Rigol MSO5000   Problems with FFT under 100 Hz RBW - Page 1
Forum section: electronics / testgear

author: hafrse
author_rank: Regular Contributor
timestamp: August 06, 2021, 07:09:26 am
content: Hello,I started to use the FFT function, it seems when scaling down under RBW 100Hz, the other half of the trace is blank! scaling down RBW to 20hz,10,hz,..,1hz the trace even stops!From RBW 100hz and above is ok.Any clues?Picture 1: 25Mhz sine wave , span 50Hz, Center t 25Mhz, RBW at 50HzPicture 1: 25Mhz sine wave , span 50Hz, Center t 25Mhz, RBW at 100HzThanks

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 06, 2021, 08:54:33 am
content: 10mV signal this is tiny, test with 100mV at least.

author: rf-loop
author_rank: Super Contributor
timestamp: August 06, 2021, 09:44:56 am
content: Of course it is blank. If not, then Rigol have bug. Very very basic FFT fundamentals...It is now good time for start learning what is FFT and how it works in theory and in practice.Your FFT sampling is 50MSa/s (50MHz) so FFT end is 25MHz and nothing more. And your screen center is 25MHz and your signal under test is 25MHz.!In other picture FFT sampling is 100MSa/s (100MHz) so there FFT trace ends 50MHz (you use full span display).. And your screen center is 25MHz.Nyquist(-Shannon) limit isfsampling/2 and here is important to note what is FFT sampling, not oscilloscope ADC sampling - of course.. Now if you keep this 50MSa/s and just as this image where it is half screen... if you rise your signal to 26MHz... think about what you see... you see now 24MHz peak... because it is folded back from Nyquist "mirror"!Some notes... try avoid signals what go to Nyquist limit and very importantly what goes over Nyquist... (exept if have enough experience and knowledge and really know what are doing and why)How ever you use FFT but you need always think where is Mr Nyquist. Now in this first image it is middle of screen so it can not display more.I do not know Rigol FFT UI/UX so I can not help how you make better settings... but remember always... there is no any FFT what can display over current Nyquist frequency in use.

author: rf-loop
author_rank: Super Contributor
timestamp: August 06, 2021, 09:52:44 am
content: Quote from: ITTSB.Europe on August 06, 2021, 08:54:33 amBull and shit... this is not relevant here. Do you see these things what Rigol screen display about FFT scale and level, how many dB this peak is over noise level...

author: hafrse
author_rank: Regular Contributor
timestamp: August 06, 2021, 03:08:45 pm
content: Quote from: rf-loop on August 06, 2021, 09:44:56 amGot it, thanks!

author: hafrse
author_rank: Regular Contributor
timestamp: August 06, 2021, 03:12:17 pm
content: Quote from: rf-loop on August 06, 2021, 09:44:56 amHi agin,The MSO5000 has a sampling freq. up to 8GSa/s but it sets samplig frequency automatically to 50MSa/s when going down to 50Hz RBW...

author: bdunham7
author_rank: Super Contributor
timestamp: August 06, 2021, 04:21:00 pm
content: Quote from: hafrse on August 06, 2021, 03:12:17 pmLower resolution frequency requires a longer FFT record length (in time units). Resolution is sample rate divided by FFT length in points. So for 50Hz RBW, they are setting the record length to 1/20 of a second or 50 milliseconds. 1 million point overall FFT length and 50ms works out to 50MSa/s. You're getting everything that can be had.

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 06, 2021, 05:46:15 pm
content: Quote from: rf-loop on August 06, 2021, 09:52:44 amYou seem in pain because the Chinese Oscilloscopes they are blind compared to Taiwanese.Throwing of bunch of theory at my face it does not cause any damage.

author: TurboTom
author_rank: Super Contributor
timestamp: August 06, 2021, 06:05:25 pm
content: The related screenshot shows a signal to noise ratio > 83dBc which can be considered quite acceptable for a nominal 8 bit front end (though you get some additional resolution from downsampling the data). GWinstek Scopes may have a nicer, easier-to-use U/I, but the plain resolution figures on this Rigol model don't appear too shabby. I guess this is what rf-loop tried to tell you in his original way...

