FFT measurement comparison, SDS824X HD vs. SR760 - Page 1
Forum section: electronics / testgear

author: graybeard
author_rank: Frequent Contributor
timestamp: April 30, 2025, 10:50:07 pm
content: Here is an interesting comparison of the FFT of a 5KHz 1Vrms sine wave from my Siglent SDG2124X AWG. I measured it using my SIGLENT SDS824X HD 12 bit oscilloscope and my Stanford Research Systems SR760 16 bit FFT analyzer. Both the Siglent and SRS are set to use Hanning windows for the FFT.The output of the SDG2124X is fed through RG58 coax into a 50Ω feed through termination. The termination measured as 49.80Ω using my BM869s. The same feed through termination was used on both instruments.I maximized the input signal on both instruments to use all of the available dynamic range. The specified dynamic range on the SR760 is >90dB. The SDS824X HD has no specification on dynamic range, but the theoretical maximum for a 12 bit signal is 74dB.This is the FFT spectrum from the SDS824X HD.This is the FFT spectrum from the SR760.This table compares the results for the first four harmonics from the two instruments.For some reason the second harmonic on the SDS824X HD is 28 dB higher. My guess is this is due to distortion in the input of the SDS824X HD. Since that harmonic is 56dB down from the fundamental the distortion will never be visible on the waveform, only the FFT.Given this measurement what would you say the useful dynamic range is of the SDS824X HD fft? I would say ~55dB.

author: newbrain
author_rank: Super Contributor
timestamp: April 30, 2025, 11:12:04 pm
content: Quote from: graybeard, snips by newbrain on April 30, 2025, 10:50:07 pmSDS804 HD's front end suffers (as clearly shown in your capture) of a fair amount of distortion.To get the best result, you shouldnotmaximize the input signal, as demonstrated by many (possibly me included?) in this forum.Could you be so kind to repeat the measure with the signal taking 50% or less of the vertical span?I'm quite sure you can get a substantial reduction of the harmonics.EtA: I just went and did it. SDG1022 X Plus (plus...), Amazon cheap BNC/SMA adapters and 50 Ω load, decent SMA coax. I tried to reproduce your settings, except for the vertical scale.EtA,again: crap, I did not notice FFT was in max-hold mode, picture updated. In any case, in addition to the obvious noise floor trade off, there's also a harmonic value one: while the 2nd has shrunk >10 dB, the others grew...

author: egonotto
author_rank: Super Contributor
timestamp: May 01, 2025, 12:14:16 am
content: Hello,I did this with the SDS3000X HD and checked it with the 16-bit PicoScope 4262.Best regardsegonotto

author: hpw
author_rank: Frequent Contributor
timestamp: May 01, 2025, 08:19:41 am
content: You should start using a 5kHz Notch, otherwise you will never know whether the THD comes from Hen or Egg (DAC or ADC).Some measurement gear have for that purpose a tracking notch or build by yourself an 5kHz notchAs for the generator some use the well know Victor LDO (low distortion oscillator about -150dBFS, but some higher close-in noise) as seen on diyaudio.com/equipment as with freq. 1kHz or 10kHz. 5kHz altered by yourself.Hp

author: gf
author_rank: Super Contributor
timestamp: May 01, 2025, 08:53:29 am
content: Quote from: SDS800X HD datasheetSo what you see seems to be still within the specs.

author: mawyatt
author_rank: Super Contributor
timestamp: May 01, 2025, 12:48:27 pm
content: Quote from: egonotto on May 01, 2025, 12:14:16 amSince you have the 4262 you could use the 4262 internal generator for the 5KHz signal and compare the FFT results. The 4262 has a high quality low harmonic signal source which is good as a reference source.Best

author: szajsiwo
author_rank: Newbie
timestamp: May 01, 2025, 01:45:46 pm
content: Same problem here wiith SDS2000xHD.Using built in wave gen and straght BNC cable loop from gen to channel 1. Three different signal ampltudes:I wasn't sure is this wave gen or front end nonlinearity but it seems it is UNFORTUNETALLY the latter one.. (from you are saying)I bought this oscilloscope for audio measurments especially FFT measurments of signal transformers and this makes it useless...Is this problem with all scope manufacturers??

