Siglent SDG1032X sine distortion at 1 kHz ? - Page 1
Forum section: electronics / testgear

author: JBeale
author_rank: Frequent Contributor
timestamp: December 24, 2022, 07:02:14 am
content: I'm curious if anyone with a Siglent SDG1032X has measured the sinewave distortion at 1 kHz. I don't have one myself.On page 7 of the spec sheet athttps://siglentna.com/wp-content/uploads/dlm_uploads/2022/06/SDG1000X_DataSheet_DS0201X_E01I.pdfit says the sine distortion spec is -60 dBc from 0-10 MHz, at 0 dBm output. On p.3 there is a plot of 2nd and 3rd harmonic levels that suggests -65 dB at 100 kHz, but it doesn't show lower frequencies.I tried building a fixed 1 kHz Wien-bridge oscillator using a NE5534A opamp with the old-school filament lamp for AGC, and that works OK. The sinewave looks unclipped on my scope (which has only 8 bits of resolution I think) but my example barely reaches -60 dB THD, even after adding a few poles of RC low-pass on the output. Now I suppose the audio-frequency ADC I'm using to measure that with might also be far worse than its spec, or my garden-variety 0.1 uF ceramic caps are somehow nonlinear at that level (??) or something else, but so far I can't prove it.

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 08:50:56 am
content: QuoteCould do this later on..This threads reminds me of the kit I´ve bought over a year ago and still haven´t assembled yet.It generates a sinewave of 1khz with a claimed thd of round about appx -150dB, theoretically.

author: Performa01
author_rank: Super Contributor
timestamp: December 24, 2022, 12:58:29 pm
content: I do not have an SDG1032X, but I stumbled across an old measurement of the SAG1021, which should be the same circuitry as the integrated AWG of an SDS2000X Plus. In any case this is the cheapest AWG solution from Siglent, so the SDG1000X should be at least as good.The attached image shows the THD from 10 Hz to 1 MHz at 2 Vpp output into 50 ohms. 0.1% equals -60 dBc.

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 01:37:40 pm
content: Made FFT measures with both generators I currently got, more in the comparison thread...Here single pics, showing the SDG1062X and 2122X, generating a 1khz sinewave with 2.85Vpp.Martin

author: MrAl
author_rank: Super Contributor
timestamp: December 24, 2022, 01:58:06 pm
content: Just to note, my earlier remarks were not intended to suggest that small LCR meters were not useful at all, just not useful for some applications.For example, someone i know talks about getting a small LCR meter for testing inductors. However, he works mainly on DC to DC converters.What would happen if he bought one would be he would be disappointed that they do not work well with converter applications and inductors because inductors used for most converter circuits have more variable characteristics that required a certain DC bias. What this means is the small LCR meter may be of no use at all to him.For more information take a look at an anisotropic curve for any metal core used for converter circuit inductors and also transformers. The inductance may look like 10uH at 10ma but 100uH at 100ma and 1000uH at 1000ma. Then, as the current increases to 2 amps, the inductance may go down to 50uH, then finally down to some very low value maybe almost a short.What this means is that there is no way you can determine how useful a particular inductor is for a converter circuit using a small LCR meter.As far as capacitance, that's a different story because they dont usually need a large current to test properly. Maybe some of the really big ones though like 100000uf and like that, to test for ESR.We could look at some examples.

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 02:00:32 pm
content: Hmmm.....

author: rf-loop
author_rank: Super Contributor
timestamp: December 24, 2022, 02:01:01 pm
content: SDG1000X series sinewave charasteristrics (THD, harmonics, non harmonic spurs) are specified for 0dBm signal level.Here 1kHz sine out from SDG1000X and level 0dBm (load 50ohm).Test using SDS2504X HDFFT window of course "flat top"For better visibility time domain average 512 and freq domain average 8. (for reduce random noise level)I'm lazy and do not want calculate total from harmonics.Also only 8 markers (latest public FW).ETA:There is NOT visible SDG1000X harmonics alone. Here can see: SDG + oscilloscope front end generated and FFT "lies"!

author: Martin72
author_rank: Super Contributor
timestamp: December 24, 2022, 02:09:43 pm
content: Ah, dBm...

author: Hexley
author_rank: Regular Contributor
timestamp: December 24, 2022, 06:04:26 pm
content: For what it's worth, I measured my SDG1062X with an old HP331A distortion analyzer. The frequency was set to 1 kHz, and the output level was set to 0 dBm with a 600 ohm load (the generator load parameter was set to match this load).The analyzer's meter read one needle's width below 0.1% THD. Call it 0.095%, maybe.