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 06, 2021, 06:15:20 pm
content: Quote from: TurboTom on August 06, 2021, 06:05:25 pmNowadays I have no problem with the original way of others.Frequency domain analysis this is a sector that has fresh equipment to demonstrate (2018~2021) with much more available definition and features.I do make thoughts to invest at something new in favor of power analysis.But the FFT of old scopes this is not the solution..

author: 2N3055
author_rank: Super Contributor
timestamp: August 06, 2021, 06:36:21 pm
content: Quote from: ITTSB.Europe on August 06, 2021, 06:15:20 pm

author: rf-loop
author_rank: Super Contributor
timestamp: August 07, 2021, 03:57:23 am
content: Quote from: ITTSB.Europe on August 06, 2021, 05:46:15 pmFirst, what a hell is country meaning here. We talk oscilloscope fundamentals and features here. If it is made in Germany, USA, China, Korean, Japan, India, Russia and so on... do you somehow think scope fundamentals and math is different based to origin.Thank god my homeland, Finland, do not manufacture scopes... if do they are more than 10 times too expensive.---For what was my answer if this need bend using iron rod, lets look....OP question with images.Quote from: hafrse on August 06, 2021, 07:09:26 amAnd your answer for help OP's "problem" he ask.Quote from: ITTSB.Europe on August 06, 2021, 08:54:33 amHow this relates to the matter raised by the questioner. I think it seems more to the fact that you didn’t understand the whole question at all and not the pictures presented but still you want write answer due to some reason.What do you think in the pictures is that the signal level should be raised 10 times and in what way that signal level is in any way even in the slightest way related to the questioner’s problem.Did you try to lead the questioner to the siding. (rhetorical question)

author: rf-loop
author_rank: Super Contributor
timestamp: August 07, 2021, 05:13:34 am
content: Quote from: bdunham7 on August 06, 2021, 04:21:00 pmOmg. yes you are right... there read 50HzRBW. And now I have checked this scope data sheet and it say FFT length is 1M.If look this OP image information scope setup is 2ms/div and 1GSa/s and scope capture memory length is 20M.If I think your calculation, how it capture 50ms (FFT data length in time)I'm a little surprised and confused.Also they use RBW what is normally used in spectrum analyzers telling RBW what isusuallydefined so that it means RBW filter -3dB width and rest depending filter shape factor.Now it rise question, what is RBW meaning here in this Rigol FFT. Your explanation tell it is FFT Δf aka ( fsample/length).But there in Rigol screen read RBW !Also Rigol MSO5000 User Guide (May 2020) tell:"FFT resolution is the quotient of the sample rateand the number of FFT points. If the number of FFT points is a fixed value(65535 at most), then the lower the sample rate, the higher the resolution."When I read this User Guide whole chapter about FFT... I'm speechless.I do not have this Rigol so I can not define and evaluate how it actually works.But independent of any other things my opinion is that FFT Δf can be named as RBW but my personal opinion is that it is bit bad practice when it mean just FFT Δf. When RBW is used just for tell (FFT) bin size user need be very careful and know how it differ from normal conventional SA RBW. RBW has been established for quite another use for many decades. Here resolution band width depends FFT window in use of course, not only FFT Δf. For avoid confusion why not just tell its is FFT Δf. Resolution depends what window is in use. Also if in mind think it is like SA RBW and its -3dB points. Flattop is very different here than example Hamming.So do not think RBW here is same as RBW in conventional SA. With example Flattop window -3dB width may be something like 3.5*FFT Δf "RBW" when Hamming may be somewhere 1.35 (and level error may be nearly 2dB depending signal position related to FFT bins f). Rectangle have most big change. If signal is halfway between FFT bin frequencies level error may be over 3dB and if then measure -3dB points from its top it is more wide than Hamming but if signal is perfecxtly just same as FFT one bin f then it is very very sharp peak.This is why I do not like use RBW when we talk about normal simple FFT.So, what is truth with this in Rigol, least I do not know.Who knows how it really is and how it is not?(but this is bit far from OP's question)

author: gf
author_rank: Super Contributor
timestamp: August 07, 2021, 09:42:20 am
content: Quote from: bdunham7 on August 06, 2021, 04:21:00 pmThe FFT size limitation and its implications are obvious. But I'm confused that it still show 1GSa/s at the top of the screen if the sample rate was actually reduced to to 50MSa/s. Or does it mean that the samples are still captured at 1GSa/s to sample memory, and only the FFT processing decimates to 50MSa/s first, before calculating the FFT from the decimated data?Edit:Quote from: rf-loop on August 07, 2021, 05:13:34 amIndeed, the spacing of the frequency bins is not the same as the -3dB bandwidth of the bandpass filter that corresponds to the selected window function. Insofar it is misleading to call the frequency resolution RBW. If the term RBW is used, it were IMO better to display the actual -3dB bandwidth of the selected window function, and not the FFT frequency resolution.

