KHz.The SDS2000X+ was set to 50 ohm input Z and the FFT performed, see PNG 153.Then the special purpose PicoScope 4262 was used with an external 50 ohm terminator added, see Pico_AWG4.Note the added "artifacts" added by the SDS relative to the PS, and also note the difference in the IMD products as shown.This clearly shows that a typical DSO is not good enough to show the waveform quality of a quality AWG (get similar results with SDG6000X). A special purpose DSO is required to revel the signal quality of these 16 bit DAC based AWGs, even when in this case we were using the Wave Combine feature of the SDG2000X (and 6000) which likely adds a little non-linearity to the resultant waveform. Does anyone know if this combining is done in the digital domain (pre DAC) rather than at the analog post DAC output?Would expect the HD version of the SDS2000X+ with it's core 12 bit ADC to have a better result, and interested to "See" how well the new Rigol 12 bit ADC DSO behaves (we asked awhile ago for a Two Tone IMD, but no now has offered yet).Edit: Added a result from our Spectrum Analyzer with a 10dB PAD in front (almost always keep a PAD on front end, saves replacing SA front ends), see PNG18. The SA amplitude hasn't been calibrated, but the relative IMD readings are good. Note the lack of artifacts and the level of the IMD Products. It's interesting that the PicoScope shows even better IMD results than the SA, a tribute to the PicoScope and of course the SDG2000X+ AWG signal source!!Best,

author: Martin72
author_rank: Super Contributor
timestamp: December 29, 2022, 06:00:50 pm
content: Maybe I should give this thread a new name and start a new comparison thread..Serious, this topic about the limits of a scope is too important IMO..

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 06:13:22 pm
content: Well it is showing just how good these AWGs areAgree tho, the DSO FFT issue certainly belongs in it's own thread.Best

author: baldurn
author_rank: Regular Contributor
timestamp: December 29, 2022, 06:21:02 pm
content: Quote from: mawyatt on December 29, 2022, 05:37:00 pmHow do we know that the extra IMD products shown on the Pico are real?

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 06:34:03 pm
content: Quote from: baldurn on December 29, 2022, 06:21:02 pmWe don't, but they are lowest (~ -100dBc ref to 0dBm signal peaks, noise floor ~ -115dBm)) of the group of tests, so would baseline them as the resultant of the PS and AWG. Please note the SA doesn't show some of these artifacts, but the PS shows some that are below the SA noise floor ( ~ -103dBm, signal peaks is ~ -11dBm due to input PAD). Since we can't "assume" the PS contributed these, truth would be of course both contributed but we can't separate either, so safe assumption it's the AWG if one is looking for the signal source "quality".Of course if we only had the general purpose DSO results, then this would "paint" an entirely different picture of the AWG, since without another "good" measurement instrument we couldn't defer/question the DSO results. Please remember in engineering measurements, one is always looking for a source significantly better than the measuring instrument, this helps to "verify" that the measuring instrument is good for the task. Also applies to evaluating a DUT, where one wants the "input signal" to be significantly better than DUT, so the end resultant measurement is mostly due to the DUT effects and not the poor quality input signal (or measuring instrument).Best,

author: baldurn
author_rank: Regular Contributor
timestamp: December 29, 2022, 07:38:41 pm
content: IMD is very common in receivers so I could easily see all three instruments generating those IMDs internally. Maybe generate and measure each signal independently before combining them externally using a passive power combiner. If that shows the same IMD products it should be the measuring instrument doing it.I would try it but I do not have a power combiner available.

author: mawyatt
author_rank: Super Contributor
timestamp: December 29, 2022, 07:50:32 pm
content: Agree, the resistive combiner should produce the best overall IMD from the signal various sources. However, these AWGs under discussion have a built-in combiner, whether it's digital or analog don't know, but produces a Two Tone Signal without any additional components or cables, just one cable, and produces very respectable results (better than we expected) as shown by the various previous plots.As always YMMV,Best

author: blackdog
author_rank: Frequent Contributor
timestamp: December 29, 2022, 09:16:18 pm
content: Hi,I did some measurements on the function generators available here in my lab.I have limited myself to the audio region at 2V RMS output at a 50 Ohm load.These THD measurements were done with Sine signals, if time is available, I will show from the generators also the square wave representation, but I will do that in a separate post.All equipment used had been on for at least 1 hour before I started measuring.These types of measurements take a fair amount of time and I have to do this in between my work. :-)The equipment used:Hameg /R&S:HMF2525Siglent:SDG1032Siglent:SDG2042Rigol:DG4162The scope used for the THD pictures is a Hameg /R&S:HMO3004And the THD meter is an Audio Precision:Portable One PlusThe spectrum picturers are made with aSiglent SDS2104 PlusFirst the SDG1032X14Bit GeneratorThis is the display of the Audio precision Analyser and it shows the measured THD at a bandwidth of 400Hz to 80KHZ, rounded off the THD is 0.07%..This scope picture shows the measured 10KHz Sinus in the color yellow and the green trace is the residue of the Analyzer output.Do not look at the size of the green trace, this is because the analyzer output is auto scale.I only show the green trace to get an idea of what the THD looks like..The following picture shows what the FFT looks like done with the Siglent scope.Keep in mind that the table always varies a bit, especially at the somewhat higher harmonics.Keep in mind the table value of -+1dB.****************************************************************************************************************************************************************************************************************************************************************Now the Siglent SDG2042X16Bit GeneratorThis is the display of the Audio Precision Analyser and it shows the measured THD at a bandwidth of 400Hz to 80KHZ, rounded off the THD is 0.007%..This scope picture shows the measured 10KHz Sinus in the color yellow and the green trace is the residue of the Analyzer output.Do not look at the size of the green trace, this is because the analyzer output is auto scale.I only show the green trace to get an idea of what the THD looks like..The following picture shows what the FFT looks like done with the Siglent scope.Keep in mind that the table always varies a bit, especially at the somewhat higher harmonics.Keep in mind the table value of -+1dB.****************************************************************************************************************************************************************************************************************************************************************Now the Hameg HMF252514Bit GeneratorThis is the display of the Audio Precision Analyser and it shows the measured THD at a bandwidth of 400Hz to 80KHZ, rounded off the THD is 0.023%..This scope picture shows the measured 10KHz Sinus in the color yellow and the green trace is the residue of the Analyzer output.Do not look at the size of the green trace, this is because the analyzer output is auto scale.I only show the green trace to get an idea of what the THD looks like..The following picture shows what the FFT looks like done with the Siglent scope.Keep in mind that the table always varies a bit, especially at the somewhat higher harmonics.Keep in mind the table value of -+1dB.****************************************************************************************************************************************************************************************************************************************************************Now the Rigol DG416214Bit GeneratorThis is the display of the Audio Precision Analyser and it shows the measured THD at a bandwidth of 400Hz to 80KHZ, rounded off the THD is 0.011%..This scope picture shows the measured 10KHz Sinus in the color yellow and the green trace is the residue of the Analyzer output.Do not look at the size of the green trace, this is because the analyzer output is auto scale.I only show the green trace to get an idea of what the THD looks like..The following picture shows what the FFT looks like done with the Siglent scope.Keep in mind that the table always varies a bit, especially at the somewhat higher harmonics.Keep in mind the table value of -+1dB.****************************************************************************************************************************************************************************************************************************************************************Now the Audio Precision: Portable One Plus - GeneratorAnalog GeneratorThis is the display of the Audio Precision Analyser and it shows the measured THD at a bandwidth of 400Hz to 80KHZ, rounded off the THD is 0.0008%.THD with my PC and a verry good Soundcart is about 0.0002..This scope picture shows the measured 10KHz Sinus in the color yellow and the green trace is the residue of the Analyzer output.Do not look at the size of the green trace, this is because the analyzer output is auto scale.I only show the green trace to get an idea of what the THD looks like, this is almost all noise....The following picture shows what the FFT looks like done with the Siglent scope.Keep in mind that the table always varies a bit, especially at the somewhat higher harmonics.Keep in mind the table value of -+1dB..And what does the last picture, which is the FFT from de AP generator on the Siglent Scoop show us now....The Siglent scope I used, does not have sufficient capabilities to make good FFT measurements below say 50dB.The slightly newer 12Bit Siglent series scoops will do 15 to 20dB better I think, the SDS2104X is a basic 8Bit scoop with 10Bit oversampling.Later I will show some pictures of the square wave the generators can make.Kind regards,Bram

author: Martin72
author_rank: Super Contributor
timestamp: December 29, 2022, 10:41:35 pm
content: As I said, I should rename the thread..But nice work, tomorrow when the XLR plugs arrived I´ll follow with the neutrik A1.And the scope FFT pictures are nearly same looking...

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2022, 11:04:37 pm
content: Quote from: Martin72 on December 29, 2022, 10:41:35 pmYou can, just go back to the OP and do it there with a Modify.The topic URL remains the same and we will all see new posts flags and/or get notifications if we are setup for them.

author: Martin72
author_rank: Super Contributor
timestamp: December 29, 2022, 11:51:09 pm
content: I know that, but I have no idea yet what the new title might be.Currently, two threads are "hijacked" by the FFT thing..

author: rf-loop
author_rank: Super Contributor
timestamp: December 30, 2022, 06:43:25 am
content: Quote from: Martin72 on December 29, 2022, 11:51:09 pmLeast one reason is here:Quote from: Martin72 on December 26, 2022, 08:22:32 pmI think it was very important to handle this FFT thing because this kind of instruments FFT is wrong tool for evaluating these said generators signal quality by measuring, for example, THD and comparing them.The result is almost garbage.Which has now been proven in this context as well.

author: Performa01
author_rank: Super Contributor
timestamp: December 30, 2022, 10:40:04 am
content: Quote from: mawyatt on December 29, 2022, 07:50:32 pmThe question about the combining method has been dealt with multiple times already, see for example reply #159 here:https://www.eevblog.com/forum/testgear/two-tone-test-with-scope-and-sa/msg4250476/#msg4250476I am amazed about your stellar results. Maybe Siglent have been able to achieve some improvement for this function since the last time I've checked it...