author: TimFox
author_rank: Super Contributor
timestamp: December 24, 2022, 06:08:28 pm
content: Quote from: Hexley on December 24, 2022, 06:04:26 pmThat's roughly the "residual distortion" of the 331-334 family.My 339 can measure down to maybe 0.002% THD.

author: JBeale
author_rank: Frequent Contributor
timestamp: December 25, 2022, 01:01:26 am
content: Thanks everyone for the useful input. Looks like this model has reasonable performance and a decent spec, plus an array of other features that are probably useful in other ways, so I placed an order for one. Somehow I had the idea that -60 dBc was easy to obtain with a simple analog circuit, maybe with the right one but I've found that not just any circuit will necessarily do that.

author: Martin72
author_rank: Super Contributor
timestamp: December 25, 2022, 11:09:54 pm
content: Quote from: rf-loop on December 24, 2022, 02:01:01 pmHopefully the siglent will do it for us someday.What your measure concerns:Using flat top makes sense to me, as it got the best amplitude accuracy.Also using averaging.I see the values displayed in the screen upper right..To be honest, in all the years I´ve used fft as it comes, adjusted as long as the output seems plausible to me, without thinking about it.Something I want to change, so I´ve read the application notes from lecroy about setting up the fft.Span = half of samplerate, OK got it."Delta"-f = 1/time per division *10 divisions, OK got it...But there must be more when I look at your screenshot.Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?Is it because of the amount of fft points, we can choose up to 2M ?And what about delta-f in general, the lower the better ?

author: radiolistener
author_rank: Super Contributor
timestamp: December 26, 2022, 05:18:34 am
content: Quote from: rf-loop on December 24, 2022, 02:01:01 pmLooks much better than Chinese generators which have about -40 dBc

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2022, 05:58:55 am
content: Quote from: JBeale on December 24, 2022, 07:02:14 amAn audio frequency ADC should be a lot better than -60dB THD, so that must be your Wein bridge oscillator.Starting on page 29 ofLinear Technology application note 43, Jim Williams discusses the distortion of various audio Wein bridge oscillator configurations. The most basic with a single good operational amplifier and a lamp for AGC achieves 0.007% or -83dB at 1 kHz with distortion limited by the lamp itself.The NE5534A is pretty good so I suspect your capacitors are the problem. This is definitely a place for C0G or NP0 ceramic capacitors if you do not use film capacitors except for decoupling.

author: Performa01
author_rank: Super Contributor
timestamp: December 26, 2022, 08:05:38 am
content: Quote from: Martin72 on December 25, 2022, 11:09:54 pmI have tried to explain the basics of the FFT setup several times before, e.g. here, Reply #23:https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832Quote

