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).

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

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 RBW

_{3dB} 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 RBW

_{Bin} 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 RBW

_{3dB} 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 RBW

_{3dB} and RBW

_{Noise}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