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Rigol DHO804 Test and Compare Thread
Mechatrommer:
--- Quote from: gf on December 02, 2023, 09:52:24 am ---When I see screenshots showing 62.5MSa/s, 625000 points, and RBW 100 (assuming that 100Hz is not really the -3dB RBW, but actually the frequency bin spacing), then I get the impression that the FFT size is the full record length of 625000 points, which would also imply that this FFT implementation can handle sizes that are not a power of two. Of course, there has to be a limit to the maximum FFT size, so the question remains what happens if the acquired records are longer than the limit. What subset of samples is selected? Start of the record? Center? Or are long records even split into multiple chunks of the maximum FFT size?
If we distrust the numbers displayed on the screen, ist it possible to export the FFT math trace to a text file? If yes, how many points does it export if full span (0...fs/2) is selected? The number of saved points and the difference between adjacent frequencies should give an indication for the actual FFT size and frequency bin spacing (unless only decimated screen data are saved :( - that's useless, of course).
--- End quote ---
you people maybe right about bin size vs RBW, but that probably due to some advanced feature of FFT that may not accessible to normal people, such as spectrum leakage and varying window function effect on actual spectrum power, until you people can provide more reliable materials to read, i think rigol refering RBW as bin spacing as defined by link i provided. if rigol implementing your peole's definition of RBW, then there is no way of knowing how many points. but then... what practical benefit to know how many points? other than swinging dick contest this brand is lower points than the other. just read the goat damned FFT/SA the like old people did it... no need number of pts or question this brand is more reliable than others... ymmv.
(attached image is for illustration or amusement only, dont be too serious! life is short. name erased to avoid swinging dick contest, source: https://www.youtube.com/watch?app=desktop&v=S8jrpCoZyx8&t=1953s) i cannot erase voice sorry ;D
Martin72:
--- Quote from: Martin72 on December 02, 2023, 10:32:54 am ---
--- Quote ---If we distrust the numbers displayed on the screen, ist it possible to export the FFT math trace to a text file?
--- End quote ---
Directly from the FFT menu not, only the peak/marker table.
But I'll try another thing, maybe that works.
--- End quote ---
Unfortunately this (save/recall-->save-->from memory, after this from screen) did not work, everything is recorded and the maths display is probably not included.
What I also noticed is that if you call up such system functions while maths (FFT) is active, the scope starts to sweat a bit and is paralysed by the processing of the inputs.
rf-loop:
--- Quote from: Mechatrommer on December 02, 2023, 10:58:43 am ---
--- Quote from: gf on December 02, 2023, 09:52:24 am ---When I see screenshots showing 62.5MSa/s, 625000 points, and RBW 100 (assuming that 100Hz is not really the -3dB RBW, but actually the frequency bin spacing), then I get the impression that the FFT size is the full record length of 625000 points, which would also imply that this FFT implementation can handle sizes that are not a power of two. Of course, there has to be a limit to the maximum FFT size, so the question remains what happens if the acquired records are longer than the limit. What subset of samples is selected? Start of the record? Center? Or are long records even split into multiple chunks of the maximum FFT size?
If we distrust the numbers displayed on the screen, ist it possible to export the FFT math trace to a text file? If yes, how many points does it export if full span (0...fs/2) is selected? The number of saved points and the difference between adjacent frequencies should give an indication for the actual FFT size and frequency bin spacing (unless only decimated screen data are saved :( - that's useless, of course).
--- End quote ---
you people maybe right about bin size vs RBW, but that probably due to some advanced feature of FFT that may not accessible to normal people, such as spectrum leakage and varying window function effect on actual spectrum power, until you people can provide more reliable materials to read, i think rigol refering RBW as bin spacing as defined by link i provided. if rigol implementing your peole's definition of RBW, then there is no way of knowing how many points. but then... what practical benefit to know how many points? other than swinging dick contest this brand is lower points than the other. just read the goat damned FFT/SA the like old people did it... no need number of pts or question this brand is more reliable than others... ymmv.
(attached image is for illustration or amusement only, dont be too serious! life is short. name erased to avoid swinging dick contest, source: https://www.youtube.com/watch?app=desktop&v=S8jrpCoZyx8&t=1953s) i cannot erase voice sorry ;D
--- End quote ---
Not perfectly to you even when I quote you.
1st.
Bins are intervals between samples in frequency domain.
Frequency domain include fact that there need be 2 samples.
All know that if we have example 1GSa/s (sampling frequency 1GHz) FFT full scale in not 1GHz. It is 0-500MHz... 2 samples means frequency interval in frequency domain... 1 Bin, one Δf. One FFT bin need two sample (in time domain)
(example for carify: 1GSa/s and 524288pts Δf=1.9073kHz. Siglent display Δf 1.91kHz) (If you use FFT full span frequency for Δf (for Bin) you need use FFTpts/2)
Spectrum analyzers, old analog... normally RBW mean filter 3dB width.
This RBW is so commonly used like it in many places just for meaning filter width as it is used tens of years in many instruments. It is confusing if some use RBW as Bin frequency interval. Better way imho, is to tell Δf and example Siglent have selected this way.
But as can see in my previous one image some Siglent oscilloscope may tell both, Δf and RBW and my opinion is that this is good way to avoid confusion. In this case RBW change when user change FFT window, because RBW3dB really change depending filter parameters. It looks like also one filter type, example Hanning may have bit different filter factor in different implements depending some deeper details. Example Siglent hanning RBW3dB is not just 1.44 x Δf (bin).
My opinion is that if use RBW as RBWBin it need also tell somehow for avoid confusion.
Least I have default in my mind after tens of years using many kind of instruments that when I see frequency domain and there RBW I will automatically asume it is RBW3dB until specially noted exception. Because this have been so common practice. And it is same also in these SA what have full digital IF and example FFT (more or less wide realtime or step swept FFT) instead of analog filters. Newer seen there RBW is FFT bin).
One maestro for this mess is National (NI) when they simply say: "The resolution bandwidth (RBW) determines the fast-Fourier transform (FFT) bin size, or the smallest frequency that can be resolved."
In this Keysight page there is some good example about different FFT window and RBW3dB and RBWNoise
GW Instek is one example about good information in this matter.
MDO2000A data sheet have nice detail.
FFT Windows, FFT Factor:
Hanning 1.44 ; Rectangular 0.89 ; Hamming 1.30 ; Blackman 1.68
Martin72:
Yeah, I was just playing around, this time with my flir i60... ;D
...The scope after about 2 hours of operation.
Martin72:
FFT again, going through several times/div.
Timebase Samplerate Memory FFT-SR RBW
10µs 1.25G 125Kpt 1.25G 10K
20µs 1.25G 250Kpt 1.25G 5K
50µs 1.25G 625Kpt 1.25G 2K
100µs 625M 625Kpt 625M 1K
200µs 312.5M 625Kpt 312.5M 500
500µs 156.25M 781.25Kpt 156.4M 199.9
1ms 62.5M 625Kpt 62.5M 100
2ms 31.25M 625Kpt 31.25M 50
And so on.
I don't understand the values at 500µs, these "crooked" values in the FFT, nor the memory, which is 625Kpt before and after, but 781.25kpt in this time base.
Rigol specifies the maximum number of FFT points as "up to 1Mpt".
But how could you find out how many points are currently being used if this value is not displayed?
BTW,
Not only is the RBW not updated at times, the graphically displayed span is not updated either.
Both can be refreshed by moving the vertical offset.
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