author: rf-loop
author_rank: Super Contributor
timestamp: December 26, 2022, 10:26:18 am
content: Quote from: Martin72 on December 25, 2022, 11:09:54 pm"Span = half of samplerate, OK got it."FFT full span (0Hz to FFT max frequency) = sampling frequency / 2As you can see in my image displayed span is not at all this FFT full span. There is displayed 20kHz span just because I have adjusted it for this span. This particular FFT full span is in this case 25MHz(but if user know what he is doing he can also look higher frequencies... If I now feed her 50.001MHz signal I can also see 1kHz but lets leave this story to other time and place...)"Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz.... "Please try to unlearn this wrong habit...Lets look it right way."Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?"Δf =FFTsampling frequency / FFT lengthIn this case (my picture) sampling frequency is 50MSa/s (look FFT sampling not time domain sampling)FFT length is 2097152 pts.50000000Hz / 2097152 = 23,84185791Hz. So we can display informantion Δf = 23.84Hz"And what about delta-f in general, the lower the better ?"There is no single simple answer at all. Because it depends.... better for what?Reducing FFT points give less resolution (higher Δf) and more speed again depending our othewr settings... t/div, memory length, time domain sampling speed (in some cases FFT use same sampling speed and on other cases it use samplerate what is decimated from time domain samplerate what also can be decimated from real ADC samplerate.So there is many things what affect each others. What is optimal setup for different needs - simply answer is: Lowest Δf is not always best but some times best is as as low Δf as possible.But naturally resolution depends what is Δf (aka Bin width) and what is used FFT window function.If we think normal spectrum analyzer we talk resolution using RBW (And if we dive to bit more deep we need also know filter shape factors)In spectrum analyzer RBW is defined (mostly) so that it mean filter -3dBc width. So when you want more or less resolution you adjust RBWIn this kind of FFT here, you adjust resolution by changing Δf and also FFT window function and here need understand that this is bit more complex depending what user need in different cases. Here I have used window "FlatTop" because it give best level accuracy because its top is wide and there is minimal if even any level drop between these single FFT points. (starting from 0Hz there is FFT points just using Δf interval up to max freq what is sampling freq/2)Roughly rounded it can say that Flat Top window "RBW" width is 3.7*Δf (and shape factor is roughly 2.5) (-60 dBc width roughly bit over 9*Δf)Flattop window scallop loss is roughly 0 so it is level accurate also with signals between FFT points (what exist Δf intervals starting from 0f).Other windows have different "RBW" width and different shape factor and scalloping lossWhat are optimal FFT settings depends what we are doing. In some case we need speed but not high resolution... in some case we want look some carrier with weak modulation so we need perhaps maximal resolution and perhaps still good level accuracy for measure modulation levels.Example if look 7.05MHz 0dBm carrier with 0.2% AM modulation there need "all resolution what can get" (SDS2kX HD) for look and try measure this for get somehow acceptable level accuracy for this modulation level. (-60dBc peaks 120Hz distace from carrier).So there need select 20MHz sampling freq for FFT (full span is then 10MHz) and 2Mpts FFT length. So we get Δf 9.54Hz. Now we can roughly think Flat Top window RBW is <40Hz (perhaps near 35Hz (-3dBc))Then set center to carrier and some example 1 or 2k span. With FFT FlatTop window it give roughly -60dBc level for modulation peaks. Of course if modulation depth is more, example 10 or 50% it goes very easy. (as we now know Flat Top window "width" at -60dBc is around 85Hz we have enough resolution for measure these 120Hz modulation peaks.)ETA: oh yes Performa01 answer allready when I have been slowly writing answer multitasking with many other things with family etc.

author: Martin72
author_rank: Super Contributor
timestamp: December 26, 2022, 10:36:25 am
content: Thank you both !

author: gf
author_rank: Super Contributor
timestamp: December 26, 2022, 11:30:59 am
content: Quote from: Martin72 on December 24, 2022, 02:09:43 pmNo need to bang the head. The unit (dBV, dBm,...) does not really matter. What matters is therelativeamplitude of the harmonics w.r.t. the fundamental, which is usually denoted dBc.BTW, another thing which does influence the SRDF and harmonic distortion is the utilization of the DAC's and ADC's full scale range. SFDR of a DAC or ADC is usually specified in dBFSunits, i.e. relative to afull-scalesine wave. I.e. the specified numbers apply to full-scale sine waves. For lower amplitudes the numbers are correspondingly worse.

author: 2N3055
author_rank: Super Contributor
timestamp: December 26, 2022, 11:34:50 am
content: Quote from: gf on December 26, 2022, 11:30:59 amRespectfully, it matters because distortion won't be same at different amplitudes. Relative dB will be the same of course...

author: markone
author_rank: Frequent Contributor
timestamp: December 26, 2022, 11:59:45 am
content: Quote from: rf-loop on December 24, 2022, 02:01:01 pmJust out of curiosity, what are (and where are specified) DANL and Phase Noise numbers for SDS2504X HD in FFT mode ?

author: gf
author_rank: Super Contributor
timestamp: December 26, 2022, 12:02:04 pm
content: Quote from: 2N3055 on December 26, 2022, 11:34:50 amAgreed. I mean, it does not matter, whether the FFT results aredisplayedin dBm or dBV units.The amplitude of the signal does of course matter, due to different distortion at different amplitudes.