author: blackdog
author_rank: Frequent Contributor
timestamp: December 30, 2022, 01:57:29 pm
content: Hi,In order to get a good understanding about the THD of different function generators,I started up my measurement computer to do some measurements with a very good Soundcard, this is the:M-Audio Delta AP 192.The yellow dashed line in the FFT window, indicates the bandwidth within which the THD is measured.The Sinus of the scope picture does not look very clean, this is partly due to the RDC session I use to operate the measuring computer, so these are image artifacts and not a problem with the test signal.The orange box shows the RMS level of he measurement signal.For each generator this is set to 2V RMS, there are some exceptions, which I will indicate later.Also a good sound card has a "sweet spot" this is only important when measuring the generator distortion of the Audio Precision measurement set.All my function generators are well above the distortion and noise of the sound card.The settings of this soundcard are these:192Khz sampling24-Bit1-ChannelFFT = 1048576 pointsWindow = Kaiser-7Averaging = 4xMeasuring software =Multi-Instruments pro 3.9============================================================================Let's start with the Siglent SDG1032x just like the previous measurements, here the measurement signal is 10KHz Sine at 2V RMS.============================================================================The bigger brother Siglent SDG2042x just like the previous measurements, here the measurement signal is 10KHz Sine at 2V RMS.============================================================================This is the HMF2525, measurement signal is 10KHz Sine at 2V RMS.============================================================================This is the Rigol DG4162 Chanel-1, measurement signal is 10KHz Sine at 2V RMS.============================================================================This is the Rigol DG4162 Chanel-2, measurement signal is 10KHz Sine at 2V RMS, the distortion of this channel is lower...============================================================================This is the generator output of a OWON SDS272s, measurement signal is 10KHz Sine at 855mV RMS, the max. level.============================================================================This is the generator output of a Silent SDS2100x plus, measurement signal is 10KHz Sine at 1.05V RMS, the max. level.============================================================================And now to show the limits of the measurement system two measurements of the distortion of the generator in the Audio Precision measurement set, the first measurement is again 10KHz and then performed now with a measurement signal that is the "Sweet Spot" of the total system, the level is just below 3V RMS.============================================================================And de last one, now at 1KHz, Sweet Spot is 2.5V RMS============================================================================Good, now it's time to start working on my business administration again, which I like a lot less, but it has to be done.Kind regards.BramPSThanks mawyatt for finding the error, yes SDG1042X should beSDG2042X!, my bad

author: mawyatt
author_rank: Super Contributor
timestamp: December 30, 2022, 02:36:36 pm
content: Bram,Nice, you have a few AWGsIs that a SDG1042X or should it be a SDG2042X ?Best

author: mawyatt
author_rank: Super Contributor
timestamp: December 30, 2022, 02:51:27 pm
content: Quote from: Performa01 on December 30, 2022, 10:40:04 amYeah, kinda figured there was a previous thread(s) on this, but memory is fadingNo complaints here about these SDG2000X & SDG6000X AWGs, altho one might complain about the UI, but we're more concerned/interested in the actual output and why we purchased such. As has been shown by many, these AWGs are pretty good in that respectBest,

author: Martin72
author_rank: Super Contributor
timestamp: December 30, 2022, 05:36:47 pm
content: So..I´ve made tests with both gens on my neutrik A1 analyzer(First time use since buying..).First: Don´t laugh about the pics, it´s display didn´t got a backlight function, so I must use a pocket torch...Something I have to upgrade on the A1, no doubt, also I should make the software running on a PC of today..Pics are showing THD+N at 10Khz.Interesting: Using an isolated transformer decreases the THD on the 1062X, but not on the SDS2122X.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 30, 2022, 06:07:35 pm
content: Hi Martin72Interesting: Using an isolated transformer decreases the THD on the 1062X, but not on the SDS2122XThis is where learning how to make reliable measurements begins, commonmode problems ar a Bitch!I have several isolation transformers and various battery powered measuring instruments here, this is to minimize common mode interference, for reliable measurements always double check is my advice.Kind regards,BramPSI have tuned a lot of tape recorders and other equipment with a Neutrik A2.

author: Martin72
author_rank: Super Contributor
timestamp: December 30, 2022, 06:25:03 pm
content: QuoteNeutrik mentioned it in the manual ( using earth-free connection) so I thought ok give it a try.As written, nothing changed on the 2122X, but 1062X.Neutrik also explaining in the manual, how THD will be measured in the A1.A notchfilter filters out the fundamental frequency.

author: rf-loop
author_rank: Super Contributor
timestamp: December 30, 2022, 07:12:20 pm
content: Martin, you can try one small thing with SDG1000XLoad 50ohm.1kHzSet next SDG displayed output levels exactlyThese are only some examples.111.8 mVrms* look what THD you get.Voltage level band change here (you can hear relay)111.9 mVrms* look THD353.4 mVrms* look THDVoltage level band change here (you can hear relay)353.5 mVrms* look THD1.118 Vrms* look THDVoltage level band change here (you can hear relay)1.119 Vrms*up to 3.536Vrms**Listen relay! for make sure V band border.I do not have any idea why it change some times example 353.4 or .5 and 353.5 or .6 and so on (in other V band change same, some times just tiny amount different)Example just with my Keithley 2015 THD what is not so stellar but better than nothing. Yes I have many tables done long time ago but they are only on paper notebook.(Setup for this Kethley 2015 THD: Mode Just plain THD (not THD+N) , freq.set manually 1kHz (instead of auto tune), Harmonics: up to 9th, SFIL:NONE (available NONE, A, C, CCITT, CCIRARM), Average 100rdgs repeating (front panel button Filter, Keithley input feed thru 50ohm.Examples353.4 Vrms* my Keithley display 0.0196% (this voltage band bottom and lowest THD) one V band top value353.5 Vrms* my Keithley display 0.0918% (this voltage band top and highest THD ) next V band bottom valueThis THD changing behavior repeats through all level bands (but naturally with bit different numbers).So THD is not just some value... it is many different values depending many things.If user know his generator he can perhaps some times select better signal quality. Just example if want use 4dBm. Use 3.97dBm and THD is better. THD is better when use internal voltage bands top values. And most bad when use just internal voltage bands bottom values.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 31, 2022, 11:22:10 am
content: Hi rf-loop,Thank you very much for explaining about the difference in behavior due to the output level of a function generator.I wanted to do this in the third part based on the aberrations that occur on the flanks of the square wave signals from the function generators I have here.The variation in THD also occurs with analog sine wave generators when additional gain stages are turned on or off there.But the differences are usually much smaller.My setup for the many measurements I have chosen (2V RMS at 50 Ohm load) where possible so that you have a bit of certainty,that 2V RMS you can then externally with a potentiometer or with 50 Ohm attenuators back to a lower level, but the "Sweet Spot" of every function generator can and wil be different.Look at my Rigol DG4162 chanel 1 and 2 distortion levels, same level, frequency but 40% different THD, probably has at to do with the linearity of the DA converters used in the DG4162 function generator.This can happen with all function generators with two or more outputs.In the measurements I showed here I assumed the audio frequency range.The distortion that occurs with the multiple function generators I have is fairly constant over the audio range.This is because the design of most function generators covers a much larger frequency range than the audio band and the opamps used i n the amplification stages are very broadband.Distortion meters that cannot show a residual always give uncertainty about the indicated value of THD.All THD meters implemented with some form of NOTCH filter almost always show the THD and many other forms of residue that have no relation to the test signal.Many analog THD meters therefore have a number of filters built in to provide more certainty about the displayed THD value.Often it is also referred to as THD + N which stands for Total Harmonic Distortion and Noise.I purposely showed the original signal in yellow in my measurements on the scope photos with the output of my Audio Precision THD function in green.So that a reasonable estimate can be made of what the residue looks like.I am aware that a scope picture of the residue can poorly show that e.g. a lot of 1/F noise is present.That is what the measurements I did last with my measurement computer and the proper sound card are for.The user of one function generator will have to look carefully at what the datasheet says in terms of distortion at a given frequency range.Almost always the distortion data are better than specified.And as rf-loop points out, by playing with the output level you can often get the distortion lower because then there are less amplifier stages/relays in the signal way and the amplifier stages are more optimally in their working range.But be aware that if you generate e.g. signals around 10MHz, then the DA and the amplifier stages will have more errors and the distortion will be much higher because of e.g. the slew rate of the opamps used in the output section of the function generator.As almost always, figure out what your good and not-so-good points are of your used equipment and think about those points when you start taking measurements.Several times I have done measurements on amplifier stages driven by the HMF2525 function generator which is very clean for a 14Bit model, see computer FFT for this.And I was expecting during the measurements I was doing what a THD around 0.002% or less and I was stuck at 0.02% THD.I had forgotten several times that I was using the THD meter in the Audio Precision, but the HMF2525 as a generator, before I made the THD measurements,I had tested my D.U.T with a square signal from the HMF2525, for measuring low THD, I had forgotten to switch generators....Brief conclusion regarding audio measurements with a sine signal.The Siglent SDG1032X is quite useful for tube equipment, this is because with most tube equipment the THD is quite high.For significantly lower THD, it is better to choose a Siglent SDG2042X, because this generator has a 16Bit DAC.But please note that I am talking about sine wave signals, the SDG1032X has a very good point and I will soon show it in measurements of pulse signals from the function generators.This time i do not remove the text below, now you now why "some times" mijn Englis is that bad.Translated withwww.DeepL.com/Translator(free version)Kind regards,Bram

author: rf-loop
author_rank: Super Contributor
timestamp: December 31, 2022, 11:57:16 am
content: Here is one example.Level do not change nearly anything but SDG change internal "voltage band" (whole range have many voltage bands)I take here just one example. Previous one level band ends (highest in this band) at 353.5mVrms and next step go to next band most low level what is 353.6mVrms. (both are very close 1Vpp and roughly 4dBm (bit under).I do not have good and steep filters for audio frequencies so my SA noise level and max input level give some limits what I can do. If I have good filter I can go more down because of course there can find more harmonics and also some spurs. Also Spectrum analyzer mixer level need keep enough low for keep its own harmonics down enough. So in this case -110dBm level is border.After generator output there is external attenuator set for drop SA input level to -25dBm. And because SA internal Att is 0dB "mixer level" is -25dBm (this SA is specified for -30dBm mixer level but with my cross check this 5dB did not give any readable difference in harmonics levels in this case. It can use higher mixer levels but I want keep SA own generated things low)It is fun to see how much harmonics levels change. Yes it is natural if look how SDG works and not go deeper inside now.But fore some users this behavior is good to know.And, this around same happen between every internal voltage band change. This is just one of them.Note that these THD % are calculated using only 1st (fundamental), 2nd and 3th harmonic levels

author: Martin72
author_rank: Super Contributor
timestamp: December 31, 2022, 05:40:19 pm
content: Quote from: rf-loop on December 30, 2022, 07:12:20 pmSetup: SDG1062X with 50 ohm load termination connected to the A1.Values with * = relay switch point111.7mVrms 0.044%111.8mVrms* 0.117%..353.4mVrms 0.043%353.5mVrms* 0.116%..1.116Vrms 0.042%1.118Vrms* 0.114%..1.2Vrms 0.110%1.4Vrms 0.100%1.6Vrms 0.091%1.8Vrms 0.083%...2.9Vrms 0.050%3.0Vrms 0.048%3.056Vrms* 0.047%3.535Vrms 0.041%On every "relais switch point" THD increases, then decreasing until the next switch point with one exception: The last switch point won´t be affected(or it is a different relais from a different circuit).Interesting things, thank you rf-loop

author: rf-loop
author_rank: Super Contributor
timestamp: December 31, 2022, 06:35:40 pm
content: Quote from: Martin72 on December 31, 2022, 05:40:19 pmAre these THD (Not THD+N) values plain THD without filtering and how many harmonics included in counting. (looks bit high values if plain THD and No filters.)

