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Trying to display bandwith via Math on Siglent SDS2k+/2kHD/800X HD

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gf:

--- Quote from: Martin72 on July 23, 2023, 09:49:13 pm ---

--- Quote ---Btw, I don't know where this comes from, but the 0.5 dB drop in the DC bin doesn't look right either.
--- End quote ---

I´ve "zoomed in" using a span of 500Mhz, when the Delta f is a bin (?), than it "happens" in exact the first one, 0Hz..24.41Mhz.
From 24.41(24.5 marked)Mhz down to 0 it´s -3dB.

--- End quote ---

-3dB? Then I think it displays the power spectrum and not the FFT spectrum "as is".
This means that negative frequencies are discarded and instead the power of positive frequencies (f > 0) is doubled.
Consequently, the power spectrum of an impulse response does not reflect the corresponding frequency response at/near DC, but only at higher frequencies.

EDIT: The problematic nature of DC in conjunction with power spectrum calculation (with rectangular and other window functions) is nicely discussed here (chapters 11 and 9).

Performa01:

--- Quote from: Martin72 on July 23, 2023, 09:49:13 pm ---I´ve "zoomed in" using a span of 500Mhz, when the Delta f is a bin (?), than it "happens" in exact the first one, 0Hz..24.41Mhz.
From 24.41(24.5 marked)Mhz down to 0 it´s -3dB.

--- End quote ---
Delta-f, frequency step, frequency bin ... it's all the same.


--- Quote from: Martin72 on July 23, 2023, 09:49:13 pm ---(possibility to change the axis-labeling left/right would be very, very useful for a better viewing.)

--- End quote ---
Why not just get the cursor info out of the way? Like I did it on all my screenshots?
In some rare cases I even had to move the FFT info in order to tidy up the screen.

Martin72:
Hi....



With the axis-labeling on right side you could see it better what "happens" on the left side of the FFT window, has nothing to do with the cursors or the cursor info.
Btw, lecroy put the cursor-infos into the channelboxes.

Martin72:

--- Quote from: gf on July 24, 2023, 06:25:37 am ----3dB? Then I think it displays the power spectrum and not the FFT spectrum "as is".

--- End quote ---

And it is independent of which window you use, I had just tried it out.

Martin72:
I'm digging up this thread so as not to clutter up the other 800X thread.
It didn't work with the 2000Xplus back then, with the 2000X HD it was OK, now with the 800X HD...
Using the "known" formula in the editor, the following is output, which has nothing to do with the frequency response I previously recorded using Sweep and FFT-Max hold.

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