author: rf-loop
author_rank: Super Contributor
timestamp: December 26, 2022, 02:52:26 pm
content: Quote from: markone on December 26, 2022, 11:59:45 amNowhere.Also it is not so straightforward in this case how to setup example for get DANL. It depends... example here in image you can see also one kind of Displayed Average Noise Level in just this used setup./window used in this image is Flat Top. Input Signal is -40dBm ~1MHzNote: during this long averaging there have been input signal frequency drift so marker signal displayed level is dropped nearly 1dB . True signal is ~-40dBm

author: Hexley
author_rank: Regular Contributor
timestamp: December 26, 2022, 05:47:36 pm
content: Quote from: David Hess on December 26, 2022, 05:58:55 amIn case the OP is still looking for a homebrew solution, perhaps this would be of interest:There is a clever alternative to the Wien bridge -- the "Phase Shift Oscillator" -- that can offer somewhat lower distortion. Roger Rosens published a description in Wireless World in 1982https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/80s/Wireless-World-1982-02-S-OCR-IDX-40.pdfof an implementation that uses a distortion cancellation technique to reduce the third harmonic component.A later version of the circuit may be found herehttp://www.redcircuits.com/Page82.htm. This one replaces the gain-stabilizing thermistor with an LED/photo resistor setup. I was able to get a measured 0.00031% THD at 2 kHz with this circuit.

author: Martin72
author_rank: Super Contributor
timestamp: December 26, 2022, 05:55:59 pm
content: Quote from: Martin72 on December 26, 2022, 10:36:25 amSo some things have become clearer to me now, the first result is the re-enactment of the FFT measurement.Looks like I got higher distortions but still it´s good.My new SDG2122X looks worse with the same settings.(Post it in the comparison thread)

author: JBeale
author_rank: Frequent Contributor
timestamp: December 27, 2022, 03:55:57 am
content: Quote from: David Hess on December 26, 2022, 05:58:55 amWords of wisdom! I had paid no attention to capacitor type. I was guilty of using X7R for the RC tuningKYOCERA AVX SR215C104KAA (MLCC - Leaded 50V 0.1 uF X7R )and simply swapping those out forPANASONIC ECW-FD2W104KQ (Polypropylene Film 450VDC 0.1uF)gives me a far better result: Audacity spectrum plot now shows around -90 dBc, and my cheap laptop audio input may start to become a factor here. This is as good a number as I had hoped to achieve. I had not realized how nonlinear the X7R could get in such an application, but thank you for pointing that out!

author: rf-loop
author_rank: Super Contributor
timestamp: December 27, 2022, 12:18:58 pm
content: My previous image.Bullshit warning aka fakenews warning!I do not believe these all harmonics and/or these harmonics levels are true out from SDG1032X/62X output!Real spectrum analyzer image isverydifferent! Later about this. Now other things keep busy.

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 12:33:05 pm
content: Interesting that it could be "fake".If you were to add up the thd from your picture, they would probably be in the specification.In an ideal world, the harmonics would fall continuously after the fundamental wave.That they do not do so here has several reasons - but that everything you see could then be "fake"...I'm looking forward to seeing what you pull out of your hat later.

author: 2N3055
author_rank: Super Contributor
timestamp: December 27, 2022, 01:25:50 pm
content: I don't have SDG1000X/2000X...This is for SDG6000X

author: rf-loop
author_rank: Super Contributor
timestamp: December 27, 2022, 02:35:28 pm
content: Quote from: Martin72 on December 27, 2022, 12:33:05 pmNaturally "all" is not fake but... I do not like oscilloscope generate so much these "fake" harmonics.So if someone ask/hope that Siglent implement automatic THD measurement.... please do not. Garbage in leads always to garbage out and this garbage we do not need, even if salesmen can add one row to features list..Due to some reasons I use here more low signal level. Signal level is -20dBm. (it is also attenuated because it is not designed for under 9kHz so I believe mixer level is not problem)Then other thing. SSA specs start 9Khz. Using SSA trace B, I have done some kind of "level correction" curve. Yes this is not now rocket science... only just for fun and make SDS FFT bit questionable for this purpose. Look these "harmonics" in SDS image specially after 7th . What generate these all peaks. It is sure they do not come from generator - least not even close this level.This requires further research. It also requires different methods and tools.Note: there need correct levels specially under 3kHz. B trace input level was -21dBm (swept with MaxHold). A trace input level -20dBm. Fixed 1kHz -20dBmCH1 input level -20dBm. Fixed 1kHz Sine -20dBm fom SDG1032/62X(-20dBm is 63.3mVpp)

author: Calvin
author_rank: Regular Contributor
timestamp: December 27, 2022, 02:42:41 pm
content: Hi,just a number of quickn´dirty measurements of my SDG2042X with my QuantAsylum QA401 24Bit audio analyzer.Yellow traces are from the analyzer, red traces from the SDG2042X.Frequencies: 1kHz and 100Hz, Amplitude: 0dBV (note the analyzer´s ADC has its THD-minimum at -16dBV input level)It shows that the analyzer´s own generator creates a cleaner Signal and at lower noise due to much lower bandwidth.regardsCalvin