author: hpw
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:21:04 pm
content: Quote from: szajsiwo on May 01, 2025, 01:45:46 pmAs I mentioned, simple use Victors LDO 1kHz or 10kHz (H2 & -150dBFS) and a related notch. This once is made for Audio measurements.

author: shabaz
author_rank: Frequent Contributor
timestamp: May 01, 2025, 03:06:57 pm
content: Hi,Usually this is no issue at all. It's more a matter of recognising the instrument distortion and then using that finding to either ignore, or remove the effects of it. Instrument distortion of various types within the dynamic range is quite common. One trick is to keep a few attenuators to eliminate needing to adjust DUT or instrument, to see if the spectrum alters when the attenuator is inserted/removed. If some of the spectrum looks different, then you know it's an artifact from the test instrument and you can ignore those parts. Alternatively you can then attenuate down to reduce or eliminate the artifacts.Note that sometimes all you care about is seeing a fundamental plus harmonics, but other times you care about seeing the content very close around the fundamental, and what you do depends on your needs. Sometimes you might even want to go to extremes and externally filter out the fundamental just so you don't get distortion (or sustantially reduce it), if all you care about is in the remainder of the spectrum.

author: 2N3055
author_rank: Super Contributor
timestamp: May 01, 2025, 03:20:47 pm
content: Quote from: szajsiwo on May 01, 2025, 01:45:46 pmAudio analysers use 24/32 bit ADC and special low noise front ends.How do you expect a 12 bit scope made for 500MHz BW could do the same job..Simple physics dictates those are two completely different instruments.There is no scope of similar BW (as in it cannot be made) that would do what you expect it.I am afraid it is not a "problem with all scope manufacturers" but your misunderstanding.Actually SDS2000xHD does a decent job for a 12bit 500MHz scope.HPW actually has a point, a use of notch filter would "suppress" fundamental tone, and you could use higher sensitivity on scope and gain a lot of range.This scope does not have any measurements specific to audio work in a first place.It is a great general purpose scope to work on audio equipment electronics (together with decoding protocols) but for audio measurements you need to use something else. Even if you had full analytic capability of audio analyzer, 12 bit ADC is not enough.

author: Martin72
author_rank: Super Contributor
timestamp: May 01, 2025, 03:47:21 pm
content: QuoteThis.The 12 bits show their strengths over 8 bits in combination with a low-noise front end when measuring small voltages (ripple).FFT is still useful in the audio field if you are aware of its limitations and want to perform rough troubleshooting, or if the distortion factor of the object being measured is of secondary importance.At work, we use the FFT function in the “Audio” range to get a quick overview of the purity of the 60/400 Hz sine wave from our inverters.The THD can be up to 3%, so using an oscilloscope is no problem.

author: KungFuJosh
author_rank: Super Contributor
timestamp: May 01, 2025, 04:27:19 pm
content: The QA403 is much better for audio. Anybody have a good link for the Victor LDO? I'm interested to check it out since Istilldon't have a QA403.

author: 2N3055
author_rank: Super Contributor
timestamp: May 01, 2025, 04:50:50 pm
content: Quote from: KungFuJosh on May 01, 2025, 04:27:19 pmLook here:http://www.janascard.cz/cj_Download.html

author: Martin72
author_rank: Super Contributor
timestamp: May 01, 2025, 05:03:15 pm
content: And here (Supply is missing)....

