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FFT1 Resolution Bandwidth - back to basics
- Subject: FFT1 Resolution Bandwidth - back to basics
- From: VK2KU <clearmail.com.au; vk2ku@xxxxxxxxxxxxxxxx>
- Date: Thu, 27 Dec 2007 00:02:55 -0800 (PST)
I quite understand that provided FFT2 is enabled, the main function of
FFT1 is to enable the removal of strong signals before the Noise
Blankers. This does not need a particularly narrow resolution
bandwidth, as Leif has often written, e.g.
> For fft1 it is a good idea to set a high bandwidth
> like 200 Hz with a sin 3 window.
> It will give a rather wide peak but with very good shape factor .....
Now if you choose a particular window power (like 2 or 3), and input a
particular bandwidth, you will actually get a rather lower value of
BW, often much lower, because the FFT size has to be an exact power of
I take it that Leif's comment really refers to the bandwidth you
actually get rather than the one you ask for.
With 96 kHz sampling, as with the Delta-44, we find that:
FFT size 1024 gives a BW of 187Hz for a sine^2 window, and a BW of
225Hz for a sine^3 window.
No other FFT size comes even close to 200 Hz, and either of these
seems ok, though I think Leif would choose the higher power window.
With 111.111 kHz sampling, as I am currently using with my SDR-IQ, we
FFT size 1024 gives a BW of 217Hz with a sine^2 window, and a BW of
260Hz with a sine^3 window.
FFT size 2048 gives a BW of 130Hz with a sine^3 window, and a BW of
149Hz with a sine^4 window.
Now which one would Leif recommend?
The sine^2 window with 217Hz, sine^3 window with 260Hz or 130Hz, or
the sine^4 window with 149Hz?
My guess is the sine^3 window with 260Hz! Leif?
Footnote to Leif:
Thanks for the C-coding suggestion re the attenuator in the SDR-IQ.
I understand the change, I would have tried this already myself, but I
have no C-compiler!
You may remember that I am running under Windows!
I will get back to you on the other suggestions when I have time to
Grandchildren visiting tomorrow for 2 days (Friday and Saturday).
73 Guy VK2KU
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