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 03:39:49 pm
content: QuoteIf you (or others) can figure out why the scope is outputting "wrong" harmonics (only in amplitude?), then the THD feature for the FFT function would be one of the most useful features ever.I know far too little about FFT, but my little layman's understanding finds it remarkable that the higher harmonics look almost mirrored.It would be interesting to see what our HDO6034A "makes" of the signal with its SA function.Unfortunately I am on holiday...

author: rf-loop
author_rank: Super Contributor
timestamp: December 27, 2022, 04:49:26 pm
content: Quote from: Martin72 on December 27, 2022, 03:39:49 pmExample in my last image FFT full span is 25MHz.Only over 25MHz signals are mirrored. Of course if there is (ref my image) 49.992 MHz signal it is then displayed as 8kHz and same if there is 50.008MHz signal and so on but using SA I believe these are produced mainly inside SDS unfortunately. With SA I can not find anything what explain these when I look SDG output. Because explanation is not there in SDG signal, then, imho, one possible guilty is oscilloscope.Do you think SDG generate these when it is set for 1kHz sine. With spectrum there can see one bit higher spur at 16.4MHz but even it is still inside FFT span and its level is around -85dBm and other spurs are more weak least up to 200MHz.

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2022, 04:51:03 pm
content: Quote from: rf-loop on December 27, 2022, 02:35:28 pmWe must remember that the DSO FFT is created from data gathered from the ADC, be it 8, 10 or 12 bit core. Whereas the SA is from a swept type measurement. Recall most SA that use the swept mode also feature a log amp which has a large DR, more so than conventional linear amps, and possible input scaling after the passive LPF or BPF in the signal processing chain.Also, the FFT is not benefiting from the dynamic scaling like we see with the FRA/Bode modes of these DSOs, it just the preamp and ADC at a fixed scale factor, and thus limited by such since the "log" dB scale is just a computation from the ADC raw data and not "preprocessed" before ADC conversion like the Log Amps & Scaling in a conventional SA.I'm not surprised the FFT shows additional "artifacts" & various different harmonic levels over the conventional SA with it's swept capability.Agree, the THD may not be an attractive added feature for signals with low THD, likely limited to something on the order of the core ADC and input amplifier linearity, so maybe 40~50dB, and thus not very attractive for any low level distortion characterization.Like the use of theTrace B correction displayto revel the error when the SA is operated below the lower frequency spec limitEdit: Should also add that these signals in question here originate from a 14 or 16 bit core DAC, whereas the DSO has a 8, 10 or 12 bit core ADC. In signal processing terms this is bass-ackwards (old engineering term to highlight a backwards condition), the measurement should have a higher effective resolution/precision than the signal of interest if one expects the results to be meaningfull regarding the "quality" of the original signal.Best

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 05:15:13 pm
content: QuoteYou could also choose a frequency that is in the range, 10khz for example.

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2022, 08:25:32 pm
content: Quote from: Calvin on December 27, 2022, 02:42:41 pmDid some similar tests over here at 10KHz with Pico Scope which has a 16 bit ADC.https://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/25/Best,

author: electr_peter
author_rank: Unknown
timestamp: December 27, 2022, 10:18:01 pm
content: Quote from: rf-loop on December 27, 2022, 12:18:58 pmYou are right to be suspicious. Harmonics are an effect (mostly) of digitization in ADC which creates additional spurs due to "sharpness" of digitization levels and low internal noise in ADC. Additional dither noise before ADC helps to smooth out these spurs and achieve better SFDR.This is an issue with a scope, not with AWG.Sources (to name a few):ADC Input Noise: The Good, The Bad, and The Ugly. Is No Noise Good Noise?Dithering in Analog-to-digital ConversionAN-804 Improving A/D Converter Performance Using Dither

author: Martin72
author_rank: Super Contributor
timestamp: December 27, 2022, 10:23:16 pm
content: FFT, two different vertical settings:https://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/msg4604809/#msg4604809

author: electr_peter
author_rank: Unknown
timestamp: December 28, 2022, 07:33:42 am
content: Can you try this measuring setup:SDG1032X 1 kHz signal with added controllable random noise level which has RMS of around 1/3-1/2 of LSB (LSB w.r.t. to SDS scope)SDS2000X input with FFTWith zero noise FFT looks like a comb (which is BS). With added noise noise floor should rise, but most higher harmonics should disappear. Noise level should be controllable to see the effect. If dither hypothesis is correct, dither noise should help to improve FFT view.Problem without dither is that averaging does not fully work in time domain, which results in "sharp" corners on a waveform, which are shown as a comb on FFT. If signal is 0.8LSB, ADC will always show 1LSB and average will be 1LSB. With dither, ADC sometimes will show 0LSB and average as correct 0.8LSB.