author: Martin72
author_rank: Super Contributor
timestamp: December 31, 2022, 06:48:04 pm
content: Hi,No filters were set (forget, there is a high-pass filter from 400Hz and a band-pass filter that lets through everything from 22Hz to 22Khz), the A1 has only THD+N (IMHO), bandwidth up to 130Khz.The data sheet of the SDG1000X series gives a THD of 0.075% (10Hz...20Khz), the measured values are significantly lower.

author: rf-loop
author_rank: Super Contributor
timestamp: December 31, 2022, 07:07:20 pm
content: Here my some older measurements and also some today data (perhaps these need do again, data partially from very old paper notebook.)Revised, supplemented and updated table.Plain THD (Not THD+N) %SFILT: NOHarmonics up to 7th.Signal 1kHz.Measured only using 50 ohm load.Meter is Keithley 2015 THDEdit: image changed. New revised image:

author: markone
author_rank: Frequent Contributor
timestamp: December 31, 2022, 08:06:18 pm
content: Quote from: rf-loop on December 31, 2022, 07:07:20 pmI would say that those numbers are great for a generator that goes up to tens of megahertz @ few hundreds of euros, i personally own a SDG2042x "upgraded" and i never expected a spectral purity at low frequencies on par with fancy audio 24 bits DACs for the simple reason that it's not its job.At the same time i do not expect that a 12 bits DSO is able to provide an actual dynamic range over 73 dB, regardless of any mathematical process in place, at this frequencies a good soundcard does a much better job.

author: Martin72
author_rank: Super Contributor
timestamp: January 01, 2023, 01:02:18 am
content: Quote from: rf-loop on December 31, 2022, 07:07:20 pmHi,The THD specification of the A1 says nothing about the number of measured harmonics, but the measuring range is given, 20hz to 40Khz.Whether this also means that at a fundamental frequency of 1Khz it also measures up to the 40th harmonic, I don't know.I will ask Neutrik."Tomorrow" I repeat the measurements, then with the 20Khz filter, possibly the values decrease.Either way, both measurement series are far below the specifications mentioned - a very good result for such a "cheap" device.

author: rf-loop
author_rank: Super Contributor
timestamp: January 01, 2023, 09:41:25 am
content: Quote from: Martin72 on December 31, 2022, 06:48:04 pmSDG1000X_DataSheet_DS0201X_E01I:QuoteNote that Min or Typical is not listed, only Max.Example I have not at all tested all frequencies between 10Hz to 20kHz.And then in datasheet introduction and advertising text:Quote0.15% is specified only up to 20kHz!Some may read this text so that 0.15% is also for entire badwith, if not calculate what this -40dBc mean.Example if look harmonics levels graph there can find that 60MHz have THD roughly 0.75% if look (only) 2nd and 3th levels alone.If look again this image and there 100kHz 2nd and 3th levels it give roughly ~0.05%But then, there is also some source for mess when we talk about THD.Example your A1 do not measure harmonics itself at all. THis is why it do not have THD mode at all, it have only THD+N bacause its working principle and this N also include noise but also all non harmonic spurs of course... everything in band except fundamental filtered away using notch filter, if I understand its user manual right.It looks like it just measure whole all in its bandwidth just with notch filter over fundamental. So there is all, noise, all spurs and all harmonics.And example Keithley 2015 measure harmonics. Because THD. It is TotalHarmonicsDistortion. If there is 2kHz signal and some spur in 5kHz. It is NOT harmonic at all. It is other tone, not part of pure THD.There can also be some filters, as example Flat aka NONE and then C, CCIR A, CCITT, CCIR ARM etc but these available in Keithley.With every setup I get different result. Also depending what all harmonics are counted. (Meters like A1 can not select harmonics at all. It can not measure harmonics, lt looks like it measure only just whole all in band. (explained in its manual)Then, example Keithley have level ranges 100mV, 1V, 10V etc. Result also depends if test signal is near used range full scale or near bottom before lower range. (bit same as with SDG voltage bands behavior relative to THD)Keithley specification say 0.004% residual distortion for THD and individual harmonics. THD+N it say 0.056% (-65dBc) residual .When someone tell just "THD" we need ask "what THD?", how measured and including what - if want avoid mess.Btw, if I set Keithley for max harmonics up to 20kHz for 1kHz test signal and I select THD+N and filter CCIT ARM I get around same "THD" values as you. (just fast looked only with two levels)Quote from: Martin72 on January 01, 2023, 01:02:18 amIt is somehow told in its manual. It looks like it have nothing for measure individual harmonics. This is why it have only mode THD+N. It have different working principle. Roughly saying: It measure fundamental and measure what is total without fundamental in measuring bandwidth. THD+N, so it measure all harmonics distortion and added with (+N) all non harmonics and noise in band.If I set signal 1kHz and level SDG1000X voltage band IV top level (3.978dBm)Then Keithley for THD+N, harmonics up to 20th and No filter aka flat. 0.0355 % (100 rdgs average)Exactly same setup but THD alone: 0.0208 %Just for fun:Then continue same THD alone with CCIR ARM weighting filter: 0.0518 %Then same but THD+N with CCIR ARM weighting: 0.0565 %

author: Martin72
author_rank: Super Contributor
timestamp: January 01, 2023, 11:41:20 am
content: Hi,The A1 is an all in one audio measurement system that can also measure tape decks, not a dedicated harmonic analyzer.And as always when you have everything in one device, sometimes something may be omitted.But what it does, it does well.QuoteI got this here:https://www.batronix.com/files/Siglent/Funktionsgeneratoren/SDG1000X/SDG1000X_Datasheet_EN.pdfWhere the 0.075% for 10Hz....20Khz is stated.

author: blurpy
author_rank: Regular Contributor
timestamp: January 01, 2023, 11:58:14 am
content: I did some THD measurements using a 24 bit audio analyzer a while ago, of the SDG2000X:https://www.eevblog.com/forum/testgear/the-siglent-sdg2042x-thread/msg3606484/?topicseen#msg3606484It does quite well I think.

author: rf-loop
author_rank: Super Contributor
timestamp: January 01, 2023, 01:52:03 pm
content: Quote from: Martin72 on January 01, 2023, 11:41:20 amYes A1 is nice "all in one" instrument for repair-tune-service etc audio things.It is shame Batronix still display old obsolete data sheet from year 2017.Even TrueArb feature is missing.(but yes THD is there Max 0.075% for 0dBm for 10Hz to 20kHz)I can go down to 20Hz and Keithley tell THD 0.0293 % (100 rdgs average)And up to 16kHz if want include 3th harmonic. (bacause it have 50kHz band max for harmonics, so with 16kHz it can still calculate 2nd and 3th): Result 0.0136%Now need note thgat my SDG1000X is old when this old datasheet have been ok.Why Siglent have changed this THD specification to double.What is your SDG1000X hardware version?If some one have more new HW and have possibility to measure somehow reliable THD... plese.I'm curious to see if something has changed that would explain the change in specs.

author: Martin72
author_rank: Super Contributor
timestamp: January 01, 2023, 04:32:33 pm
content: Hi,Quote02-01-00-24-00Interesting that this specification has changed so "drastically" in the last 5 years.

author: rf-loop
author_rank: Super Contributor
timestamp: January 01, 2023, 05:21:36 pm
content: Quote from: Martin72 on January 01, 2023, 04:32:33 pmWe have same HW.I also wonder what have lead to this change.I hope that some of the users who have latest HW can do comparable tests so that we can see if there have really been significant changes in this matter, or if the specification has just been changed due to some borderline case so that no one can complain under any circumstances that it does not meet the specifications. .

author: pdenisowski
author_rank: Super Contributor
timestamp: January 02, 2023, 06:23:32 pm
content: Quote from: rf-loop on January 01, 2023, 09:41:25 amAgreed. This is how I would describe it as well:THDis only the power in the (sum of N) harmonics. The power of each harmonic is individually measured with a sufficiently narrow resolution bandwidth to minimize power from noise, spurs, etc. In this case you have to specify the fundamental frequency, the number of harmonics you want to include in the measurement, and the starting RBW (this increases in a predictable way for each harmonic, so only need to specify it for the fundamental).ForTHD+N,you essentially measure all power (harmonics, spurs, noise, etc.) over a bandwidth with the fundamental notched out (again, as narrowly as possible). THD+N requires that you specify the bandwidth of interest. Traditionally, THD+N was an easier measurement to make than THD, and I often see people confusing THD and THD+N.I recently did two videos on this. They are primarily focused on spec ans and RF (not AF), but the basic concepts are still the sameUnderstanding Harmonic Distortion MeasurementsMeasuring Harmonic Distortion with the FSW (which talks about the role of RBW)

author: Martin72
author_rank: Super Contributor
timestamp: January 02, 2023, 08:01:28 pm
content: QuoteTo me it sounds more plausible to measure everything than "only" the harmonics - Or do I have a thinking error?BTW, here is an excerpt from the service manual on how to perform the measurement(pic below).(The Googledrive link with the docs for the A1 is still active - If it´s interesting I could post it)

author: RoV
author_rank: Regular Contributor
timestamp: January 02, 2023, 09:09:33 pm
content: Hi, I share also here results of a measurement I posted under the topic "Re: Siglent SDG1032X sine distortion at 1 kHz ?"Made a test on my SDG2042X (ok, 2122X).Used an E-MU 0202 connected to my PC, acquiring ~20 s at 48000 Hz sampling, 24 bit resolution. Generator at 1 kHz sine, 1 Vpp output. Acquisition gain set to have about 40% level on sin peaks.Processed with Welch method, using a Blackman window of length 4800 to minimize sidelobes and 50% overlap between data chunks: result in figure, with fundamental normalized to 0 dB.2nd harmonic is at -103.5 dB, 3rd at -105.5. Worst is 10th at -101.9 dB.THD, computed up to the 10th harmonic, is -97.5 dBc, or 0.0013%.

author: Martin72
author_rank: Super Contributor
timestamp: January 02, 2023, 09:54:33 pm
content: Looks too fantastic...Seriously, good thing.@rf-loop:Concerning thd specs of sdg1000x: I´ve read the specs of the sdg2000x, in the old sheet (2017) and in the new, a thd of 0.075% is mentioned.Same as in the specs of the sdg1000x in 2017...

