LeCroy 3034 - has anyone got the FFT to do anything? - Page 1
Forum section: electronics / testgear

author: peter-h
author_rank: Super Contributor
timestamp: December 10, 2021, 03:43:35 am
content: It just doesn't seem to work. It does display a spectrum but the spectrum is obviously nonsense. If I feed the signal to an old TDS2004B (which certainly has no "deep" config for that feature) I see the expected spectrum but that scope is quite noisy so the dynamic range is poor.I wonder if there is a specific config somewhere, but I can't find any. The youtube videos they have (for similar scopes) also show nothing.The signal in question is a sinewave made up of say 64 steps - herehttps://www.eevblog.com/forum/projects/suggested-low-pass-filter-for-a-dac/

author: tautech
author_rank: Super Contributor
timestamp: December 10, 2021, 04:27:10 am
content: Screenshots of FFT and FFT menu settings ?

author: Martin72
author_rank: Super Contributor
timestamp: December 10, 2021, 05:56:14 pm
content: Hi,I guess you got a WS3034 ?QuoteI´ve read this post at work and went to our lecroy ws3024...It was the first time I´ve used the fft function...The fft menu is very simple, no tablefunction, peakmarkers, etc....Pics : 1khz sinewave, 10khz sinewave, fft menu...Martin

author: tautech
author_rank: Super Contributor
timestamp: December 10, 2021, 06:15:48 pm
content: Just as I suspected, FFT results are strongly related to the # of cycles displayed just like in Siglents which is in turn governed by the memory management which is different to the OP's Tek.Thanks Martin.

author: peter-h
author_rank: Super Contributor
timestamp: December 10, 2021, 08:21:21 pm
content: That's an amazing find. Yes I was displaying no more than one or two cycles on the LeCroy, and then switching to the Math menu to do the FFT. It showed just one nondestript and broad peak, instead of the line spectrum showing the fundamental and the various harmonics. I will try it again when I am back near the scope in a week's time.

author: Martin72
author_rank: Super Contributor
timestamp: December 10, 2021, 09:04:49 pm
content: Hi,Have a look at this fft post also:https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg3782555/#msg3782555

author: peter-h
author_rank: Super Contributor
timestamp: December 12, 2021, 05:09:38 pm
content: I have a Anritsu spectrum analyser which has more knobs and config options that I ever worked out what they do, whereas these digital scopes seem to have almost none... If it wasn't so damn heavy I would have used it on this project, but also it only goes down to 10kHz.

author: peter-h
author_rank: Super Contributor
timestamp: December 18, 2021, 05:41:49 pm
content: No, no luck, the 3024 FFT does almost absolutely nothing, no matter what x scale I use.

author: Martin72
author_rank: Super Contributor
timestamp: December 18, 2021, 11:20:57 pm
content: I can only reply what tautech said:Quote

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: December 19, 2021, 02:31:48 am
content: I designed my own USB PC scope.I wanted a spectrum analyser screen too so looked up FFT's source code online.Plenty around.Sadly it took five different versions before I got one that worked well.While some display spectrums they were very poor or simply wrong.

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 07:49:58 am
content: OK here are screenshots. 500Hz 3V peak square wave, and FFT selected on the math menuThis shows the FFT configIf I change the Output type from Amplitude to Power Spectrum I get thisPlaying around with the "zoom" values (centre frequency and freq per division) I get more complete nonsenseWhat I am expecting is the standard spectrum of a square wave, which is not exactly rocket science! The TDS2004B shows this immediately.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2021, 08:17:39 am
content: Slow your main timebase down so to have several hundred periods on the display.Refer to replies 2 and 3.

author: gf
author_rank: Super Contributor
timestamp: December 19, 2021, 10:08:43 am
content: Quote from: peter-h on December 19, 2021, 07:49:58 amThe yellow trace in your 3rd screeshot suggests a frequency resolution of ~4kHz.With 4kHz resolution you can't expect that the trace can resolve a peak every 500 Hz.In order to get a (significantly) better frequency resolution you need to reduce the sample rate and/or increase the number of FFT points.4kHz resolution at 2GSa/s implies that you were already using 500k FFT points. Don't know if the scope supports more points at all.So likely the sample rate is the only wheel you can turn. A 0...50kHz span is well sufficient for 500Hz up to the 100th harmonic. You don't need a 0...1GHz span for this signal.And 50kHz Nyquist implies a sample rate of only 100kSa/s, i.e. a factor of 20,000 lower than your current sample rate of 2GSa/s.As tautech said, try to slow down the timebase until sample rate drops to say 100kSa/s (or maybe even lower).