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 08:46:17 am
content: Quote from: electr_peter on December 28, 2022, 07:33:42 amThere is not zero noise in ADC input. There is "lot" of wide band random noise from different parts in front end.As can see in this image below what include noise before ADC and ADC itself.Input CH1 no signal (just open BNC, AC 50ohm)100mV/div. Zoomed in 2mV/div for display noise level. (every horizontal line is one ADC step)Display have 30s persistence.Then there is FFT. (FFT split window mode and then time domain zoomed in (time domain zoom displayed overlayed FFT display area) zoomed in horizontally and vertically zoomed in enough for count ADC steps (in this display area either the time domain scale or the FFT scale is visible, not both)Upper FFT cursor is 2dBm level. It is same as oscilloscope vertical displayed full scale (if go more details ADC full scale is tiny bit more but nonsense here)FFT (using these used settings) noise floor top is around -99dB down from full scale. (2dBm to -97dBm)When look Ch1 noise peak level it is roughly 4.8mVpp what looks like 24 steps. (if it is just 0.2mV/step then full scale is 819mV (display vertical 800mV+ bit overlap ))

author: electr_peter
author_rank: Unknown
timestamp: December 28, 2022, 09:36:12 am
content: Noise is definitely non zero. Does 20MHz limit has an effect on this noise?This leaves the question what is going on with the FFT comb. DNL error or something funky inside SDS?

author: rf-loop
author_rank: Super Contributor
timestamp: December 28, 2022, 10:33:52 am
content: Quote from: electr_peter on December 28, 2022, 09:36:12 amYes, naturally. Not directly just only reduced by BW ratio, there is so many different noise sources and noise is not pure straight white noise coming to then BW filtering.Example just I measured (very roughly and same 100mV/div and all just as in previous image) full BW 600uVacrms (stdev) and 20MHz BW 450uVacrms for example. Also we know this noise is not flat white noise.Here nice info: Intersil (Renesas)/ "Understanding Noise in the Signal Chain"https://www.renesas.com/us/en/document/ppt/understanding-noise-signal-chain

author: electr_peter
author_rank: Unknown
timestamp: December 28, 2022, 04:05:59 pm
content: Alternative title for this topic and relatedcomparison-between-siglent-sdg1000x-and-2000x:"SiglentSDG*000X low noise output highlights limitations ofSiglentSDS2000X HD scope". Oh, the irony

author: 2N3055
author_rank: Super Contributor
timestamp: December 28, 2022, 04:30:44 pm
content: Quote from: electr_peter on December 28, 2022, 04:05:59 pmYou are so funny... You make no sense, but funny....So it's a limitation that oscilloscope has spurs 20 db below it's native dynamic range... Who knew it works that way...

author: electr_peter
author_rank: Unknown
timestamp: December 28, 2022, 04:45:42 pm
content: Quote from: 2N3055 on December 28, 2022, 04:30:44 pmHumor is different for everyone.To be serious, it is clear from specs and experiments that SDG has very low noise and SDS HD is great 12-bit low noise scope. FFT stuff is just a reminder to be careful with readings near noise floor. Just the fact that this nuance gets attention signifies bright future for scopes with 10-12 bit ADCs and low noise AFEs.

author: electr_peter
author_rank: Unknown
timestamp: December 29, 2022, 08:20:49 am
content: Simulations showed that ADC dither is not an issue (noise is high enough to hide quantization effect), but small non-linearities in AFE could cause FFT to show "comb". Assuming non-linear behavior is stable over time, it can be estimated by downloading raw time domain samples and comparing with "ideal" input signal. Not sure if at such low distortion level it is possible to do a digital non-linear correction after the fact or whether it is worth the effort, this requires more modeling. Of course, correction would be scope and input setting specific and would require very pure input signal.FFT view at different input levels and SDS HD scope settings could get small insight at which AFE part causes distortion, maybe there is a sweet spot somewhere.FFT shows something non-linear from AFE because noise is so low. IMO that's better than to drown (hide) distortion in high noise floor.