author: pdenisowski
author_rank: Super Contributor
timestamp: January 03, 2023, 01:26:52 am
content: Quote from: Martin72 on January 02, 2023, 08:01:28 pmNo, that's absolutely correct: measuring "everything" over a bandwidth (minus the fundamental), i.e. THD+N, is relatively easy: just notch the fundamental out and integrate over the bandwidth, so to speak.THD (just the harmonics, not noise, spurs, etc.) was a lot harder to measure before modern spec ans with automated measurement personalities. You first have to measure the power of the fundamental - this is usually done in zero span mode with a RBW just slightly wider than the signal. Then you have to change frequency to each harmonic (calculated based on the frequency of the fundamental) and measure those in zero span. But since the width of the harmonics grows with harmonic order, you have to scale up RBW at each harmonic to make sure you're measuring the entire (wider) harmonic signal.But again, modern spec ans with an automated harmonic distortion measurment function can do all of this very easily -- measuring THD "by hand" can be very time-consuming and error-prone. I would say that THD is actually an easier measurement than THD+N (at least with a spec an). THD+N requires a tunable notch filter, whereas THD with a modern spec an doesn't require any additional hardware.It might also be worth mentioning that THD+N is essentially the inverse of SINAD, which is a measurement often supported on audio and some radio testers, but not common on RF spectrum analyzers.

author: Performa01
author_rank: Super Contributor
timestamp: January 03, 2023, 05:11:19 am
content: Quote from: Martin72 on January 02, 2023, 08:01:28 pmI guess so…Of course it used to be a lot easier to measure THD+N, as it just required an RMS voltmeter and a notch filter. This is how the traditional distortion meters work. No way to measure (and calculate!) THD without a spectrum analyzer.But just because something was much easier to accomplish in the past, it is not automatically more plausible as well. You might think so as an end user ("I don't care what it is, it just doesn't belong there!"), but even as an end user you are affected differently by the different unwanted signals.Consider the most popular area where distortion is a big thing: Audio. So you have, say, -50 dBc THD+N and you think this is fairly bad and it does not matter where all the unwanted signals stem from. But:• If it's pure noise, then your signal is essentially distortion-free but the signal to noise ratio is just 50 dB. Some folks won't even notice that noise, at least not with low dynamic pop/rock music…• If it's pure harmonics, then you might notice it because of a slightly altered sound. If the harmonics happen to be predominately even numbered, then the audiophools will rave about the "warm and fuzzy" sound as they are used to it from their single ended tube amplifiers - using a (highly linear) light bulb as a pullup resistor• If it's only spurs, then it will sound rather disturbing and folks who wouldn't have taken notice in one of the two previous cases might start to complain all of a sudden.So even the end user would prefer separate specifications for THD, S/N and spurs.For the designer of such gear it's of course all the more important to know where the unwanted signals come from. Needless to say that the investigation as well as the final countermeasures look very different for the three different types of unwanted signals mentioned before.

author: rf-loop
author_rank: Super Contributor
timestamp: January 03, 2023, 06:28:22 am
content: THD is Total Harmonic Distortion. It is just it, nothing else. It do not include any other things but harmonics (naturally there is never absolutely ideal meters).When we measure other things, example harmonics and non harmonics and noise this is not anymore THD. It is THD+N where N include everything (if not more specified) so why we even talk then about THD (if not just for make things messy), it is more like TD (total distortion).Problem is that if we have measured THD+N and result is example 0.1 % and then we tell "THD" is 0.1%. It is then just basic lie.Also there is other things. One important is band width we have used for measurement or example what all harmonics we have included to measurement. Also some other things what make this whole "THD" thing quite messy - if we do not use right name and include other parameters. Some may example use also some weighting filter what may affect quite lot.Many times peoples talk just about THD. I will repeat this. After someone tell to you that some equipment, example audio amplifier or signal generator have THD x%.If not told anything more than "THD", then many times may need ask this question: "What THD or is it something else?"

author: Performa01
author_rank: Super Contributor
timestamp: January 03, 2023, 09:17:23 am
content: Quote from: rf-loop on January 03, 2023, 06:28:22 amI like this term (TD)! Very appropriate!Quote from: rf-loop on January 03, 2023, 06:28:22 amYes. If a more sophisticated spectrum analyzer calculates THD for us, it takes he number of the highest harmonic as a parameter. The traditional distortion analyzers usually have a fixed bandwidth, around 80 kHz as far as I remember. But with the many alternative solutions today, like soundcards plus software, we get rather dubious conditions. A soundcard that can provide 192 kSa/s could maintain an 80 kHz analysis bandwidth, but we don't know what analog (AA) filters are there in place to reduce the bandwidth to something more suitable for audio. Ultimately, there are many soundcard solutions where the attempt of measuring THD at 20 kHz would just be a joke because of insufficient bandwidth. Yet serious designers of audio gear want to know the distortion at 20 kHz, because they have to specify THD over the whole audio range – and at 20 kHz distortion figures usually don't look that good anymore (if measured correctly)…Quote from: rf-loop on January 03, 2023, 06:28:22 amI hate weighting filters in general. No wonder, they are supposed to make measurements representative for the user experience, but are confusing and even misleading for the designer. They'll just get abused by the manufacturers of audio gear in order to make the banner specs look good.Back in the mid-seventies of last century I was looking for a decent tape recorder. There have been many brands back then and I compared the specs as I wanted the highest possible S/N performance. Of course I had a favorite as well, and this machine specified about 55 dB S/N at 19 cm/s. Of course this was a weighted value after some standard of the DIN 45500 family.One year later, in the newest catalog, they suddenly specified 62 dB under the same conditions. Only in the fine print you could find that it was now measured after IEC-A or something –I don't recall exactly anymore. In any case, I was a young teenager back then, overlooked the fine print and thought "wow, they have improved their machine by quite a bit!" – and you guess it, the new numbers went into the comparison table and made this machine the winner.

author: Martin72
author_rank: Super Contributor
timestamp: January 03, 2023, 10:49:29 am
content: Thankyou guys, interesting stuff as always..Seen through the test field glasses, the matter would be relatively simple:Target : Max. 0.15% THDActual : approx. 0.04% THD(+N)Approx. 0.02% THD onlyTest passed...But it is of course very great to look behind the scenes at what is behind the measurements.But the thing with the 0.15% is still suspect.Sure, rather too much than too little - But that the measured values are so drastically below....

author: Martin72
author_rank: Super Contributor
timestamp: January 03, 2023, 06:36:03 pm
content: Just for curiosity I´ve measured our rigol DG1022Z with the Lecroy HDO6034A.Using it´s spectrum option.THD according to the spec 0.075% (seems to be a magic number...) 10Hz...20Khz.What I really like on the HDO is the performance, it´s soo fast in everything...Nevertheless, pics below.

author: markone
author_rank: Frequent Contributor
timestamp: January 03, 2023, 07:10:42 pm
content: Quote from: Martin72 on January 03, 2023, 06:36:03 pmJust out of curiosity, how much money did your company spend for this piece of jewelry ?

author: Martin72
author_rank: Super Contributor
timestamp: January 03, 2023, 07:35:11 pm
content: If I remember it right, a little bit more than 13000€.Including the poweranalyzer option, spectrum analyzer option and severeal serial decoding options.It makes always sense to buy by lecroy directly, together with the waverunner 9054 we got 42% discount and payed appx 26k for both together.And the performance in general is really good by both.They´re pc-based systems (Intel core i5 with 4x3.2Ghz, 16GB RAM, windows10), multigrid display, etc., tons of upgrades avaible (SSD, more RAM, more memory, triggers, math, decoders and so on).And the UI is superb, having the both everyday as "references", siglent did the job with the 2000x+ and HD very well.The HDO is my personal favorite at work.

author: markone
author_rank: Frequent Contributor
timestamp: January 04, 2023, 01:08:43 am
content: Quote from: Martin72 on January 03, 2023, 07:35:11 pmI remember a smooth behavior also with an old 64Xs around 2008, it has been my favorite for 4 years (together a CP150 and ADP305) until i changed job, now it's time that i do not see/touch one and judging how things are getting here I think it will be unlikely that will happen again ...

author: Martin72
author_rank: Super Contributor
timestamp: January 05, 2023, 10:33:13 am
content: Quote from: Martin72 link=topic=357170.msg4612261#msg4612261Answer:Quote

author: rf-loop
author_rank: Super Contributor
timestamp: January 05, 2023, 12:15:00 pm
content: Quote from: Performa01 on December 29, 2022, 09:50:57 amReferring to the previous one.It is really good to keep in mind the mixing results generated by the oscilloscope itself, especially when working with FFT settings where the processing amplify is high and we are looking at weak signal levels, such as, for example, rather low harmonics and nonharmonics spurs of the signal source being examined.Here is just small part of observations.In the test, I looked at the dual tone signal.Throughout the test, the signal from the signal generator was kept completely unchanged. . The only variable was the CH1 V/div setting on the oscilloscope. I did a test for all possible values between 40mV/div to 100mV/div. Both signals had a level of about -12dBm. Signals freq. 101.3kHz 158mVpp and 103.3kHz 158mVpp. At this level, dual tone peak-peak total is close to full scale (at 40mV/div).I have full set of results but here just few of them. (there is really lot of changes when go through all possible fine steps, just one fine step and change may be over 10dB.. 15dB in 3rd order IM products)img 71 just 100mV/div and img 21 just 50mV/div.In images 17 and 35 there is perhaps most high and most low 3th order im levels (64dBc and 84dBc, 20dB change is quite big)image 40 some kind of one just random medium example.ThenLast image without number is just how this dual tone looks in time domainETA: Added image. With spectrum analyzer exactly same signal what was used with SDS2504X HD images.This somehow proof that dual tone from generator is least not worse than in this SA image. (for avoid SA own generated distortion Attenuator was set for 23dB so that mixer level stay enough low for keep SA's 3rd order IMD as low as possible. SA's performance is not enough for analyze SDG 1000X in this matter.

author: blackdog
author_rank: Frequent Contributor
timestamp: January 05, 2023, 01:10:20 pm
content: Hi rf-loopYour measurements are a good example of what I often say on forums,know your instruments!In one of my posts I mentioned that I used 2V RMS as the output signal and then terminated with a 50 Ohm termination resistor.This is because with most generators, THD is fairly favorable and then you can then attenuate externally as needed.This level may of course just not good for the particular generator used, therefore know your instrument!I have tested it on five of my function generators where possible at the 2V RMS.And then you get the second step that you will have to perform as a user, this as you did with your last measurements, which area of the input attenuator from here the osciloscope is the most linear.There are different parts in the osciloscope where the gain can be controlled, these include these, relay input attenuator, " Voltage Control Amplifier" and digital.So then you can also have a fairly large difference between the steps when you look at the THD and as in your last measurements at the third order products.It's quite complex to take all those variables into account....Technicians who do a lot of measurements with a Spectrum Analyser are used to using e.g. a passive attenuator to attenuate their generator signal in steps to see if they do not overload the input of their Spectrum Analyser.I think most users of modern oscilosscopes are not always used to that when doing FFT measurements.I think it's good to bring this to people's attention here.Thanks for the nice measurements.Kind regards,Bram

author: pdenisowski
author_rank: Super Contributor
timestamp: January 05, 2023, 02:04:12 pm
content: Quote from: blackdog on January 05, 2023, 01:10:20 pmYes. I worked as a field applications engineer in test and measurement for > 20 years and this was my experience as well.It bears repeating that whenever making any type of "distortion" measurement (IP3, THD, etc.) it's very important to ensure that the measured distortion is not being created within the measuring instrument itself.

