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Rigol DHO804 Test and Compare Thread
ebastler:
--- Quote from: gf on December 03, 2023, 08:36:23 pm ---Stopband rejection is > 68 dB. I'm surprised that you seem to get more :-//
--- End quote ---
Well, it rolls off to -80 dB or a bit better in your MatLab plot as you move further away from the peak. And that's about what TurboTom's screenshot also shows, right?
gf:
--- Quote from: ebastler on December 04, 2023, 12:31:03 pm ---
--- Quote from: gf on December 03, 2023, 08:36:23 pm ---Stopband rejection is > 68 dB. I'm surprised that you seem to get more :-//
--- End quote ---
Well, it rolls off to -80 dB or a bit better in your MatLab plot as you move further away from the peak. And that's about what TurboTom's screenshot also shows, right?
--- End quote ---
That's not the reason. But my fault for not seeing immediately what was happening. Yes, the first side lobe is about -68dB. But the frequency response of the window function has zeros at +/-N*delta_f, where N is an integer >= 5. If the signal frequency is an integer multiple of delta_f, then the side lobes don't show up because the signal frequency hits the zeros between the lobes, so outside the main lobe, we only see the noise floor in the discretely sampled frequency spectrum. For signal frequencies which do hit the side lobes, we'll see the resulting "leakage".
Btw, the Siglent flattop window is possibly a different variant, having lower side lobes (handbook sais -93 dB; due to lack of the hardware I can't verify). This would also imply a wider main lobe.
Performa01:
--- Quote from: gf on December 03, 2023, 11:40:31 pm ---
--- Quote from: Martin72 on December 03, 2023, 09:15:56 pm ---Earlier I took the 1khz reference signal as the source and went through all the windows.
--- End quote ---
Looks broken on your Rigol. And not just Flattop, the others look weird, too.
In order to show maximum leakage, don't choose exactly 1kHz, but rather 997.5 Hz in conjunction with your Rigol settings, and 998.97 Hz with your Siglent settings. IOW, us a frequency f = (N+0.5)*delta_f, where N is an integer. Calculate with enough decimal places, i.e. delta_f = 4.76837 and not just 4.77 for your Siglent settings.
--- End quote ---
FOr the SDS2000X Plus, this has already been done a while ago (reply #3561):
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg4318822/#msg4318822
Since the FFT core implementation is identical, I expect quite similar results from the SDS1104X-E.
Performa01:
--- Quote from: gf on December 04, 2023, 01:12:55 pm ---Btw, the Siglent flattop window is possibly a different variant, having lower side lobes (handbook sais -93 dB; due to lack of the hardware I can't verify). This would also imply a wider main lobe.
--- End quote ---
Indeed. The RBW of the Flattop window is 3.73 times the frequency step.
gf:
--- Quote from: Performa01 on December 04, 2023, 01:30:35 pm ---FOr the SDS2000X Plus, this has already been done a while ago (reply #3561):
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg4318822/#msg4318822
--- End quote ---
Very nice! :-+
[ I obviously had a peek at it at the time, but I did not remember. ]
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