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2025, 05:29:44 pm
content: Is the Victor low-distortion oscillator still available?I can’t find it, searching on Google.

author: hpw
author_rank: Frequent Contributor
timestamp: May 01, 2025, 05:47:50 pm
content: Quote from: TimFox on May 01, 2025, 05:29:44 pmAll about onhttps://www.diyaudio.com/community/threads/low-distortion-audio-range-oscillator.205304/as lengthy threadSome last messages:https://www.diyaudio.com/community/threads/low-distortion-audio-range-oscillator.205304/post-7929838andabout sending PM to him at he is at Riga & Lativaon diyaudio forumhttps://www.diyaudio.com/community/threads/low-distortion-audio-range-oscillator.205304/post-7932651

author: Martin72
author_rank: Super Contributor
timestamp: May 01, 2025, 06:25:39 pm
content: Here is the FFT from the Magnova Scope.First 1Vrms, flat top, 1Mpts, 4-times averaging.Then again 0dBm, from Hann, 8 Mts, 4-times averaging.The source is an SDG6052X, impedance 50 ohms.

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2025, 06:54:09 pm
content: Quote from: hpw on May 01, 2025, 05:47:50 pmDo you have a direct contact e-mail for him? I don't subscribe to diyaudio.

author: egonotto
author_rank: Super Contributor
timestamp: May 02, 2025, 01:49:53 am
content: Hello,another picture of the Micsig MHO3.Best regardsegonotto

author: graybeard
author_rank: Frequent Contributor
timestamp: May 02, 2025, 07:50:14 am
content: Quote from: newbrain on April 30, 2025, 11:12:04 pmSince my first post I have thought about this a bit more and I believe the actual story is more nuanced.To start with, I got a better measurement with my SR760. By changing the grounding, I was able to gain 10dB of dynamic range. Here is the improved measurement of the 0 dBV sine signal from my SDG2124X. This result is included in the following graphs.I looked at the specifications for the SDS800X HD series oscilloscopes. In those specifications Siglent specifies the Equivalent Number of Bits (ENOB) at 8.4. This reduction ineffective bits from 12 is due to noise and distortion.The ENOB for any ADC system is always less than the theoretical limit for the number of bits.For a given number of bits, the dynamic range = 20 * log(2^(N-1))The minus one comes from the fact that we are measuring an AC signal, so the maximum amplitude is only half of the input range. Based on this formula the following graph shows the expected dynamic range based on the number of bits.The following graph shows the minimum signal that can be accurately measured. For a given vertical range setting in Volts/division of a SDS800X HD vertical channel.So as requested I went back and remeasured the FFT spectrum at various input settings, this time for all four channels. In the following graphs I also include the accurate measurement limits for each input setting. For every case the SDS824X HD exceeds is published specification by 10 dB or more.In these measurements you will notice the measured value of the second harmonic drops. As you increase V/div you are using fewer effective bits for the measurement. So is the apparent decrease in the second harmonic due to less distortion, is at an artifact of the reduced sensitivity, or both? I contend it doesn’t matter since all of the measurements 10dB or more are better than the stated accuracy of the instrument.

author: hpw
author_rank: Frequent Contributor
timestamp: May 02, 2025, 10:09:27 am
content: Quote from: TimFox on May 01, 2025, 06:54:09 pmmay look at his web sidehttps://viccc42.wixsite.com/uld-audio... there will be an email.Important to know:- the PCB requires boxing, otherwise large hum pickup- can be connected using symmetric connections- at best, requires batteries powering

author: shabaz
author_rank: Frequent Contributor
timestamp: May 02, 2025, 08:13:00 pm
content: Quote from: graybeard on May 02, 2025, 07:50:14 amHi,Whether it's 10 dB below a dynamic range derived from ENOB is not really the issue, because it's visible on your screen in a very usable area of FFT-derived dynamic range (which will be a larger range, partially adjustable by your FFT settings), therefore your aim is to (at least initially) simply determine if it's instrument caused, or really existing in your DUT signal.You do that by either (possibly) making amplitude adjustments on the DUT or attenuation/gain/volts/div adjustments on the test instrument, or by physically inserting attenuation into the system (all may work, but inserting attenuation means you're guaranteeing most closely that you're comparing things without any other unexpected changes to the rest of the system).Once you've done that, you'll have the knowledge whether the content is something you can :(A) use as displayed (i.e. even if it's 10 dB below, you can and should still use if you can, because it's a valid part of the content, and one key advantage of the spectrum display is simply that one benefits from that extra dynamic range), or(B) something you can safely ignore in the specific frequencies that you've determined are derived from instrument distortion, or(C) something that you may desire to measure differently, for instance adjusting the spectrum to reduce or eliminate the instrument distortion (such as inserting a filter to remove the fundamental).