author: Performa01
author_rank: Super Contributor
timestamp: January 05, 2023, 03:30:49 pm
content: Quote from: rf-loop on January 05, 2023, 12:15:00 pmYes, that was my observation as well. Sometimes I have cheated a little and taken advantage of this when presenting test results in the following thread - especially when I was talking about "sweet spots"https://www.eevblog.com/forum/testgear/two-tone-test-with-scope-and-sa/

author: mawyatt
author_rank: Super Contributor
timestamp: January 05, 2023, 05:17:20 pm
content: Since the DSO generally isn't designed as a Spectrum Analyzer, think what rf-loop has shown with the Linearity dependance on actual Input Signal Chain Gain/Attenuation Distribution is an important point in our DSO usage as a SA (FFT). These DSOs are really Data Acquisition Systems, and a SA is just input signal data processing, same as with a 'Scope usage. However, a Scope doesn't really need much Display DR or Linearity beyond 1% (~40dB) because we can't really "see" the effects on the display since this is a linear display, however when utilized as a SA in FFT Mode the display is now in dB and thus much higher on screen overall signal amplitude dynamic range, and subtle signal artifacts begin to show as do measurement instrument non-linear channel effects.The quality modern DSO must possess a higher Signal Channel DR for use with the FFT Mode, and this imposes a higher initial design and build cost.Overall Signal Chain "Sweet Spot" is an interesting topic. There was a unique class of amplifiers we employed long ago that had very interesting properties, one of which was the linearity "Sweet Spot" with a theoretical infinite dynamic range. We utilized very fast SiGe 400GHz Ft devices and later planned on >600GHz Ft InP devices for implementing special purpose custom ASICs. One of these had another unique property with gain expansion rather than compression with increasing input signal levels, all of these prior efforts were to try and "squeeze" as much DR from a given supply rail as possible and thus were operated mostly in current domain. So we were trying to place the Input signal within the "Sweet Spot" of the Signal Channel Dynamics to allow further signal processing, similar to what has been shown by changing the DSO Gain/Attenuation Distribution.Best,

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 05, 2023, 07:18:50 pm
content: If you have aharmonic distortion analyzer(or tuneable notch filter), then it is incredibly useful to use the oscilloscope to view theoutput signal from the THD analyzer,which contains the distortion and noise components alone, without the fundamental. Today many digital 'scopes can perform FFT which will clearly display the amplitude of each harmonic. The first time I saw an FFT of THD analyzer output (in 1978!) I thought"this explains why some amplifiers sound better than others". But at that time FFT analyzers were strictly unaffordable for ordinary people like me. Times have changed!In terms of perceived audio quality: The best sounding amplifiers have mostly 3rd harmonic (3x fundamental frequency), with equal or lower 2nd harmonic. Higher order harmonics should drop off rapidly. Ideally the levels of 4th, 5th, and 6th order harmonics are -60dB (or lower) compared to the fundamental. The best amplifier circuits have unmeasurably low levels of 7th and higher order harmonics, at least -80dB below the fundamental. There shouldneverbe any high order harmonics with levels above -20dB compared to amplitude of 2nd or 3rd harmonic.Another"THD analyzer with oscilloscope"view which I have used for many years is toput the oscilloscope into X-Y mode.This is extremely useful even when using only amodest qualityTHD analyzer (Examples: HP 331A, 332A, 333A, 334A, Heathkit IM-5258).Apply the output signal from the amplifier under test to X axis.Apply output signal from THD analyzer to Y axis.Ideally the X-Y display will be a flat lineuntilthe amplifier under test begins to clip.The onset of clipping will immediately raise large vertical components at the extreme left and right edges of the X-Y display.More important,"crossover notch distortion"will appear as a single vertical line at the middle (0V) of the X axis. Crossover notch distortion isextremely audible, even in tiny amounts. Manyearlytransistor amplifiers hadplentyof crossover notch distortion. Especially Crown and Phase Linear. But improved solid-state circuit designs largely eliminated this issue after 1980. In contrast, tube (valve) amplifiers rarely have significant crossover notch distortion. In my opinion this is a big reason why people have been saying (for many years) that tube amps sound better than solid-state amps.Rough diagram of X-Y displays for different conditions:No significant distortion: ---------------------Clipping: |--------------------|Crossover notch: ----------|----------I'll post images from actual measurements if anyone here is interested.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 05, 2023, 07:28:22 pm
content: Why does this topic have two names?1) Re: Measuring Distortions with the Scope:What you see is not what you really have2) Re: Comparison between Siglent SDG1000X and 2000X

author: tautech
author_rank: Super Contributor
timestamp: January 05, 2023, 07:32:30 pm
content: Quote from: elecdonia on January 05, 2023, 07:28:22 pmSee reply #63, the OP chose to change it to better reflect the content.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 05, 2023, 07:52:28 pm
content: Several posts in this topic display the FFT harmonics from the output signal of a"function generator."Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. Thisisn'tsuitable for testing audio gear. It corresponds to THD of .5% to 1%. This is far higher than the THD of any decent audio amplifier circuit. In order to make useful measurements of distortion, the "residual" distortion of the signal source (oscillator or generator) should be at least 10 times lower than the expected THD of the amplifier under test.Function generators always produce plenty of harmonics, especially higher order harmonics. The reason for this is that function generators internally start out by generating either a ramp or triangle wave shape. This is then filtered and/or processed with non-linear diode clipping circuits to "approximate" a sine wave. The best function generators have harmonics of -40dB to -60dB below the level of their fundamental. Function generators also produce extensive high-order harmonics up to and above 10th order. Adding up all these harmonics corresponds to about 1% THD, with the very best function generators approaching .25% THD.In contrast,"sine wave audio oscillator"circuits actually generate a sine wave as their starting point. Some generators may offer a square-wave output also, but that is produced by running the pure sine signal through a comparator. The best sine-wave oscillators have THD as low as .0001% (after background noise is filtered out).Pure digital generation of sine waves by high-grade 24-bit D-A converters can also achieve very low THD.I recommend that future FFT data shown in this topic should use an "ultra-low distortion" sine wave oscillator (or 24-bit D-A) for the signal source.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 05, 2023, 08:12:47 pm
content: Theoretically a very high-quality ultra-low-distortion sine wave oscillator could be phase-locked to the output signal from a function generator. But this isn't how most function generators are made.Function generator:Available output signals: Sine, square, ramp, triangle, pulses of controllable widthOutput frequency range can be as large as .01Hz to 10MHzOutput signal amplitude is tightly defined (changing the frequency doesn't alter output amplitude)Distortion for sine-wave output: .25% to 2% THDSome models offer voltage-control of output frequencyLow-distortion sine wave oscillator:Available output signals: Sine (and optionally square)Typical output frequency range is 20Hz to 100kHz or sometimes 1MHzOutput signal amplitude may vary by +/- 1-3dB as frequency changesDistortion for sine-wave output: .0001% to .1% except older tube (valve) sine wave oscillators may have distortion spec of .5-1% THDSome sine wave oscillator circuits provide only a single fixed output frequency (typically 1kHz), or a limited number of fixed output frequencies

author: mawyatt
author_rank: Super Contributor
timestamp: January 05, 2023, 08:14:28 pm
content: Think the context here where "Function Generator" is mentioned more than likely infers a AWG, which can have a respectable THD+N metric as shown by a few.Best,

author: Performa01
author_rank: Super Contributor
timestamp: January 05, 2023, 08:21:21 pm
content: Quote from: elecdonia on January 05, 2023, 07:52:28 pmThank you so much for giving us valuable insight in the obscure field of audio engineering. We learn something new every day!In case you did not understand Tautechs response: This thread originally was (and somehow still is) about the distortion in AWGs (Arbitrary Waveform Generators), which have been shown to provide decent distortion levels down to -80 dBc.In case you did not read the posts within this thread: unsurprisingly, it has been found that the linearity of the frontend of an average general purpose DSO cannot keep up with the modern AWGs, hence we get much worse distortion figures than what the generators are actually capable of.Hint: the latter fact was the reason why the OP decided to change the thread title.

author: Martin72
author_rank: Super Contributor
timestamp: January 05, 2023, 09:36:40 pm
content: QuoteExactly.It actually started "harmlessly", with an FFT comparison between SDG1000X and 2000X.rf-loop noticed (in a neighboring thread) that something could not be right and so it began...What I noticed (later), the FFT of the two looked pretty much the same, which possibly already indicates an "error".I find that very interesting what happens there and must read me again the pages here to create perhaps a summary of why you can not trust your eyes (apart from that you know the actual values according to the data sheet).Also, we have not yet illuminated whether it gets "better" when you feed in a signal that has a not so low THD.I had briefly addressed this at work or discussed with a senior developer and he said flatly, that comes from when you measure "digital" with "digital"....Oh well, the display of my Neutrik A1 has an illumination, it is only totally weak:EL foil, over 20 years old...Replacement bought, should come next week and then I need no more flashlight.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 05, 2023, 09:57:15 pm
content: Quote from: Performa01 on January 05, 2023, 08:21:21 pmSomehow I missed the word"arbitrary"when I first read this topic. I must re-read all 5 pages more carefully."Arbitrary waveform" implies a digital system with D-A converter at the output. Today 16 and 24-bit DACs with excellent linearity and very low noise are widely available and cheap. I'm not surprised that -80dB distortion ( .01% THD ) is feasible for an AWG. Perhaps I'm showing my (advanced) age because my concept of "function generator" applies to analog devices limited to producing sine, square, ramp, & triangle waveforms?AWG systems are a whole different animal.QuoteI fully agree. This is one reason why it is so useful to connect the oscilloscope to theoutput signalfrom a THD analyzer (or notch filter) so that the oscilloscope isn't required to process the fundamental.In any case one must first measure the signal source (AWG) with the analysis device (DSO) to determine the baseline distortion/noise level before putting adevice-under-testinto the signal chain.

author: Performa01
author_rank: Super Contributor
timestamp: January 05, 2023, 11:41:32 pm
content: Now we are on the same page!