author: bdunham7
author_rank: Super Contributor
timestamp: August 07, 2021, 01:56:47 pm
content: Quote from: rf-loop on August 07, 2021, 05:13:34 amI tried to use the term 'resolution', meaning Δf, but I see an 'RBW' snuck in there, probably because the OP used the term as it appears on the scope. Given the basic math and the nature of FFTs, I doubt the term 'RBW' here is anything other than a somewhat mistaken reference to the frequency bin spacing, Δf. If their FFT had some extra special secret sauce, they'd be bragging about it somewhere.

author: 2N3055
author_rank: Super Contributor
timestamp: August 07, 2021, 11:35:25 pm
content: @rf-loop, @GF,At one time I took a closer look at how DS7000 did this, and I believe MSO5000 might be the same.They erroneously call bin width a RBW. There is no any kind of proper calculation of RBW as is defined in industry.They sample at sample rate stated on top of the screen and then either take only part of the sample buffer or donwnsample. That is not clear. I presume they simple use sub-buffer because that is simpler.I find that, half SA, half FFT user interface confusing too. Regardless bin width being wrongly called RBW, fact that it is neither here or there is bothersome.I can handle FFT logic, and SA logic, this mix is just, well, not right...AFAIK, even R&S in it's RTB2000 (that doesn't have proper Spectrum view like it's bigger brothers), FFT is also some kind of hybrid between SA and FFT all extracted from scope's acquisitions.But them being R&S they made it less confusing. Although, I seem to remember an user here being confused with that one too at one point.Good read:http://www.site2241.net/march2021.htm

author: iMo
author_rank: Super Contributor
timestamp: August 08, 2021, 01:14:22 am
content: It seems to me (I do not own the scope) the RBW depends on the sampling rate and number of points only (see inside the top "H"orizontal window), and RBW is set independently from the freq "Span", most probably by the horizontal time scale knob (in ms/us etc.).RBW = Sampling_rate/Number_of_PointsRBW = 1GSa/s / 20MPts = 50HzRBW = 2GSa/s / 20MPts = 100HzThe "Span" (the resulting spectra freq scale spanning "from..to" in MHz) is set in the acquisition menu on the right, items "Start" and "End" in MHz, ie.Start = 0End = 25MHz or any other "end of the frequency spectra" shown on the display..

author: normi
author_rank: Regular Contributor
timestamp: August 08, 2021, 02:38:45 am
content: Quote from: hafrse on August 06, 2021, 07:09:26 amBased on your focus on RBW I am assuming that is what is most important to you. Based on how the RBW or Frequency resolution is calculated, you will be limited to 25Mhz maximum frequency if you require 50Hz resolution. This is due to a sample rate which should be more than twice the frequency being sampled. If you were to move the RBW to 25Hz the sample rate required would be 25MSa and the max frequency under test would be half that at 12.5Mhz. Move RBW to various values and you will observe. To get the frequency up to 50Mhz would require 2Mpts FFT and sample of 100MSa, however the screen would be halved again once you go down to 50MSa.The scope's channel sample rate has no effect on this, if you want the 8GSa sample rate just disable the auto setting and move memory depth to 200M.Fs/n =50MSa/1Mpts=50Hz RBW

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 08, 2021, 12:04:28 pm
content: Quote from: rf-loopOf which year of manufacturing?Quote from: rf-loopI care to inform people for the correct tools which can do the job about the delivery of meaningful measurements.Even dedicated power analyzers developed five years ago, today they are trash.This is why professionals they do upgrade their tools much faster than hobbyist do.If the OP is after work that pays, he should get something modern.