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2025, 04:22:29 am
content: Here's something from 8-bit land to think about. First, the 5kHz 1Vrms, then the same but with a 500mV offset, then with the offset but the input set to AC. All using an external 50R feed-through terminator.You can also significantly change the level of the 2nd harmonic by adjusting the vertical position knob on the scope trace. Using 1V/div so that the entire signal could be above or below the center line, I was able to get a 10dB difference depending on where I put it on the screen.

author: Someone
author_rank: Super Contributor
timestamp: May 03, 2025, 05:52:01 am
content: Quote from: graybeard on May 02, 2025, 07:50:14 amThat is a gross oversimplification, dynamic range is generally not used in this context:https://www.analog.com/media/en/training-seminars/tutorials/MT-003.pdfAs soon as you start measuring on the FFT then you have introduced bandwidth to the noise measurement, and the usable noise floor can be well below the "dynamic range" of a single ADC conversion because the FFT is the aggregate result of many many ADC conversions.shabaz provides some excellent practical pointers on how to validate your noise floor

author: Someone
author_rank: Super Contributor
timestamp: May 03, 2025, 06:06:48 am
content: Quote from: bdunham7 on May 03, 2025, 04:22:29 amVery interesting, repeated it here on a scope and had significant variation (>20dB for 2nd harmonic, >30dB for 3rd) depending on where the acquisition was vertically (be that external DC offset or AC coupling and internal DC offset). Interestingly the variation was non monotonic suggesting ADC calibration rather than amplifier non linearity in that case.

author: mawyatt
author_rank: Super Contributor
timestamp: May 03, 2025, 01:47:39 pm
content: Quote from: Someone on May 03, 2025, 05:52:01 amWalt's short FFT tutorial is one of the best, especially wrt DSO useBest

author: mawyatt
author_rank: Super Contributor
timestamp: May 03, 2025, 02:57:04 pm
content: Quote from: bdunham7 on May 03, 2025, 04:22:29 amWe don't see as much 2nd Harmonic "improvement" with the 12 bitters. However, you can find a few "spots" where you can effectively "null out" the 3rd Harmonic with DC Bias !!Best

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2025, 03:07:47 pm
content: Quote from: Someone on May 03, 2025, 05:52:01 amADC conversion plus noise! Here are three screenshots at 1V/div, the first showing a full-range input of 2.83Vrms, 8Vp-por ~+9dBV. The second is 500µVrmsor -6366dBv. This is about 1/64 LSB! You can clearly see the signal although it is a bit low. The third is the same 500µVrmswith a superimposed 50mVp-ptriangle wave at 1MHz (triangle dithering--sort of). That shows the -6366dBV signal quite accurately, so with a proper dither you can trade BW for dynamic range, 72dB in this case. 500µV is as low as my AWG will go and attenuators won't work because the AWG scaling won't allow me to increase the triangle wave, but it appears there's a bit more room to go lower. So with 8 bits and enough BW you probably can get 80dB of dynamic range.

author: gf
author_rank: Super Contributor
timestamp: May 03, 2025, 07:47:20 pm
content: Quote from: bdunham7 on May 03, 2025, 03:07:47 pmA few remarks:Instead of "enough BW", I guess you mean "narrow enough (measurement) BW"? (in your case ~25Hz)I wonder why the noise floor in image #18 is higher than in #16? The scope settings seem to be the same. Is the generator noisier in #18?If a 1MHz triangle is sampled at 2MSa/s, then we get only a 2-level dithering and in the worst case (when the triangle is sampled at phases is 0 and PI) no dithering at all.FFT size was only 128k. So there may still be room for up to 12dB SNR improvement when the full FFT size is used.

