Products > Test Equipment
Rigol DHO804 Test and Compare Thread
TurboTom:
--- Quote from: gf on December 01, 2023, 07:25:59 pm ---
--- Quote from: TurboTom on December 01, 2023, 11:15:28 am ---Wouldn't worry too much about this since the most relevant window functions von Hann, Blackman-Harris and rectangular are available and appear to work correctly.
--- End quote ---
I would. A flattop window is important to get good amplitude accuracy for frequencies that are not integer multiples of the bin spacing. The maximum amplitude error of the flattop window 1) is only 0.016 dB. Hann, Blackman and various others still have significant scalopping loss for frequencies that fall in the middle between two bins. Blackman-Harris has also ~0.83 dB. And a rectangular window leaks as hell if the frequency is not an integer multiple of the bin spacing.
1) I consider the Matlab variant here, but there exist several other variants too
--- End quote ---
Whatsoever, it shouldn't be too difficult to correct this: In the scope's directory /rigol/resource/window/ there are all the window functions as binary arrays of 2^20 single precision (float32, little endian) numbers stored. Obviously, the file "flatTop_1048576.hex" is defective since it starts and ends with some value of -0.853 while it should be something very close to 0.
But since it isn't too complicated to generate a correct file, those who need the proper flattop window shape could easily modify it and replace the file. Nevertheless, this should have been Rigol's job. :-//
Martin72:
--- Quote ---Nevertheless, this should have been Rigol's job.
--- End quote ---
That's exactly what it should be.
If you're still wondering how easy it is to hack the new generation scopes....
These are evaluation boards with a screen and a basic software framework and if something is wrong, you can fix it yourself. :P
TurboTom:
I just generated a new Flat Top window file as per the formula described here. The result looks much better than the original but there's still some side lobes emerging from the noise. I cannot tell for sure how flat the top actually is, but if someone wants to try, here's the download link. :D
Edit: Sorry I had to update the download link since I found a small error in my math... Now it's corrected and the FFT with the FlatTop window looks really nice ;D
See attached screenshot: UR - FlatTop; LL - Blackman-Harris; LR - von Hann
P.S. I could just barely control myself not to use an AM signal. >:D
Edit: Download link updated. Alternativeley, you can download a windows EXE file that will generate the window file here.
gf:
Here is the frequency response of the spectrum analysis filter corresponding to Matlab's flattopwin().
The flatness in the range -0.5...+0.5 is important.
Stopband rejection is > 68 dB. I'm surprised that you seem to get more :-//
ebastler:
--- Quote from: TurboTom on December 03, 2023, 07:53:42 pm ---I just generated a new Flat Top window file as per the formula described here. The result looks much better than the original but there's still some side lobes emerging from the noise. I cannot tell for sure how flat the top actually is, but if someone wants to try, here's the download link. :D
--- End quote ---
Very nice work, both finding the file and fixing it -- thank you!
Is it possible that this messed-up Flat Top window has been a Rigol tradition forever? My old DS1054Z shows very similar "leaky" spectral lines when using the Flat Top window. No easy way to swap out the window function on that scope, I'm afraid...
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version