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*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 2. 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 find that: 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 try them. Grandchildren visiting tomorrow for 2 days (Friday and Saturday). 73 Guy VK2KU --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Linrad" group. To post to this group, send email to linrad@xxxxxxxxxxxxxxxx To unsubscribe from this group, send email to linrad-unsubscribe@xxxxxxxxxxxxxxxx For more options, visit this group at http://groups.google.com/group/linrad -~----------~----~----~----~------~----~------~--~---LINRADDARNIL