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 10:34:44 am
content: Yeah - that did eventually produce something that looks about right, but a) the spectrum depiction is still crap and b) it is not obvious to me at which point (how many cycles on screen) are needed to get some sort of result.This is one result - you can see the calculation is producing very few samples horizontally, and drawing straight line segments between themNow let's have a lot more cycles on the screen - not any betterFurther experimenting shows that showing any fewer cycles than the uppermost image, destroys the whole spectrum.Going back a bit but this time looking at a sinewave, you get garbage again; in this case anything faster than 5ms/div on the timebase stops it workingNow with a different waveform. This is 400Hz, with 128 DAC samples per cycle, so we expect harmonics every 51.2kHz, and this is shown correctly, even at 5ms/divThis is the same waveform through a Sallen-Key 3rd order lowpass filter - appearing as one would expectThe worrying thing is that thing scope works if you know what waveform to expect and you tweak it until it shows itIt appears to need around 40 cycles per screen to work at all, though it depends somewhat on the signal. Is there some objective procedure to set this up? The parameters available don't resemble the config on the Anritsu spectrum analyser I have.Just spotted gf's post. Here is another image, with the sample memory reduced from 10ms to 1ms, but I am not sure what this has achieved apart from making the floor much more noisyIs there no way to store images on the EEVBLOG server? Fortunately I have my own FTP server, and a screenshot program which uploads to it, but most people don't have that, and I already have 1GB+ of images stored on it.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2021, 11:10:17 am
content: Much better Peter.Quote from: peter-h on December 19, 2021, 10:34:44 amYes, it's the way LeCroys' and Siglent's work and primarily due to their WYSIWYG memory management.You need have a large sample of data to get the bins for FFT to give meaningful results.In LeCroy's it seems you don't have Start, Stop and Span adjustments although I may have missed them so you probably need to work with a Hz/div setting to get the info you need.Just a step back and some thought about what is staring at you on the display can help immensely to get worthwhile results.QuoteYes use the Attachments and other options on the posting page to add files for them to appear as thumbnails when you post. Use the (more attachments) link and not the first as it's known to give trouble and later you can embed images anywhere in your post by placing the thumbnail URL between [ img ] image URL [ /img ] flags.

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 11:16:34 am
content: QuoteHave a look at his last pic on post#10..

author: gf
author_rank: Super Contributor
timestamp: December 19, 2021, 11:21:30 am
content: Quote from: peter-h on December 19, 2021, 10:34:44 amAt 100MSa/s, Nyquist is 50MHz, thus the FFT covers a 0...50MHz span. I.e. your 0..500kHz display shows only 1% of the calculated FFT spectrum, and 99% of the FFT spectrum is not used.OTOH, at 10MSa/s, the FFT covers a 0...5MHz span, i.e. your 0..500kHz display now shows 10% of the calculated FFT spectrum (still 90% are unused).Assuming 500k FFT points1), 100MSa/s lead to a frequency resolution (bin width) of 200Hz (= 100MHz / 500,000), while 10MSa/s lead to a 10x better resolution of 20Hz.The "apparently more noisy" is likely a display artifact, caused by the way how the scope maps/squeezes 25000 horizontal frequency points to a significantly lower number of screen pixels.When you zoom in, you should eventully notice the better frequency resolution at 10MSa/s, compared to 100MSa/s.1)Check the actual number of FFT points in you FFT setup

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 12:38:46 pm
content: I am trying to get my head around how to produce a "menu" for setting up this scope...Presumably the 8 bit ADCs will be the limiting factor; unless you take many readings and average them (and assume the LSB is random) the dynamic range will be just 1/256, which is how many db?The spec says: 8-bits; up to 11-bits with enhanced resolution (ERES); ERES appears to be a software key activated option.

author: gf
author_rank: Super Contributor
timestamp: December 19, 2021, 01:26:52 pm
content: If enough "dithering noise" is present at the input you can overcome/enhance the 8-bit resolution.(Without dithering noise there is a risk that the ADC quantization noise may not be white for particular input signals.)Both, ERES and FFT eventually benefit from processing gain and lower the noise floor.See also here(figure 6).If the scope lets you combine both, without reducing the number of FFT points, then it is even better.Even w/o ERES, FFT still benefits from processing gain, depending on the number of points (-> 10*log10(M/2) dB, where M is the number of FFT points).