author: graybeard
author_rank: Frequent Contributor
timestamp: May 03, 2025, 08:28:57 pm
content: I changed the waveform being measured from a sine wave to a square wave. I made no other changes on the setting of the SDG2124X. Notice the waveform is not perfectly symmetrical since the rise and fall times differ slightly.This is the 0-100 KHz spectrum I measured using the SR750.This is the 0-100 KHz spectrum I measured using the SDS824X HD.This table details the readings of all the harmonics from the two instruments.Notice that on the SDS824X HD spectrum has very strange “skirts” on the waveform below -60dBV and does to really show the even order harmonics below -60dBV shown by the SR760. So I decided to investigate the skirts of the 9th harmonic (45KHz).First look at 45KHz peak on the SR760. As expected, it shows a very narrow single peak with good looking skirt which is only 1.5Hz wide at the measurement floor of -110dBV.Now look at the zoomed in view of the 45KHz peak and skirt from the SDS824X HD. This skirt is 1.6KHz wide at -110dBV. I have no idea where the ripple on the skirt comes from, but based on the SR760 measurement that signal is being generated internally in the oscilloscope.I believe you should use the full range of the scope ADC when making measurements. However, I would not take anything below the 10 bit line as a valid measurement. The data below the 10dB line could be useful for detecting changes in a waveform, but not useful for absolute measurement.

author: bdunham7
author_rank: Super Contributor
timestamp: May 03, 2025, 08:37:49 pm
content: Quote from: gf on May 03, 2025, 07:47:20 pmBy "enough BW" I was referring to the BW of the scope being much higher than the BW we're looking at. As in 500MHz vs 50kHz.The noise floor is about the same as #15, the settings are the same for all three screenshots. Why the one with a very small input has a lower noise floor is an interesting question that I'm guessing is related to amplifier nonlinearity.The dithering issue would only happen if the sample clock and AWG clock were perfectly correlated. Properly implementing triangle dithering is a bit more complex than what I've done here. If I pick a triangle wave fundamental too far from a submultiple of the sample clock then I get spurs everywhere. If it were exactly a submultiple, then we'd have the issue you mention. If I make it 1M + 50Hz, then I get a little spur at 50Hz which I can live with. The trick is to push all the effects from the dithering either up above the area of interest or down near DC. As it happens, my first attempt here just used whatever natural difference there is between the AWG clock and the scope sample clock, plus 64 averages. The FFT looks less pretty if you use less averaging.Slowing down the scope and going to a 2M FFT did lower the noise a bit, but not 12dB!

author: graybeard
author_rank: Frequent Contributor
timestamp: May 03, 2025, 10:46:34 pm
content: I like to experiment with my test instruments to understand their capabilities and limitations. The SDS824X HD is my newest “toy.” That is why I started this experiment and thread.The Stanford Research SR760 is a 16 bit instrument with 100 KHz of bandwidth. It specifies 90dB of dynamic range but typically does a bit more. It is optimized for doing low frequency FFTs.The Siglent SDS824C HD is a 12 bit instrument with 200 MHz of bandwidth. It is optimized for the display of time domain waveforms.The SDS824X HD has 2000× the bandwidth and four less 4 less bits (24dB) of native resolution than the SR760. Expecting the 200 MHz scope to deliver the same audio 90dB+ dynamic range and spectral results as an optimized instrument is fantasy.The Siglent scope is a useful tool for FFT spectrums but has limitations. Hopefully this thread will help you realize those limitations as you utilize your SDS800X HD series scope.

