Products > Test Equipment
Rigol HDO1000 and HDO4000 12bit oscilloscopes launched in China
switchabl:
Maybe they are trying to line up the bins of a power-of-two FFT with the center/span setting? I kind of hate that approach though, CZT is a much cleaner way to get arbitrary spans (even at the cost of somewhat longer FFTs).
I hadn't looked at the fourth image properly. As far as I can tell the only difference between third and fourth is sample rate. So this is genuinely worrying.
It might make sense to download the raw samples at those settings and compare against an FFT in Matlab or Python.
Martin72:
--- Quote from: Martin72 on January 30, 2023, 09:22:35 pm ---Although...if batronix would lend me one....
--- End quote ---
Request made... ;)
aurel:
I tried to reproduce this FFT issue on my HDO1K and I think I nailed down the bug.
First of, in Auto memory mode I don't see any issue, so I did all the tests in 10k manual memory mode and centered the FFT at 100 kHz.
- Setting the time base to 2 ms/div results in a 500 kSa/s for both the main sampling rate and the FFT window => no visible issue
- Setting the time base to 5 ms/div results in a 200 kSa/s for both the main sampling rate and the FFT window => FFT display is cut after the 100 kHz point (above Nyquist) => no visible issue
- Setting the time base anywhere between 2 and 5 ms/div is the interesting part ! For example setting to 4.5 ms/div results in a 200 kSa/s main sampling rate but a 222.22 kSa/s in the FFT window => FFT display is cut after the 111 kHz point (above Nyquist of the wrong sampling frequency in the FFT window) => there's your issue !
So the bug is that the sampling rate used in the FFT window is computed straight from the timebase, rather than computer from a "rounded up" timebase like it is done for the main sampling frequency.
This should be quite an easy fix for Rigol (let see how much time it takes them !)
For now, to use the FFT in a safe way, you should just avoid manually setting any non "round" timebase, just stay on a 1-2-5 timebase (which is what most users will do anyway).
Fungus:
So you have to manually set the time base to something weird for it to appear?
Fungus:
--- Quote from: aurel on January 31, 2023, 11:00:46 am ---- Setting the time base to 5 ms/div results in a 200 kSa/s for both the main sampling rate and the FFT window => FFT display is cut after the 100 kHz point (above Nyquist)
--- End quote ---
You mean 'cut' like in this screenshot, nothing displayed?
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