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 19, 2021, 03:59:23 pm
content: few variables you need to look closely (or better have some knowledge and experience) when using FFT...1) sample rate (SR): this will decide your spectrum span (FS).. larger span will means low RBW, ie higher bin's freq span..2) number of FFT points (NP): see below3) number of FFT bin points (BP): BP = NP / 24) so... we use formula RBW= FS / BPso once these variables in mind, we can organize where our fundamental (freq of interest) should show up in FFT, we try to avoid it being displayed in far right or far left... because we risk losing the signal displaycase 1 example:SR = 2GS/sBP = 200Khence...FS = (2GS/s) / 2 = 1GHzRBW = 2G / 200K = 10KHzthat means any fundamental signal below 10KHz will not show up satisfactorily in FFT, even 20-30KHz will show up at far left (not ideal), the same to signals near 1GHz range... we want our fundamental like 5-10% from left.. ie at FFT point 10% x 200Kpts = 20Kth point.. bin freq for that point is 20K x RBW = 20K x 10KHz = 200MHz... so for the above case 1... suitable freq under observation should be 200MHz...case 2 example:i have 50Hz signal and i want to see up to 10th harmonics (500Hz) what sample rate should i use? using SR in case 1 will result in fundamental and all its harmonics lumped in FFT bin/elements #1 which is not good. to get a good span, apparently we want a span of 500Hz (full screen) or 1KHz span (10th harmonics will be shown at center screen), lets take that we want 1KHz, so whats the suitable SR... we use above formula FS = SR / 2... or SR = FS x 2 = 1KHz x 2 = 2KS/sthats it! with your 500Hz square (maybe up to 5KHz 10th harmonics) using ~200Kpts bin at 2GS/s sample rate, surely you'll get a funny feeling when you try to punch in those formulas. how many cycles/repetition signal shown on DSO screen is irrelevant as long as you try to achieve the suitable SR and BP for your freq of interest... please also take note what gf tried to tell you in earlier posts, because he's trying to tell you the same as what i tried to say here.... after you punch in variables into these formula to organize your display but the DSO still doesnt show what should be, then you can safely say the DSO's FFT implementation is garbage.. iirc my Lecroy SDA6000 is not, while playing with its FFT few months ago... until then...

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 04:43:46 pm
content: Quote from: peter-h on December 19, 2021, 12:38:46 pmNo, it´s almost in there...Math--->Eres---> 0.5 to 3bit

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 06:08:43 pm
content: Not in mine

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 08:00:07 pm
content: Go into the math setup or I´ll look tomorrow at work at our ws3000, telling it step by step in the evening here...(We got the WS3024 without "Z", plus serial decoding option, nothing else and it´s surely in the math setup - NOTE:How to use Eres function is NOT mentioned in the operators manual for the 3000 series, but in the operators manual for the 3000Zseries....)

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 08:25:04 pm
content: I found it!It produces interesting effects unless the waveform is totally steady - as one might expectOne can set ERES on a max of two signals, using F1 for one and F2 for the other.

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 08:32:28 pm
content: Quote

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 08:46:32 pm
content: ERES isn't worth a lot IMHO.The display is still very stepped, despite the extra 3 bits claimed gain. The effect is a lot worse than the limit of the LCD resolution, too, so I think it just doesn't work well. Also, since the ERES'd versions of the waveform are displayed as F1/F2, the timebase knob doesn't work anymore as a "scope"; you have to adjust each of F1/F2 individually for the timebase. It is nothing like a digital scope with an 11-bit ADC, and I wonder why they didn't simply do that.Nothing beats a £200 ex-Ebay Tek 2465B if you want to see detailAnd the 3034 ERES is completely useless with any varying signals, it is even worse than an analog scope which is almost useless for varying signals

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2021, 08:56:09 pm
content: Quote from: peter-h on December 19, 2021, 08:46:32 pmAs is Averaging on ANY scope.There are uses for which ERES/HiRes shines which is why it's incorporated into modern DSO's.There are several tricks to cleaning up waveforms that a DSO shows but a CRO won't and only time behind them will let you become familiar with them.

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 08:59:58 pm
content: QuoteIt depends a lot from what you want to do with it - and how high your expectations are...Saw my last pic ?It shows ripple of an dc-dc converter, in mv range...You won´t see that so clearly without using ERES, because the signal was too noisy.