author: gf
author_rank: Super Contributor
timestamp: May 04, 2025, 07:56:59 am
content: Quote from: graybeard on May 03, 2025, 08:28:57 pmThe first sidelobes of a Hann window are only about -31 dB. Use a different window function if you want to get a spectrum analysis filter with higher selectivity and better stopband attenuation. The frequency responses of a few window functions are attached. The price you have to pay for higher selectivity is a wider main lobe and a wider ENBW, which means a higher noise floor - there is no free lunch. If you want to measure the amplitude of "peaks" of periodic signals accurately, you need a flattop window which has very low scalloping loss. The Siglent variant of the flattop window has also high selectivity, I think the side lobes are lower than -90 dB. The price is, however, an even wider ENBW.Also keep in mind that the FFT bin spacing on your SR760 screenshot is more than 100x narrower (30,5mHz) than on your SDS824X screenshot (3,81Hz). While the FFT size of the SR760 is very limited, it does a zoom FFT via downconversion if you zoom into a narrow span, while most scopes only support baseband FFT.

author: gf
author_rank: Super Contributor
timestamp: May 04, 2025, 08:21:24 am
content: Quote from: bdunham7 on May 03, 2025, 08:37:49 pmAgreed, that's true, of course.QuoteHonestly, I did not expect to get the full 12dB, because the noise isn't white/flat. That's why I wrote "up to 12dB".

author: graybeard
author_rank: Frequent Contributor
timestamp: May 04, 2025, 11:33:05 am
content: Here is the same square wave as previous measured with various window functions.Blackman:Blackman-Harris:Flat top:Gaussian:Hamming:Hanning:Rectangle:None of these spectra show the correct even order harmonics and all look questionable below -60dBV.

author: graybeard
author_rank: Frequent Contributor
timestamp: May 04, 2025, 11:49:08 am
content: Here are the same window functions zoomed in on the 9th harmonic (45KHz) from 44KHz to 46KHz.Blackman:Blackman-Harris:Flat top:Gaussian:Hamming:Hanning:Rectangle:

author: Martin72
author_rank: Super Contributor
timestamp: May 04, 2025, 02:08:31 pm
content: Hi,Very interesting.I used the Magnova Scope to reproduce the last measurements, averaging 12x, otherwise I would still be measuring.In the Magnova world, there are (almost)no Even harmonics....EDIT: Pics deleted, new uploading

author: Martin72
author_rank: Super Contributor
timestamp: May 04, 2025, 02:20:19 pm
content: Further screenshots..

author: Martin72
author_rank: Super Contributor
timestamp: May 04, 2025, 04:23:04 pm
content: Finally, and then no more spamming here, measurements with the SDS3104X HD.Same settings as for graybeards' measurements(except avg, here 64), but note thedifferent RBWfor thesame delta-fcompared to the SDS800X HD.Except for “Flat Top” (3.73 in both cases), the coefficients are different.Fewer window types, as the 3000X HD has not yet received an update in this direction like the SDS800X HD.

author: Martin72
author_rank: Super Contributor
timestamp: May 04, 2025, 04:29:37 pm
content: Harmonics at 45kHz, span at 2kHz.

author: 2N3055
author_rank: Super Contributor
timestamp: May 04, 2025, 06:01:38 pm
content: Like Martin's Magnova images show, for a squarewave (at least a nicely shaped one) there should be no even harmonics.It consists exclusively from odd harmonics that have amplitude dropping by it's order : f*3 is 1/3 of amplitude, f/5 is 1/5 and to infinity.In this case SR760 is showing wrong picture. It should not show even harmonics, in theory. But, square wave is not perfect, so it might show something. But surely not at that amplitude.A quick check with (also 16 bit instrument) Picoscope 4262 (5MHz BW) shows very similar results like SDS800xHD. Von Hann window used to make it directly comparable.Even harmonics (i have faster SDG6052X AWG) are better than -100 dBV

author: graybeard
author_rank: Frequent Contributor
timestamp: May 04, 2025, 06:36:26 pm
content: Quote from: 2N3055 on May 04, 2025, 06:01:38 pmI was thinking about the even harmonics on the SR760 last night and I think those are being generated by internal slew rate limiting in the SR760. It clearly does not have the bandwidth to see the 11.3ns edges of the square wave.The square wave from the SDG2124X has difference in the rise and fall times; 11.25ns vs. 11.36ns. This difference can account for the even harmonics.