author: pdenisowski
author_rank: Super Contributor
timestamp: January 06, 2023, 12:26:02 am
content: Quote from: elecdonia on January 05, 2023, 07:18:50 pmI'm interestedAlways amazed at the different ways people find to use XY mode

author: nctnico
author_rank: Super Contributor
timestamp: January 06, 2023, 11:32:44 am
content: Quote from: Performa01 on January 05, 2023, 08:21:21 pmAfter doing some testing (with a Tektronix AFG31000 ) the question is more like: how useful is long FFT where it comes to showing valid results or not. I did some testing with the RTM3004 which is limited to 128kpoints. As a result the FFT noise floor is about -80dB and shows no distortion products. On my GW Instek with 1Mpts FFT (resulting in a lower noise floor) I can see the front-end distortion products just fine.

author: gf
author_rank: Super Contributor
timestamp: January 06, 2023, 12:14:57 pm
content: Quote from: nctnico on January 06, 2023, 11:32:44 amEven if you consider anything below (say) -50dBc as potentially invalid, a long FFT is still useful if you need to separate closely spaced frequencies.

author: 2N3055
author_rank: Super Contributor
timestamp: January 06, 2023, 08:31:05 pm
content: Quote from: nctnico on January 06, 2023, 11:32:44 am-80dB noise floor? You mean dynamic range?At what sensitivity and signal level?On SDS2000X HD noise floor is approaching -140dBm..On following capture dynamic range aproaches 100 dB (-5dBV minus -103dBV)Same number of points..No wonder nonlinearities are seen...If dynamic range were 80 dB they also would not be seen..

author: JeremyC
author_rank: Regular Contributor
timestamp: January 07, 2023, 01:48:39 am
content: Quote from: Martin72 on December 26, 2022, 08:22:32 pmI’m new to the forum, however I watch it since ~2019.In my opinion many people don’t try to understand FFT backgrounds and they do misinterpreting the results. Somebody should create new topic like “FFT good practices / primer”, independent of the equipment vendor. FFT can be confusing… FFT windows may sound confusing, almost each vendor has own interpretation of FFT, etc… But all are base on the same math formulas.For beginners:- Check your DSO ADC bits, and if:- it’s 8 bit, ignore everything below 48dB in respect to dbV should be ignored.- it’s 10 bit, ignore everything below 60dB in respect to dbV should be ignored.- it’s 12 bit, ignore everything below 72dB in respect to dbV should be ignored.- it’s 14 bit, ignore everything below 84dB in respect to dbV should be ignored.- it’s 16 bit, ignore everything below 96dB in respect to dbV should be ignored.… and so on.- Use the vernier and adjust that your wave to covers 90 - 95% of the p-2-p signal on the screen.- Don’t use any kind of averaging, ERES, highres, etc.- Use AC coupling (most scopes will be fine with frequencies > 30Hz in AC coupling, check yours).- Use plenty of samples… it will depend on your scope.- Ignore myths...As good start I suggest to check THD + Scope published by W2AEW about 3 years ago:If anybody is in high-end audio, you should ignore oscilloscope and use good quality ADC/DAC combined with software like Audio Rightmark, ARTA, or REW suite. ...Or buy specialized equipment, however it may come pricey…

author: mawyatt
author_rank: Super Contributor
timestamp: January 07, 2023, 05:24:00 am
content: To evaluate the DSOs ability to properly represent the THD of a waveform, rather than use a waveform of unknown low THD (sine-wave for example), use a waveform with known THD such as a square-wave or triangle-wave. Altho these waveforms may have an uncertainty in creation from the generator like the sine-wave they have a THD that can be computed theoretically and not ideally zero, so the results from a DSO FFT evaluation can be compared to the theoretical THD of the applied waveform.Because these waveforms naturally possess higher levels of controlled THD compared to a uncontrolled abet low THD sine-wave, they can be measured by the DSO with the lower resolution ADCs & performance Input amplifiers/attenuators within their span of limited performance.Here's a few THD for common waveforms derived from this IEEE paper:https://www.researchgate.net/publication/260672713_Analytic_Method_for_the_Computation_of_the_Total_Harmonic_Distortion_by_the_Cauchy_Method_of_ResiduesSquare-wave THD = Sqrt[(pi^2)/8 -1] ~ 48.3%Triangle-wave THD = Sqrt[(pi^4)/96 -1] ~ 12.1%Sawtooth-wave THD = Sqrt[((pi^2)/6 -1} ~ 80.3%Pulse-wave THD = Sqrt[((D(1-D)*(pi^2))/2(sin^2(pi*D))) -1] which varies from 48.3% to 191% as D (Duty Cycle) varies from 0.5 to 0.9, or 0.1 as THD function is symmetrical about D = 0.5 (Square-wave).We are unable to verify our DSO FFT against these theoretical waveform THD readings at this time, so others may want to utilize this information and report results. Hopefully we'll be able to perform these measurements ourselves and report the results soon.Of course this introduces another "uncertainty" with the waveform "quality", some of the modern AWGs likely are good sources at lower frequencies (as shown with sine-waves), and we do have an accurate by design custom low frequency squarewave source with precise duty cycle and amplitude available.Anyway, hopes this helps with the FFT evaluations regarding THD.As always, YMMV.Best,

author: _Wim_
author_rank: Super Contributor
timestamp: January 07, 2023, 06:36:23 am
content: Quote from: JeremyC on January 07, 2023, 01:48:39 amI think you need to re-watch the referenced video by W2AEW (#65: Basics of using FFT on an oscilloscope), because it perfectly makes sense to use HI-RES or ERES to increase the effective number of bits. This way an 8-bit scope can have a larger effective number of bits at a much lower bandwidth.A good comparison between ERES and HI-RES can be found here:https://teledynelecroy.com/doc/differences-between-eres-and-hires

author: markone
author_rank: Frequent Contributor
timestamp: January 07, 2023, 02:03:31 pm
content: Quote from: _Wim_ on January 07, 2023, 06:36:23 amThe point is that any math process that extend dynamic range with ADC sampling add artifacts and alter BW, so what JeremyC is saying makes perfect sense if you want to be sure to no catch phantom FFT peaks or miss real ones with BW cut.I personally agree with that, while I often use ERES equivalent functions in YT mode I have no interest to see FFT fake deep dynamic ranges in 8-12 bits scopes, I find all those small false frequency components quite distracting and when I see 100-120dB FFT screenshot taken from ordinary scopes I make a smile.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 07, 2023, 03:19:23 pm
content: Quote from: gf on January 06, 2023, 12:14:57 pmThis earlier post describes FFT measurement of the AWG signal from a Siglent which worked out very well, clearly displaying harmonic amplitudes -100dB below the fundamental amplitude. OP lists most of the measurement parameters.https://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/msg4615288/#msg4615288In my opinion the use of 24-bit A-D to analyze the signal from the AWG is significant.Until recently 24-bit A-D converters were nearly unobtainable at any price. But today they are mass-produced and affordable. My thinking is that having 24-bit resolution for the analysis system is valuable.

author: _Wim_
author_rank: Super Contributor
timestamp: January 07, 2023, 03:39:11 pm
content: Quote from: markone on January 07, 2023, 02:03:31 pmI agree that the very low noise floor shown is artifact, but if you take into account the effective number of bits (including the bits created by enhancement from HIRES or ERES), than the actual dynamic range can be calculated (20*log(2^enob)) and is in my opinion "real".Many high resolutions ADCs use oversampling to achieve their high resolution, and for me oversampling and HIRES/ERES are essentially the same. Bandwidth reduction and bit-enhancement can be seen from the attached tables (copied from the Lecroy explanation)So for a 10 bit scope a 3 bit enhancement is certainly possible for these low frequency ranges in the posts above, giving around 78db of dynamic range. Anything below that must indeed be taking with a grain of salt, but I just wanted to point out that an 8-bit scope has more that 48db of dynamic range...

author: mawyatt
author_rank: Super Contributor
timestamp: January 07, 2023, 04:06:44 pm
content: Quote from: mawyatt on January 07, 2023, 05:24:00 amGot a chance for a quick measurement. This is with SDS2000X+ in 10-bit mode using SDG2042X for waveforms.Triangle-wave at 5VPP, THD 12.0%(12.09%)using first 19 harmonics (Theoretical 12.1%),using 50 harmonics 12.10%Square-wave at 5VPP, THD 46.5%(46.64%)using first 29 harmonics (Theoretical 48.3%),using 50 harmonics 47.42%, 100 harmonics 48.14%Sawtooth-wave at 5VPP 100 harmonics 80.05% (Theoretical 80.3%)Pulse-wave 25% Duty Cycle at 5VPP, 100 harmonics, THD 91.99% (Theoretical 92.23%)We could see the results converging towards a slightly higher THD as we added more harmonics in the measurements, which is to be expected as the higher order harmonics are being truncated (not included) which yields a lower overall THD rendering. We also confirmed the Square-wave result with the mentioned precision custom source. The squarewave error is more susceptible to harmonic truncation because the harmonics fall of as 1/f, whereas the triangle wave falls off as 1/f^2.Anyway, this gives us a little more confidence in the DSO FFT ability to render a reasonable THD result for waveforms with higher THD, curious what others find utilizing these waveforms.Best,Edit: Added results from(PicoScope 4262)with 16-bit core ADC.

author: mawyatt
author_rank: Super Contributor
timestamp: January 07, 2023, 04:27:38 pm
content: Quote from: _Wim_ on January 07, 2023, 03:39:11 pmHaving been involved in ADC chip developments in the past, it's funny when folks think an ADC chip is the single conversion type one often refers to as a "Flash" single step conversion, when they really are an oversampled multi-step conversion type. The prior mentioned 24-bit resolution types are massively oversampled Delta-Sigma types, which are very CMOS friendly and yield nicely to CMOS advanced technologies, and why they are so reasonably priced (small chip size). However, no free lunch as they tend to operate at lower frequencies, but with CMOS process advancements are moving up in frequency and soon maybe into the lower RF bands. Hopefully we'll soon see some new CMOS friendly ADC architectures appear and trade off massive oversampling with waveform quantization in both amplitude and time, and support post Analog to Digital conversion Anti-Aliasing filtering dictated by the actual input waveform metrics!!Best,

author: rf-loop
author_rank: Super Contributor
timestamp: January 07, 2023, 05:58:34 pm
content: Some may say that when the ADC is 8 or 10 or 12 bit then this and that dynamic range and below this and that level FFT displayed things are more like shit.So or so. But in practical example this happens in real life with real instruments and not with student books.OscilloscopeSDS2504X HDGenerator SDG1000XInput CH1, 50ohm AC, Full BW, 50mV/div (with this setup 4dBm signal is roughly full screen (thewre is stioll small overlap after screen 8div before ADC F.S.Acquisition mode Normal,FFT also normal and window Flat Top.FFT span 300Hz (30Hz/div) and center 100020HzThere are two signals. Marker 1 signal is 99930Hz from SDG CH1 and Marker 2 signal is 100070Hz from SDG CH2 but there is ~30dB attenuator before splitter.SDG Ch2 signal is amplitude modulated using modulation depth 0.2% and modulating frequency 50Hz so sidebands are 60dBcNow think that marker 1 signal is in practice maximum level (near ADC full scale. Screen height 400mVpp responds -3.98 dBmIn image (77), there can see modulation sidebands well and even levels are measured quite well. These mod. Peaks are around -90dB below marker 1 signal (maximum level) and -60dBc (carrier is 30dB below maximum level. )As can also see there is still nice distance to noise floor.Image (78), all same but FFT trace average 8 what lightly cleans random noise to more close its average.