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 09:42:10 pm
content: This shows a sinewave (filtered with a 3rd order lowpass S-K filter, and "totally clean" on an analog scope), with the "3 bits improved" version below it.There is a difference, but not much, especially at the top and bottom.

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2021, 09:45:47 pm
content: The above screenshot has ch 3 with a 20MHz bandwidth limit. I was using that because there is a bit of noise around the board. This pic shows ch3 on the full bandwidth; a bit more difference but still not anywhere near the screen resolution

author: Martin72
author_rank: Super Contributor
timestamp: December 19, 2021, 09:49:24 pm
content: Question:What do you expect ERES will do on this ?FYI, here an abstract of ERES function from the lecroy OP-Manual:Quote

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 09:55:36 am
content: With unlimited time to average out a periodic waveform, I would have expected it to be LCD resolution limited.But fair enough - it is still a good scope.

author: gf
author_rank: Super Contributor
timestamp: December 20, 2021, 11:34:27 am
content: Presuming that the ERES trace is really calulated with 11-bit precision, I wonder whether the display processing is possibly limited to 8 bits, so that the enhanced resolusion only becomes visible when zooming in vertically?

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 02:22:42 pm
content: Sure, but if I was in Marketing, you know what I would doMost users of this feature will expect what I expected, especially given that the 3034 is just a low-end (~ 10 years old) PC motherboard, with some $100 ADCs, and some FPGAs... But then various other implementation aspects are bad... try the WaveStudio (a remote control app) which is so slow it is barely usable, even over 100mbps ethernet (=10 megabytes/sec!) with everything else being gigabit-grade. It would be a great feature if it worked well. I have used this over Remote Desktop and the RDP connection (over ADSL) is a lot faster than the WS screen updateI can see why they aren't using 12+ bit ADCshttps://www.ti.com/data-converters/adc-circuit/high-speed/products.html#p84=12;14The scope touch screen is also almost unusable. You need a "pen" to access most of the items, and I use a USB trackball the whole time.But maybe my impression of this scope is coloured by the fact that it blew up 1 year after we bought it; luckily it was in warranty. For the money one could buy three 10-15 year old Tek scopes on Ebay which have very similar functionality (but a smaller screen; not as nice) and when they pack up, just throw them away, and I reckon three of them will last much longer than this "LeCroy IBM PC"

author: 2N3055
author_rank: Super Contributor
timestamp: December 20, 2021, 04:28:22 pm
content: Quote from: peter-h on December 20, 2021, 02:22:42 pmIt is not PC based platform. It is old ARM platform, that is why it is slow...With all due respect, you need to read up on FFT theory a bit. All the FFT displays you shown would be the same on 50000 USD scope if you were to set it up the same.Wave runner 3034 is not a fast scope, but has some nice features and can be used for some nice work.

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 04:45:51 pm
content: Indeed; I am happy to learn. But somehow the Tek scope works straight off. Well, it has to, since there is almost no configI also could not see why you need so many cycles on the screen. I wrote an FFT program at univ, 1970s, Fortran of courseand it needed just ONE cycle, and you got the spectrum from that. That is how an FFT is supposed to work, and the ability to do that enables a spectrum of a single shot event to be obtained, too.

author: Martin72
author_rank: Super Contributor
timestamp: December 20, 2021, 08:38:53 pm
content: Quote from: 2N3055 on December 20, 2021, 04:28:22 pmWith "Z", it is a nice fast scope...https://www.eevblog.com/forum/testgear/teledyne-wavesurfer-3024-200mhz-oscilloscope-any-good/msg3886253/#msg3886253

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 09:20:03 pm
content: Is Z a software update or a new PCB?Apparently it runs WinCE... I didn't realise when I bought it how old this model was. But there is no way for me to get scope demos here in SE UK so I had to buy it on spec. Interesting to read the other threads reporting the touch screen as useless; I found the same and cannot understand how a well known company can release such a poor implementation.

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2021, 09:25:46 pm
content: Quote from: peter-h on December 20, 2021, 09:20:03 pmRebadged one of these:https://www.siglent.com/products-overview/sds3000x/

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 09:53:11 pm
content: Yeah, I saw, but which one? What about the 0.9pF/1M/1GHz ZS1000 active probe?