author: TimFox
author_rank: Super Contributor
timestamp: May 04, 2025, 07:06:20 pm
content: The absence of even harmonics is a result of perfect time symmetry in an ideal square wave (equal “mark” and “space” times). This is also true for other symmetric waveforms, such as triangle waves. When unpacking a new spectrum analyzer, I first played with a waveform generator to see the even harmonics appear and disappear as I adjusted the symmetry of a square wave.

author: Martin72
author_rank: Super Contributor
timestamp: May 04, 2025, 07:26:04 pm
content: I tried that right away.I left the SDG6052X at 50% duty cycle, then set it to 50.01%.(FFT Blackman, center accidentally set to 45 kHz)

author: gf
author_rank: Super Contributor
timestamp: May 04, 2025, 08:01:38 pm
content: Quote from: TimFox on May 04, 2025, 07:06:20 pmAnd the OP's square wave is really good in this respect:

author: graybeard
author_rank: Frequent Contributor
timestamp: May 04, 2025, 10:02:25 pm
content: In playing with the SDS824X HD in an attempt to optimize the FFT I found that one can do two FFTs simultaneously. However when I tried to add a third FFT the scope froze.In playing with the settings I found I got the nest results when I set the memory depth 100M points. The scope is only using 50M points when the FFT is turned on. That memory is most likely being dedicated to the FFTs.I also found in playing with the windows I get the best results using the Blackman-Harris window. So the Blackman-Harris window will be used for the following examples.As I tried different setting for the waveform I found that if I set the maximum FFT size to 128K I get the best results. They FFT results degrade if the maximum setting is made smaller or larger than 128K.This table shows the displayed FFT parameters for the four maximum memory depths I will show here.This shows the maximum setting of64K in redand128K in blue:This shows the maximum setting of256K in redand128K in blue:This shows the maximum setting of1M in redand128K in blue:

author: gf
author_rank: Super Contributor
timestamp: May 05, 2025, 06:25:54 am
content: Quote from: 2N3055 on May 04, 2025, 06:01:38 pmYes, the even harmonic levels shown by the SR760 are not plausible for the given signal.But the zoomed FFT pictures herehttps://www.eevblog.com/forum/testgear/fft-measurement-comparison-sds824x-hd-vs-sr760/msg5902968/#msg5902968cannot be correct either, w.r.t the given sample rate and number of FFT points. Is/was there a known bug?[ Compare to Martin's SDS3000 screenshots herehttps://www.eevblog.com/forum/testgear/fft-measurement-comparison-sds824x-hd-vs-sr760/msg5903286/#msg5903286which use the same parameters - they look reasonable. ]

author: graybeard
author_rank: Frequent Contributor
timestamp: May 05, 2025, 06:54:27 am
content: I want to revisit the apparent distortion. To do this I mixed a two sines waves; 10KHz & 11 KHz. Both outputs were set to 1Vrms.For the FFTs I set the maximum FFT depth to 128Kand window to Blackman-Harris.In the following GIF you can see that as the input sensitivity is reduced from 40mV/div to 1V/div the noise floor comes up. The distortion products don't magically disappear, they just get lost in the rising noise floor.As expected the best results come from when the waveform fills the screen, not when you reduce sensitivity to supposedly correct distortion.