author: 2N3055
author_rank: Super Contributor
timestamp: December 29, 2022, 09:32:11 am
content: Quote from: electr_peter on December 29, 2022, 08:20:49 amSimulations were not necessarily needed to gain that knowledge. We already knew there is enough noise for dithering.Nothing wrong with verifying. It is nice when theory and practice agree.Nonlinearities introduce harmonic and intermodulation distortions, phase delays and deform BW flatness,that is how it works, correct.Digital non-linear correction is possible in theory but not practical because of sheer volume of real-time data and needed models.As you go through ranges, various levels of attenuation and PGA gain is used in combination. Any of those combinations and different drive levels would create different distortions.There are many sweet spots in attenuation ranges, especially if you go from coarse to fine gain settings.I absolutely agree with last statement (this is what I said before): only reason why people see these spurs is because noise floor is good.

author: electr_peter
author_rank: Unknown
timestamp: December 29, 2022, 09:55:16 pm
content: Quote from: 2N3055 on December 29, 2022, 09:32:11 amChecked possibility of digital non-linearity correction, IMO it's not worth the effort.1) Correction requires non-trivial amount of math operations, proper calibration even more so. 2) Actual distortion is small, but it is non-linear and variable/unstable. Your could be chasing ghosts in AFE during calibration with questionable stability. 3) Adjustment effect is small and almost invisible in time domain for most cases, but it will have some impact on FFT at low signal levels.Too much effort/cost for small improvement in case of general purpose DSO, better to focus design effort on a more linear AFE instead.

author: electr_peter
author_rank: Unknown
timestamp: January 01, 2023, 08:53:34 pm
content: Quote from: 2N3055 on December 28, 2022, 04:30:44 pmIn case of FFT spectrum view, number of points in FFT calculations has an effect on noise floor. For \$N = 12\$ bit ADC and \$L = 4096\$ point FFT, FFT noise floor is combination of signal to noise ratio (\$SNR\$) and FFT processing gain$$FFT\hspace{1ex}noise\hspace{1ex}floor = SNR + processing\hspace{1ex}gain = 107 dB$$where$$SNR = 6.02 \cdot N + 1.76 = 74 dB$$and$$processing\hspace{1ex}gain = 10 \cdot log_{10}{(L/2)} = 33 dB$$With bigger number of data points for FFT noise floor is lower. This is related to SA where narrower \$RBW\$ results in lower noise floor. Alternative view is to think about relationship between power spectral density (\$V_{rms}/\sqrt{Hz}\$) and power spectrum (\$dB\$).Reference:Analog Devices, Taking the Mystery out of the Infamous Formula, "SNR = 6.02N + 1.76dB," and Why You Should Care

author: RoV
author_rank: Regular Contributor
timestamp: January 02, 2023, 11:50:31 am
content: 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 80% level on sin peaks.Processed with Welch method, using a Blackman window of length 4800 to minimize sidelobes: result in figure, with fundamental normalized at 0 dB.Highest harmonic is 2nd, at -96.3 dB. Then 3rd is at -105.5, 4th at -105.2, 5th at -105.9.

author: RoV
author_rank: Regular Contributor
timestamp: January 02, 2023, 12:09:19 pm
content: Test repeated with a lower (~half) gain setting on the E-MU 0202, because I was suspicious about the 2nd harmonic.Now the 2nd is at -103.5 dB, the 3rd as before. Worst is the 10th at -101.9 dB.THD, computed up to the 10th harmonic, is -97.5 dBc, or 0.0013%.

author: rf-loop
author_rank: Super Contributor
timestamp: January 02, 2023, 04:16:13 pm
content: In this topichttps://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/there was also some discussion related to this.Because this topic here now is just named for SDG1032X (SDG1000X series)I think this is good/better place also for this table:There is also one mystery.Siglent have changed SDG1000X 10Hz - 20kHz 0dBm sinewave THD % specification.It have been in old datasheet max 0.075% and in 2022 published latest data sheet (SDG1000X_DataSheet_DS0201X_E01I.pdf) it is max 0.15%Please, who ever publish somehow trusted way measured SDG1000X THD please inform also generator HW version (first two digits is enough). Least I am curious to see if there is real change in latest version. Previous and long time produced HW version was: 02-...... and 2022 started new HW version is 03-....... and 2022 also datasheet have changed.