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2021, 10:00:48 pm
content: Quote from: peter-h on December 20, 2021, 09:53:11 pmSiglent 3k models are only 500MHz or 1 GHz so the 300 MHz model is special order for LeCroy.QuoteThat's LeCroys own probe that fits their proprietary probe bus, Martin might be familiar with it.

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 10:15:47 pm
content: I could not find a Siglent scope which looked like the 3034 - so maybe the internals are same, but "rebranded"??

author: Martin72
author_rank: Super Contributor
timestamp: December 20, 2021, 10:30:08 pm
content: QuoteYep, what their currentprobes concerns - we got a "lot" from lecroy and until now it was the greatest stopper for buying scopes from other brands, because of the missing probus.But next year, I´ll break it down, order some siglents...QuoteIn my opinion, it was the beginning of the partnership with siglent by making the ws3000 series for lecroy and both, lecroy and siglent, wasn´t aware at this time, that windows ce plus the lecroy MAUI interface was a little bit too heavy for the hardware.And therefore:QuoteNew hardware, from the display/touchpanel to the board.Remember, the ws3K with Z got the option of power analyzis the ws3K without Z don´t have...By the way, you can sell your 3034 for a price you can easily buy a siglent sds2k+....If you don´t need the probus interface, the siglent will be the better choice in nearly every case and cost "only" 1400 bucks.What your version concerns.QuoteNot 300Mhz...Avaible WS3000 versions:200Mhz,350Mhz, 500Mhz, 1GhzAvaible WS3000Z versions:100Mhz, 200Mhz, 350Mhz, 500Mhz, 1GhzWhere the 100Mhz version got 2GSa/s instead 4GSa/s....

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2021, 10:33:26 pm
content: Quote from: peter-h on December 20, 2021, 10:15:47 pmWaverunner/Wavesurfer are different AFAIK and only some models are made by Siglent.Originally they were a joint venture with HW from Siglent and the Maui UI from LeCroy.We never saw the Siglent SDS3000 or later X models in the west as the agreement with LeCroy gave them exclusive western marketing rights.Siglent later developed the SDS5000X whose UI is quite similar to LeCroy's and the later SDS2000X Plus series and the new SDS6000A followed this UI.Some of these later Siglent models are rebranded by LeCroy.

author: peter-h
author_rank: Super Contributor
timestamp: December 20, 2021, 10:54:16 pm
content: Can any of these do an FFT on just one cycle of the waveform?

author: Martin72
author_rank: Super Contributor
timestamp: December 20, 2021, 10:54:47 pm
content: QuoteWaverunner models are above the wavesurfer in the lecroy model portfolio.They got features the wavesurfers will never have, like multigrid display and so on.Our older waverunner are made by IWATSU, the wavesurfer 432 too and the waveJET.It was clearly marked on the backside "Made by Iwatsu for Lecroy".Nowadays you don´t got these remarks.QuoteWhen I remember the waveACE desaster for lecroy, this is a honour for siglent and the evidence that they´re making good stuff.

author: nctnico
author_rank: Super Contributor
timestamp: December 20, 2021, 11:19:15 pm
content: Quote from: peter-h on December 20, 2021, 10:54:16 pmThat is not how FFT works. What you need to do is force the oscilloscope to acquire the amount of samples needed for the FFT depth at a given sample frequency. This requires setting the memory limit manually and using the time/div adjustment to force a specific samplerate which in turn sets the FFT frequency span. Remember that FFT also uses windowing to remove the effects of partial cycles at the beginning and end of the data fed into the FFT algorithm. If you want to look at the waveform as well, you'll need to use zoom mode.Quote from: peter-h on December 20, 2021, 09:20:03 pmI nearly bought one as well but the local Lecroy dealer didn't want to give me a good deal. Fast forward to today: being an owner of a Lecroy Wavepro 7k series I can say the touchscreen sucks on that model as well. It basically is a touchscreen bolted onto a UI which isn't meant for touchscreen; that doesn't end well. Lecroy scopes are great for complicated signal analysis but not for daily driver scopes. On top of that the 3000 series just doesn't have the processing power to make the signal analysis part work well. In the end I'm very relieved I didn't got the WS3000 and nowadays you can buy much better DSOs as well. That doesn't help you right now but is something to consider for future purchases.

author: Martin72
author_rank: Super Contributor
timestamp: December 20, 2021, 11:29:52 pm
content: Like a WS3000Z...