author: nctnico
author_rank: Super Contributor
timestamp: January 07, 2023, 06:20:37 pm
content: Quote from: markone on January 07, 2023, 02:03:31 pmAgreed. For starters there has to be enough white / Gaussion noise for eres / high resolution to work well. Eres and high-res are nice tools to make a signal look cleaner but if you zoom in far enough, you can see all kinds of weird distortions. I recall doing some testing on my good old Tektronix TDS744A. Turning the 20MHz bandwidth limit on, made the high-res produce all kinds of fantasy signals. The same goes for FFT; you need to be really carefull with interpreting what you are seeing. Deep FFT is useful to look at closely spaced frequencies (as others have mentioned as well) but the extra dynamic range might not be there at all. Or put differently: the reliability / confidence level of the measurement results that are below the dynamic range of the ADC, is very low.

author: _Wim_
author_rank: Super Contributor
timestamp: January 07, 2023, 06:52:49 pm
content: Quote from: nctnico on January 07, 2023, 06:20:37 pmI agree some fantasy signals can be shown, but this is in my (limited) experience always below the effective dynamic range (so not the dynamic range visualized, but the once calculated using the enhanced number of bits).Is there in your opinion a difference between oversampling (https://www.analog.com/media/en/technical-documentation/tech-articles/Increase-Dynamic-Range-with-SAR-ADCs-Using-Oversampling.pdf) and HIRES? For me it seems this is exactly the same, but I could be missing something.Maybe with my 12-bit pico I am closer the the actual noise floor and never had insufficient gaussian noise? I do not use the FFT on my 8-bit scope (DS1054Z) because it is a pain, but with the 12-bit Pico (and Analog discovery) I always have had quite trustworthy results by using averaging and/or ERES.

author: nctnico
author_rank: Super Contributor
timestamp: January 07, 2023, 07:16:43 pm
content: Quote from: _Wim_ on January 07, 2023, 06:52:49 pmIt is exactly the same.

author: JeremyC
author_rank: Regular Contributor
timestamp: January 08, 2023, 02:11:17 am
content: Quote from: _Wim_ on January 07, 2023, 06:36:23 amI watched this video many times and I read many FFT related publications from the “A” vendors in the last few years.Sorry, I should mention in my posts that I disagree with W2AEW about the averaging/hires modes with FFT. In my opinion W2AEW is excellent (if not the best) information source and I’m glad he’s sharing his knowledge and experience with the public. But in this topic I would disagree with his opinion about averaging combined with FFT, sorry.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2023, 02:31:25 am
content: Think of it this way: if you can make extra information appear magically from an 8 bit ADC, then why aren't we all using scopes with 1 bit ADCs? Even cheaper to make!

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2023, 04:53:56 am
content: Quote from: nctnico on January 08, 2023, 02:31:25 amThere is no "magic" in these concepts and in fact many of the slower Delta Sigma types are simply 1 bit core ADC types (1 bit comparator) with massive Oversampling and Multi-Order Modulators followed by high order Decimation Filters and easily achieve well beyond 20 ENOB, these are the common 24 bit Delta Sigma ADC chips that only cost a few $!!Many of the higher speed ADC chips employ techniques similar to the 1 bit core Delta Sigma ADC, except they utilize more than 1 bit (usually 3 ~ 4) in the "Comparator" and DAC "Feedback" path to speed the overall conversions up, and digitize the Signal - Feedback difference instead of integrating such as the DS ADC do (if they utilize integration then some of the benefits of Modulator induced "Noise Pushing" can be employed, but this slows things down).As was mentioned by another earlier post, the HIRES and ERES are effectively doing something similar to what some of the core silicon ADC chips are doing to enhance resolution, and as shown is always a tradeoff between resolution and speed. This is where the mentioned new type ADC architecture may come into play which makes amplitude quantizations but at non-uniform sample intervals which are dictated by the input signal, so simultaneous amplitude and time quantizations...but this is another topic well beyond the scope being discussed here.So there are many ways/techniques to approach the ADC, some are completely within the CMOS chip, some are external software enhanced, some external hardware enhanced, and some both hardware/software enhanced. They all are trading off speed/BW for resolution, but none rely on "magic" to achieve their respective results.What you may have interpreted as "magic" in rf-loops notes with the lower level dual unequal tones, one of which is lightly AM modulated, is the effect of the larger tone acting as a "dither" for the lower ADC bits to resolve the smaller tone with AM modulation. This was (maybe still is) a classic technique to improve the apparent resolution/linearity of a non-linear device with a non-responsive zone, called "dead zone". The ADC is non-linear across each LSB transition because it has a "dead zone" where the bit doesn't respond to the input signal, where added noise (maybe internal) or additional signal will modulate the LSB transition, effectively "dithering" across the LSB dead zone and subsequent filtering can reduce the dead zone effects and resolve into fractional LSBs revealing input signal fine details below the LSB as shown.Best,

author: Performa01
author_rank: Super Contributor
timestamp: January 08, 2023, 06:20:23 am
content: No doubt that the real deal – a true higher resolution ADC – is the superior solution. Yet there is nothing wrong with a proper ERES/HiRes implementation as long as the user thinks a little. And that should not be too much asked, since there are so many situations in T&M where we cannot trust blindingly and just copy some values from an instrument to the lab protocol without a second thought.For instance we might run into troubles when we try to measure a -60 dBm signal using FFT when the corresponding input channel is set to 100 mV/div (800 mVpp = +2 dBm full scale). This is clearly outside the genuine 8 bit dynamic range.SDS2354X Plus_LVL_10MHz_100mV_-60dBm_8bitThe measured amplitude is -57.5 dBm, hence 2.5 dB off – just remember, we try to measure a 632 µVpp signal at an 800 mVpp full scale input range. In a low noise DSO, the so called "process gain" doesn't work that well, fair enough.But then again, not many will attempt to measure such a weak signal by using a particular insensitive input range of the scope – we would rather choose 1mV/div instead:SDS2354X Plus_LVL_10MHz_1mV_-60dBm_8bitUnsurprisingly, the measured amplitude is correct now; -59.9 dBm means that the error is just 0.1 dB.For a fair comparison, now that we used a 40 dB more sensitive input range, we should also try to measure a 40 dB lower signal, i.e. -100 dBm instead of -60 dBm:SDS2354X Plus_LVL_10MHz_1mV_-100dBm_8bitAll of a sudden even the signals outside the genuine dynamic range are measured accurately: -99.9 dBm is only 0.1 dB off again. This is because at a high sensitivity of 1 mV/div we have sufficient inherent noise to make the resolution enhancement inherent to a longer FFT fully work.What if there are any stronger signals present, so we cannot select a higher sensitivity without overdriving the scope input? All the better, then even the lower sensitivities will work, because now we don't need noise as dither anymore, but have those stronger signals instead.Of course, a 12 bit SDS2000X HD doesn't have any troubles measuring -60 dBm even in the insensitive 100 mV/div (+2 dBm FS) range. After all, this is still within its genuine dynamic range of ~72 dB.SDS2504X HD_LVL_10MHz_100mV_-60dBm_NormalThe last screenshot demonstrates how we can still measure -100 dBm (6.3 µVpp, far outside the genuine dynamic range) at 100 mV/div (800 mVpp) with only 0.74 dB error:SDS2504X HD_LVL_10MHz_100mV_-100dBm_Normal

author: _Wim_
author_rank: Super Contributor
timestamp: January 08, 2023, 06:41:20 am
content: Quote from: JeremyC on January 08, 2023, 02:11:17 amNo need to say sorry. As seen from the replies above, it is clearly something were different opinions exist. I agree about W2AEW, excellent channel, have watched all of his video's, even the HAM ones as a non-HAM.

author: _Wim_
author_rank: Super Contributor
timestamp: January 08, 2023, 06:47:20 am
content: Quote from: Performa01 on January 08, 2023, 06:20:23 amThanks for this. Quite impressed by the performance of the SDS2354X!@nctnico, do you now of a signal that can be generated easily with an AWG where artifacts would appear within the effective dynamic range. Would be nice to do a side by side comparison, and also useful to see what to watch out for...

author: tautech
author_rank: Super Contributor
timestamp: January 08, 2023, 07:00:32 am
content: Quote from: _Wim_ on January 08, 2023, 06:47:20 amDo note Performa01 used 2 DSO's, SDS2354X PlusandSDS2354X HD.

author: _Wim_
author_rank: Super Contributor
timestamp: January 08, 2023, 09:05:51 am
content: Quote from: tautech on January 08, 2023, 07:00:32 amAh, missed that one. Thanks for pointing that out! Still, remains very good results with the HD. Would be a excellent replacement for my pico5000...

author: gf
author_rank: Super Contributor
timestamp: January 08, 2023, 09:19:18 am
content: Quote from: JeremyC on January 08, 2023, 02:11:17 amHiRes/Eres provides additional processing gain for a subsequent FFT (assuming that the FFT size is already maximum and cannot be increased any more) if the particular HiRes/Eres implementation doesdecimatethe data after filtering, so that the FFT is applied to the lower sample rate.OTOH, if the HiRes/Eres implementation only applies a lowpass filter (w/o decimation) then it does not increase processing gain of a subseqent FFT, but only distorts the frequency response. No need to filter, the FFT acts as filter bank anyway.And even if the HiRes implementation does decimate, the next question is whether its filter is an appropriate anti-aliasing filter for the decimation. The traditional boxcar averaging filter certainly isn't -- not at all. For the high dynamic range we are after, we would need an anti-aliasing filter with very high stopband attenuation in order that aliases folded into the 1st Nyquist region drown in the FFT noise floor.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2023, 11:48:52 am
content: Quote from: mawyatt on January 08, 2023, 04:53:56 amYes, but these ADC cores are designed to meet the specifications of the end goal which is to have and ADC with X bits and not X+'magic number' bits.Anything beyond is just luck. If you buy a whole bunch of 3.5 digit DMMs and stick 3 extra digits to them, a few may seem to be accurate beyond 3.5 digits but that is more due to luck and cirumstances (right temperature for example) rather than solid engineering. In the end measuring is about having a certain confidence level in what is being shown on screen.The same goes for measuring signal levels using an oscilloscope. The numbers Performa is listing in a posting above, have a 0.5dB error margin. IOW: a different unit may show different results within the specified error margin.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2023, 11:53:24 am
content: Quote from: _Wim_ on January 08, 2023, 06:47:20 amThe two tone test mentioned earlier on is a good one (and also industry 'standard').

author: switchabl
author_rank: Frequent Contributor
timestamp: January 08, 2023, 12:19:42 pm
content: From a DSP perspective, if you are only interested in a low frequency (relative to fs/2) sub-band, you could either do a full FFT and throw part of it away or decimate first and do a shorter FFT. The latter is of course more efficient and this is basically what happens with ERES on. So you can get more frequency resolution (and processing gain) with the same resources.Of course, caveats mentioned by gf apply (only works if ERES implementation actually does decimation, ERES filter less than perfect).

