Bernhard,

What exactly is your question?

*Originally posted by Bernhard*

**My basic settings are 2048 points and a period of 25µs, that gives me a 10 kHz window.**

I don't rightly know what you mean by "period", nor, in this case, "window"

Given that your FFT is 2048 points long, it will capture 2048/Fs seconds of audio (46.44ms in the case of 44.1kHz samplerate). The window of an FFT is always the length of the FFT itself, by definition, and doesn't have a frequency associated with it. The bin width of a 2048-point FFT at 44.1kHz samplerate is 44100/2048 = 21.533Hz.

46 * 21.533 is 990.5Hz (this is the closest bin to 1kHz, so is the best input freq to use if 1kHz is your ideal).

47 * 21.533 is 1012.1Hz, which is the next closest bin to 1kHz.

Either of those input frequencies is fine, and will allow you to use no specialised window. If you use 1kHz as your input frequency, you'll need to use a specialised window, and get a much degraded reading.