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2021, 11:37:12 pm
content: Quote from: peter-h on December 20, 2021, 10:54:16 pmNo, although not sure about the SDS6000A as I have heard it's different although I cannot say for sure until we get one next year.Screenshots below from SDS5104X with the same quickly made FFT settings and only at different timebase settings.Source = Leo Bodnar pulser....forgot to set input coupling to 50 Ohm.

author: Someone
author_rank: Super Contributor
timestamp: December 21, 2021, 12:45:57 am
content: Quote from: peter-h on December 20, 2021, 10:54:16 pmIts high time you read the manual for this scope, as it pretty much explains all your confusions in a couple of paragraphs.This is a rich information scope for analysis of signals, that also tells you all the details (usually right there on the screen since it has the space) of what is happening. Its there in your own screen captures as cropped below. If you dont want a scope that does things for you, leaves no choices, dont buy such an analysis scope? (be that Lecroy or otherwise)Quote from: nctnico on December 20, 2021, 11:19:15 pmlol, thats exactly how FFT works.Quote from: nctnico on December 20, 2021, 11:19:15 pmCan/may/might/could/possibly use windowing depending on the application. Windowing is not inherent to FFT.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 21, 2021, 06:39:52 am
content: Quote from: peter-h on December 20, 2021, 04:45:51 pmQuote from: peter-h on December 20, 2021, 10:54:16 pmwith such query, backed up by proof that you tried to observe 500Hz's FFT @ 2GS/s, added with missed formulation recap and important hints given earlier by members, i have a high confidence that you've lost your grasp totally on FFT algorithm...Quote from: Someone on December 21, 2021, 12:45:57 amyes thats how it works, but it will produce garbage of confusing readings...

author: gf
author_rank: Super Contributor
timestamp: December 21, 2021, 08:03:46 am
content: Quote from: peter-h on December 20, 2021, 10:54:16 pmThis is possible if your (periodic) signal happens to have the "right" freqency. Or more specifically, if your signal's frequency is fs / N, where fs is the selected1)sampling rate and N is the number of selected FFT points (usually a power of 2).Example: If fs=100 MSa/s and N=524288, and your signal freqency happens to be 190.73486328125 Hz, then the FFT will be calculated from exactly one cycle of the waveform.What you are asking may be nice for a school example. But when are these border conditions ever granted in practice?Another point is: You do not really want to get a frequency resolution (bin spacing) as coarse as the signal frequency, but you likely want a (significantly) higher resolution. But the former is what you get when you calculate the FFT from a single cycle only.And what the time domain display shows on the screen is still yet another issue, since the selected FFT size does not necessarily match the selected memory depth, but the FFT size is possibly just a small subset of the captured samples, while the displayed time domain traceiscan be a different subset of the captured samples.Ediit:1)On many scopes you cannot select the sampling rate directly, but only indirectly via memory depth, #channels, timebase, and maybe other parameters.

author: gf
author_rank: Super Contributor
timestamp: December 21, 2021, 10:06:01 am
content: Quote from: Someone on December 21, 2021, 12:45:57 amWell this may be a matter of the point of view2). If we consider that FT and DTFT are actually defined for aninfintetime line, then the process of picking-out a finite number of adjacent samples from the infinite time line, in order to apply the DFT only to the samples in this time window, is already "windowing". The relative weights of the points in the window is then defined by awindow function. If the weights are equal, then we are dealing with a rectangular window, but even a rectangular window is still yet another window function, still leading to a very particular1)frequency response of the spectrum analysis filter that corresponds to this window function. It just has been adopted to use "no window" as synonym for a rectangular window function.1)Btw, the frequency response of a rectangular window is of course lousy and leads to much spectral leakage. For a spectrum analyzer like display, rather a window functions with very low side lobes (in the frequency domain) is desired, which concentrates most of the energy in the main lobe, and particularly this is desired in conjunction with a large number of FFT points which lead to a high processing gain and a low noise floor.Edit:2)It is of course not inherent to the FFT per se (which is just a mathematical algorithm), but it basically applies to the context in which the FFT is used here.

author: peter-h
author_rank: Super Contributor
timestamp: December 21, 2021, 10:34:54 am
content: Obviously I don't know how to use this scope properly, which is why I posted here (it's not because I wanted people to tell me to read the manual, which I have done) but the discussion shows that it is not trivial, even for those who do understand it.What I don't get is why somebody in LC Marketing didn't suggest copying e.g. Tek and initialising the Math/FFT function with some useful defaults, so it actually does something useful. It is clearly possible to do, but despite my Q further back nobody has said how to work out the optimum settings.