author: Performa01
author_rank: Super Contributor
timestamp: May 05, 2025, 04:47:50 pm
content: Quote from: graybeard on May 04, 2025, 10:02:25 pmFor comparable results, we should make sure to have the latest FW installed, i.e. 1.1.6.5.In my review of the SDS824X HD I've demonstrated the parallel use of up to four FFTs, so got curious if something has been broken with a later FW update. But thankfully no, everything works as expected with 1.1.6.5:SDS824X_HD_FFT_Sine_L2M_512k_128k_32KSorry, but all these findings are not helpful and give the false impression of some pretty unreliable DSO function. This isn't actually the case - there are clear rules, so we can know in advance how to set up an FFT for a given task.Since the FFT length is limited to 2 Mpts, there is little use in having considerably longer records, let alone 50 or even 100 Mpts. The max. analysis length of the SDS800X HD is 10 Mpts and unlike the new higher end machines it doesn't have a global Analysis Gate (short: AG). So if the record length is 50 Mpts for example, this gets first decimated to 10 Mpts for general Math processing, then once again to 2 Mpts (or even less) for the FFT. You need to know (and allow for) that, in order to avoid some nasty aliasing traps.While Blackman Harris is probably the most popular window for audio, precise measurements demand a good Flattop window, which the Siglent scopes generally provide.For the FFT-length, a simple rule applies: the more the merrier. More FFT points mean smaller frequency steps and lower RBW, hence also better frequency resolution and lower noise floor. Only tradeoff is FFT length vs. FFT speed.Knowing all that, the findings and screenshots are just not plausible - and the reason is all too obvious: a square wave with reasonably fast edges will produce a spectrum that vastly exceeds the FFT bandwidth of 100 kHz (200 kSa/s) and also 2.5 MHz (5 MSa/s).The first screenshot in this posting has been produced with a 45 kHz sine wave and four FFTs with Flattop window and their lengths limited to 2 Mpts, 512, 128 and 32 kpts. As can be seen, with a sine source everything is fine. Now let’s switch to a 5 kHz square wave with 2 ns rise/fall time with the exact same measurement:SDS824X_HD_FFT_Square_RT2nsNow the FFT length makes a difference indeed, yet it doesn't matter, since all four traces show just nonsense. The dreaded aliasing takes its toll…We can only maximize the FFT length (2 Mpts) to get the highest possible sample rate - and if that’s not enough as in this example, we can reduce the rise time of the square wave. The 20 MHz bandwidth limiter has been engaged as an additional aid, yet not very effective, because the FFT bandwidth is still limited to 2.5 MHz. First attempt is 35 ns rise time:SDS824X_HD_FFT_Square_RT35nsThat looks a lot better already, but there are still spurious sidebands, stemming from the aliasing. We need to slow down to 350 ns rise time in order to get a halfway decent result:SDS824X_HD_FFT_Square_RT350nsFun fact: if the DSO were able to do a 20 Mpts FFT, then the results of a 35 ns edge should have been satisfactory in the same way.

author: graybeard
author_rank: Frequent Contributor
timestamp: May 05, 2025, 06:23:22 pm
content: I have been using the 1.1.6.5 firmware since it was released.I have tried three times to enable more than two FFTs. Each time the scope froze. I just tried it again and the same results. Just after I added the third math function F4 it froze. I was connected to the scope through the web server so I have an image of the last screen.This freeze state (with this combination of settings) is repeatable for me

author: mawyatt
author_rank: Super Contributor
timestamp: May 05, 2025, 07:14:54 pm
content: @ graybeardWe can also get 4 FFTs simultaneously and running 1.1.6.5 version. Try resetting to default and work up from there.Best

author: 2N3055
author_rank: Super Contributor
timestamp: May 05, 2025, 08:06:55 pm
content: Quote from: mawyatt on May 05, 2025, 07:14:54 pmI can only repeat this.3xFFT+1xAverage()4xFFTLeft it running for an over than hour with first combination, then 4xFFT for more than that, while web control was running.It keeps triggering normally. It does slow down a bit though.

author: mawyatt
author_rank: Super Contributor
timestamp: May 05, 2025, 08:14:37 pm
content: Yeah 4 math functions, be those FFTs or otherwise it seems!!Would play around with it some, but have to head for an appointment.Not too shabby for a ~$500 DSO IMO!!Best

author: Performa01
author_rank: Super Contributor
timestamp: May 06, 2025, 09:28:25 am
content: Quote from: mawyatt on May 05, 2025, 08:14:37 pmYou can still use the formula editor in order to save math channels. So e.g. FFT(Filter(C2+C4)) is perfectly usable.From this perspective, one could say they can get four FFT as a final result, yet with almost any number of standard math operations for the pre-processing of the data.