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2023, 02:55:52 pm
content: Quote from: nctnico on January 08, 2023, 11:48:52 amThisISthe way things work, whether you want to believe it's due to "magic" or "luck" is of course your prerogative.The mentioned 24 bit DS ADC just uses a 1 bit comparator (ADC if you will), yet achieves better than 20 Effective Bits (ENOB). This is achieved, even specified in the data sheets, using Oversampling and Decimation, basic Signal Processing 101 methods.The idea of "dithering" to achieve higher resolution and improve linearity is not new, this was utilized in the early 70s in the Ring Laser Gyro (RLG) to "unlock" the counter rotating photon (laser) beams at low rotation rates (dead zone), and likely utilized well before this time. Don't think folks back then would rely on "luck" or "magic" to guide/navigate an airplane, ship, submarine, satellite, rocket or missile.So the techniques being discussed are real and repeatable, not conjured up non-sense as we often see. As much as many like the Siglent DSOs (we have a couple), the results shown are not unique to these instruments, but achievable and repeatable by any quality DSO with the necessary features to achieve such, and would expect the Rigol, GW, Keysight, LeCroy, Tek and so on, would respond similarly to these techniques.Anyway, hope you'll realize this isn't "magic" nor "luck", just good old Engineering/Science Signal Processing being put to use for our (and others) benefits.Best,

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2023, 03:10:40 pm
content: Quote from: nctnico on January 08, 2023, 11:48:52 amEvery measurement ever will have result within it's certainty interval. On any instrument.0.5dB error margin for a scope is not bad at all...And scope Performa used has specified BW flatness..So I don't see any problems...

author: mawyatt
author_rank: Super Contributor
timestamp: January 09, 2023, 04:23:08 pm
content: Quote from: Performa01 on January 08, 2023, 06:20:23 amThis is the basic issue with folks utilizing an instrument beyond their understanding, as many knowledgable individuals have said before, "Know Thy Instrument", or KTI!!We've all been there, done that, at least us older folks!! Where we grab a little utilized instrument from storage to make a measurement or troubleshoot without taking the time to revisit what/how the instrument works/behaves, only to make a useless measurement that confounds the problem at hand!!Today many of these little utilized instruments/functions are now contained within the modern DSO, especially true with the ERES/HiRes, FRA/Bode, and SA(FFT) Function/Mode/Instruments, and there seems to be little understanding of how the Signal Processing, Input Amps/Attenuators and core ADCs behave (especially WRT each other) when utilized outside the usual Time Domain Scope operation, as with these mentioned functions.Anyway, thanks to the demonstrated truly knowledgable individuals like yourself and very few others, some of the less seasoned folks can sift thru all the rubbish and glean a sparkle of understanding regarding these increasingly complex data acquisition systems we now call DSO/MSOsBest,

author: Martin72
author_rank: Super Contributor
timestamp: January 09, 2023, 11:02:16 pm
content: Now I only have the SDG2122X, the SDG1062X is gone....But not far away, it landed on my work(sold to it).....This has the nice effect that I can continue to make measurements with it, but then with the HDO6034A...Not the worst option.

author: Martin72
author_rank: Super Contributor
timestamp: January 10, 2023, 09:49:19 pm
content: Today just before closing time quickly measured what, must still adjust the scaling, for better reading.But nothing new, the second harmonic dominates the spectrum, even with the HDO.Then again quickly our last analog generator, a Wavetek, connected.Then I experimented with and without ground, but these were only marginal changes.

author: 2N3055
author_rank: Super Contributor
timestamp: January 10, 2023, 09:54:48 pm
content: Quote from: Martin72 on January 10, 2023, 09:49:19 pmWise choice of level and V/div. Try similar with SDS2000X HD. Also try using fine vertical and watch for the sweet spot...

author: Martin72
author_rank: Super Contributor
timestamp: January 10, 2023, 10:35:09 pm
content: QuoteCan do but now with the SDG2122X only(or I take the HD to work...Hmmm..)Meanwhile I got an idea, we have several high precision power analyzers from ZES here.They can show THD and SHD, when their input sensivity is low enough, I would connect the SDG1032X to one of them, could be interesting.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 12, 2023, 09:07:15 pm
content: Quote from: nctnico on January 08, 2023, 11:53:24 amI recall Prof. W. Marshall Leach and others (Walt Jung?) using atwo-tone testcomprised of equal levels of 10kHz and 11kHz to demonstrate TIM (transient intermodulation distortion), aka “slew-rate induced distortion.” For systems with a very wide bandwidth the test tones may be increased in frequency up to 19kHz and 20kHz. The frequency difference between the two tones is kept at 1kHz. The resulting intermodulation distortion product always appears at 1kHz. Such a large frequency displacement makes this distortion product relatively easy to hear and measure.Prior to the discovery of TIM, the two tones in the SMPTE/DIN standard IM distortion test were 60Hz along with 7kHz at a relative level of -12dB compared to the 60Hz signal. Non-linearity will generate sideband signals at 60Hz intervals immediately above and below 7kHz. Another similar IM test signal substitutes 4kHz instead of 7kHz for applications with lower bandwidth. In this case the distortion products appear in side bands near 4kHz.

author: EA1DDO
author_rank: Newbie
timestamp: March 29, 2023, 10:36:09 am
content: If you need to measure a "Direct injection box" (aka transformer) audio characteristics, eg. attenuation, THD distorsion, etc.What piece of equipment do you recommend?Thanks

author: Martin72
author_rank: Super Contributor
timestamp: May 06, 2023, 09:44:44 pm
content: Hi...I had made a small setup today to measure the amplifier tomorrow using bode plot.Before that I had sent a 1khz sinus into the amplifier and plotted the FFT of it.Following the discussion here, the result can not be trusted...or is it?Or in other words, how could you make sure that what is displayed is coming from the amplifier and not from the scope itself...

author: pope
author_rank: Frequent Contributor
timestamp: May 06, 2023, 09:55:00 pm
content: Quote from: EA1DDO on March 29, 2023, 10:36:09 amAn audio interface with the appropriate software. Or an audio analyser if you can afford one.

author: BillyO
author_rank: Super Contributor
timestamp: May 06, 2023, 10:03:41 pm
content: Quote from: Martin72 on May 06, 2023, 09:44:44 pmSee if you can get a hold of a sine wave reference oscillator.

author: Martin72
author_rank: Super Contributor
timestamp: May 06, 2023, 10:46:03 pm
content: Well, finally it´s time to assemble this kit here..

author: mawyatt
author_rank: Super Contributor
timestamp: May 07, 2023, 12:23:11 am
content: Is that an oscillator based upon the Wien Bridge, kinda like what LT Jim Williams did long ago with the Opto-Coupler replacing the incandescent bulb in the famous HP oscillator!! Recall Jim used this oscillator to evaluate 20 bit ADCs, so should be good enough for our DSOsBTW looks like fun aheadBest,

author: Martin72
author_rank: Super Contributor
timestamp: May 07, 2023, 12:58:39 am
content: Hi,It´s this one :Ultra low distortion oscillator (below -140dB)And yes, a classical Wien-Bridge...

author: BillyO
author_rank: Super Contributor
timestamp: May 07, 2023, 01:08:32 am
content: Quote from: Martin72 on May 06, 2023, 10:46:03 pmWhere can you get those boards? Looks like a nice ting to have.

author: Martin72
author_rank: Super Contributor
timestamp: May 07, 2023, 01:15:38 am
content: Here:http://www.janascard.cz/aj_Vyrobky.htmlYou can get it completely or only the pcbs.

author: Sighound36
author_rank: Frequent Contributor
timestamp: May 07, 2023, 07:43:58 pm
content: I have only just read this thread our workload currently is quite manic so as we have a quality audio analyser UPV66 have set up the unit to measure its own internal high precision genny @ 1 khz ref voltage is 1V 20Hz-20Khz FFT 256K 4 averages, Van hann windowOn the Wavepro HD I used max bandwidth 4Ghz, NO filters ( I have ERES and digitial filter package) as close as I can get to Martin's set up here, ground is on 600Ohm output on the genny selected. SA set to Von han / power spectrum / RBW20Hz. 1Mohm input 1m long RG59U 22 AWG. with an XLR<>BNC adapter, I didn't de embed cable impedence tonight.Next image same setting etc just with 10 avergesThe following image now spots 200Ks and 1Ms sampling same HTB then the 10 avergae of that as well.Then 5MS memory points and 25MS/s sampling rate & that averageLast pairing 200Meg points of memory & 1GS/s rate.Finial image the noise sat on the genny output with it off!

author: Martin72
author_rank: Super Contributor
timestamp: May 07, 2023, 08:11:29 pm
content: Hmm..So the first pictures show the "truth", because actually there should be no harmonics in the amplitude range...

author: Sighound36
author_rank: Frequent Contributor
timestamp: May 07, 2023, 08:30:18 pm
content: Hi MartinI think it with either NTICO or Sinisa? who made the observation about what's really buried in the noise.Its Sunday evening maybe I have not been as accurate as I could have been? The analogue gennerator is the precision version so it should be pretty clean. Again we are back to how good is the set up/opertor and actual ability of the equipment to deliver a result that is as free of measurment uncertanty as possible.If you have specfic set you wished checked happy to do this.Sighound

author: Martin72
author_rank: Super Contributor
timestamp: May 07, 2023, 08:56:22 pm
content: Hi,I'll try to finish the oscillator as soon as possible or use the generator from my neutrik A1 to recreate your measurements.From my (probably too simplistic) thinking, if a signal with say -100dB thd is fed in, there should be nothing to see in the displayed FFT when it goes "down" to -80dB.

author: Martin72
author_rank: Super Contributor
timestamp: May 08, 2023, 09:25:28 pm
content: QuoteThe missing parts ordered today at Mouser, the sum has already become three digits.