author: gf
author_rank: Super Contributor
timestamp: December 21, 2021, 02:37:18 pm
content: Quote from: peter-h on December 21, 2021, 10:34:54 amMy feeling is rather that you are possibly struggling to determine your desired FFT parameters (samplig rate, number of FFT points, desired window function,...), which lead to the result you expect to see. If you don't feel able to determine these parameters, because you don't understand their impact on the result and the relation between them, then you rather need to read-up a bit about FFT/DSP topics (one can actually spend weeks or months on these topics, when diving deeply into them). But this is rather not lack of knowledge "how to use this scope properly" then.Once you know which FFT paramers you really want, I'm pretty sure you will find a way to enter them into the scope somehow, even if the sampling rate can only be selected indirectly, as a function of the selected timebase, memory depth, etc.And if your needs exceed the FFT capabilities of the scope (say you would need 10M points), then you still had the option to download the captured samples, and to do any desired FFT processing yourself offline on the PC, e.g. in Matlab, Octave, etc.Quote from: peter-h on December 21, 2021, 10:34:54 amPlease define your criteria for "the optimum". A global optimum likely does not exist, but you'll rather need to make individual compromises between different criteria, for different use cases. What exacly are you after? What exactly do you want to see or measure, and at which accuracy, resolution, etc.? Without detailed specification of the objectives it is hard to optimize. If your aim is limited to "just some spectrum display of my signal", then you already did hit this aim.

author: peter-h
author_rank: Super Contributor
timestamp: December 21, 2021, 03:22:53 pm
content: Let me put it differently: how come, on the Tek scope, you just press Math then FFT and you see a spectrum which looks about right and which is a reasonable starting point.And they don't need the timebase shrunk so it is displaying hundreds of cycles of the waveform, since nobody uses a scope like that (it is completely useless).So there must be some simple way to set up the defaults. For example, earlier in this thread you can see that if the timebase is set to any less than 10ms/div, the FFT doesn't work, but at/above 10ms/div it does work, and if you go much above 10ms/div the result doesn't change all that much.

author: Momchilo
author_rank: Regular Contributor
timestamp: December 21, 2021, 05:03:46 pm
content: Maybe the FFT tutorials from Lecroy will help you.https://teledynelecroy.com/doc/tutorial-fft-analysishttps://teledynelecroy.com/doc/setting-up-an-fftI didn't use the FFT function on modern Tek scopes. So I can't tell what they do.But here is a basic FFT video for a Tek scope made by w2aew:You have to set it up properly too like your Lecroy scope.You need some understanding of what you expect in the spectrum analysis. It's similar for a spectrum analyzer. The device doesn't know what you want to see. So you have to set up the parameters of the device like span and RBW.

author: gf
author_rank: Super Contributor
timestamp: December 21, 2021, 05:32:26 pm
content: Quote from: peter-h on December 21, 2021, 03:22:53 pmChcked the manual of the TDS2004. Seems that it has a (fixed) record size of only 2500 samples. This implies that the sampling rate must start dropping when the timebase becomes slower than 250ns/div, otherwise the buffer would overrun.On the LeCroy you obviously used a record size of 10M. That is4000 timesas much. At 2GSa/s, this means you can store 5ms of data in the buffer w/o overrun. In other words, you can slow down the timebase until 500µs/div, and the sampling rate can still stay at the maximum of 2GSa/s w/o overflowing the buffer. Only if you slow down the timebase even more, the sampling rate must drop in order to avoid overflow.Consequence: If you want to deliberately drop the sampling rate, then you need to select a pretty slow timebase.Did you set the record size to 10M, or is this the default?

author: Someone
author_rank: Super Contributor
timestamp: December 21, 2021, 09:24:07 pm
content: Quote from: peter-h on December 21, 2021, 10:34:54 amThe manual answers your exact question, directly, in the section for FFT analysis, a section that is extremely short and to the point.Quote from: peter-h on December 20, 2021, 10:54:16 pmYou have not read the manual if you are asking this.Quote from: peter-h on December 21, 2021, 03:22:53 pmShow some examples of that behavior, on other scopes if you onlycaptureseveral cycles of waveform you get the same low resolution FFT.Quote from: peter-h on December 21, 2021, 03:22:53 pmYou say useless, others say important. Different scopes have different ways of working, and you need to read the manual.Quote from: peter-h on December 21, 2021, 03:22:53 pmThe FFT is still working, its just not showing you what you want to see (and you still have not been able to explain exactly what that is you are wanting). There are no "defaults" that readjust other settings of the scope, the FFT function is taking what you have captured, and displaying what it can. You are in control of what the scope is doing.garbage in garbage out