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 08, 2021, 12:08:23 pm
content: Quote from: 2N3055 on August 06, 2021, 06:36:21 pmIf you ever come to such a moment, you are welcome to use my telephone number.Its faster than web related communication.

author: rf-loop
author_rank: Super Contributor
timestamp: August 08, 2021, 12:53:59 pm
content: Quote from: 2N3055 on August 07, 2021, 11:35:25 pmRigol is not alone here.It's also a bit confusing when National Instruments writes this:"Resolution bandwidthThe resolution bandwidth (RBW) determines the fast-Fourier Transform (FFT) bin size, or the smallest frequency that can be resolved."https://zone.ni.com/reference/en-XX/help/372058U-01/nirfsa/resolution_bandwidth/

author: normi
author_rank: Regular Contributor
timestamp: August 08, 2021, 02:33:10 pm
content: Calling it Resolution Bandwidth makes it simple for users to transition between a SA and the Oscilloscope. A user does not have to go and research what the oscilloscope is saying. Rigol documentation mentions FFT, however the brochures also mention the FFT feature as spectrum analyzer, so making the settings and wording as close as possible makes the two appear to be the same.

author: gf
author_rank: Super Contributor
timestamp: August 08, 2021, 02:49:45 pm
content: But instead of displaying the FFT bin spacing (50Hz in this example) and calling it RBW, it were also possible to display the actual -3dB bandwidth, e.g. 72 Hz if a Hanning window is selected, or 65 Hz for a Hamming window, etc. Then the RBW numbers were better comparable to the SA RBW numbers (still not perfectly comparable, of course, since the -3dB bandwidth is not the one and only characteristic of a filter).

author: 2N3055
author_rank: Super Contributor
timestamp: August 08, 2021, 03:12:57 pm
content: It would have been better if they invented term like "frequency resolution" and simply explained what it means in a manual. Redefining industry standardized terms is always confusing.It shouldhave not been called same RBW as SA, exactlybecauseof the people who know how to use SA and will haveexpectations it means the same.This whole topic is based on problem by a person whichexpectedFFT on scope to behave same as SA, confused by similar terminology. If it was using proper FFT terms and names, it would be obvious it is something else and you need to read a manual..

author: rf-loop
author_rank: Super Contributor
timestamp: August 08, 2021, 03:19:59 pm
content: Quote from: normi on August 08, 2021, 02:33:10 pmMore confusing than make simple for transition between SA and Oscilloscope.Everyone who is familiar with SA have imagine and also mostly real knowledge what it mean... there 100Hz RBW IS 100Hz (-3dB width)But now RBW used here for FFT is not at all same as RBW in SA. Only same is these three letters R, B and W.Example flattop RBW can somehow think it may be roughly 3.5 times this fake "rbw"... but when user set it for some other window it may go bit messy if try think -3dB width (too long story..)What is purpose to fool users. It is frequency step (interval) between FFT bins and just nothing - nothing - more. And it is fair and honest to tell people also in FFT info on screen and not fool that it is RBW like in SA's. Also there is many noob users and other way not so educated and experienced users. Why "fool" them to think it is RBW when it is not RBW in this context when user may think he use it as "spectrum analyzer". What it helps. Better to name it as FFT Δf.Now if user do not know it, there may happen very positive thing. He perhaps read some basic fundamental information about what is this kind of simple FFT.

author: normi
author_rank: Regular Contributor
timestamp: August 09, 2021, 01:37:06 pm
content: I figure they probable had it with some other name since the older scopes were not presented this way, then the sales and marketing guys got involved. As 2N3055 says frequency resolution would have been a better term, and this term already is used with FFTs.

author: kcbrown
author_rank: Frequent Contributor
timestamp: September 05, 2021, 12:43:37 am
content: Quote from: rf-loop on August 08, 2021, 03:19:59 pmYou could call it that. But because it's the frequency width of the FFT bins, and those bins are adjacent to each other, and the FFT's frequency resolution is directly dependent on it, I'm with 2N3055 on this one: it's probably sufficient to just call it the FFT's frequency resolution. While this terminology ("frequency resolution") is also used in the analog domain as a synonym for RBW (seehttp://www.site2241.net/docs/Understanding-spectrum-signal-analysis.pdf#page=23), here it's directly applicable with no interpretation required (at least that I can see. I'm always willing to be schooled more on such things, so if I'm wrong here then have at it!).