author: Someone
author_rank: Super Contributor
timestamp: December 21, 2021, 09:32:27 pm
content: Quote from: gf on December 21, 2021, 10:06:01 amBold assumption, when many would take an FFT as explicitly periodic.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 21, 2021, 09:53:06 pm
content: Quote from: Someone on December 21, 2021, 09:24:07 pmwell just to be fair to OP... i dont own WaveSurfer 3000 series, so i can only do screenshots... the 1st screenshot is interesting.. in zoom tab we can manually enter vertical and horizontal setting. if we established the FFT parm/variables i explained earlier, we'll have a clear mind what setting need should be.. for example if our FS is 1GHz, we can set horizontal center to 500MHz @ 100MHz/div, vertical setting to suitable signal power level, generally maybe 0dBm center @ 20dBm/div will be a good start to see all levels.. then you should have see complete view on screen of FFT from 0-1GHz.. we can then zoom in to freq of interest further by reducing horizontal setting, but we need to keep our FFT formulation in head, yes DSO is not perfect SA, so we need a clear and sharp head setting FFT parm in DSO. about why some DSO can show the correct view at first click well i guess some cars are aimed for automatic gearshift where users dont have to think and click anything, but unfortunately some manufacturers still like manual transmission setting where you need to be like a F1 driver, before electronic age... ymmv.. (manual link:http://cdn.teledynelecroy.com/files/manuals/wavesurfer-3000-om.pdf)

author: gf
author_rank: Super Contributor
timestamp: December 22, 2021, 11:17:55 am
content: Quote from: Someone on December 21, 2021, 09:32:27 pmI possibly didn't express myself well. I consider DFT itself periodic, too, of course. And yes, windowing is not inherent to FFT algorithm itself, but rather a pre-processing step. But windowingisan inherent part of a (discrete) Short Time fourier Transform (STFT), where a continuous stream of samples is split into junks by a sliding window (that's the "windowing" part of the STFT) and then DFT is applied to each chunk. And that's basically the analogy I'm thinking of: After sampling the input signal with the ADC we get a continuous stream of samples. The scope captures just a finite time window of this sample stream, and DFT is applied to the samples in this window. So what we get is more or less equivalent to a shapshot of a STFT, for one particular window on the time axis. [ What get is of coursenotthe Discrete Time Fourier Transform (DTFT) of the whole signal, since this would require an infinte number of samples. For signal components whichare knownto be periodic, the DFT of the samples in the window may be a good enough approximation for their DTFT, though, depending on the chosen window size and window function. ]

author: voltsandjolts
author_rank: Unknown
timestamp: December 22, 2021, 12:23:54 pm
content: If you haven't explicity applied any windowing function then you are are using the default rectangular window of all 1's

author: gf
author_rank: Super Contributor
timestamp: December 22, 2021, 12:48:02 pm
content: Yes of course, that's what I alredy mentioned earlier. And even the rectangular window corresponds to a particular frequency reponse of the spectrum analysis filter, when we consider the windowed DFT from a filter bank point of view. In fact the corresponding bandpass (which gets centered at and applied to each frequency bin) has high side lobes and much spectral leakage.

author: peter-h
author_rank: Super Contributor
timestamp: December 26, 2021, 10:42:00 am
content: I have read the manual for the FFT and I can't say I would be any wiser if it wasn't for this thread.The key is to have lots of cycles on the screen; 100+. After that, everything is secondary.Now I am trying to get something else to work: ARINC429. Nothing in the manual. I will start a new thread.

author: 2N3055
author_rank: Super Contributor
timestamp: December 26, 2021, 11:09:47 am
content: Quote from: peter-h on December 26, 2021, 10:42:00 amI recommend to anybody learning FFT on a scope these two short writeups.They are simple but explain basic relations between sample size, sample rate, sampled data time period, and basics of why windowing is needed and fact that different windows give different results. They are great for building basic intuition of how to use FFT on a scope.

author: peter-h
author_rank: Super Contributor
timestamp: January 06, 2022, 09:43:00 pm
content: Thank you 2N3055. That 1st one is a good summary of what one has to do.