Hello,
Currently i'm trying to use the FFT functions of the GSL to filter
measuring signals in frequency domain. The transformation works fine and
i have the amplitude but how do i get the corresponding frequencies for
the amplitudes?
The excerpt of the GSL documentation how to calculate the frequencies
confuses me a bit. Delta is the time-step in the time domain, right? For
example 1/2s. So the amplitude of index 0 represents 0Hz sounds good to
me. But how to calculate the other ones? Please take a look at my
example how i think it works at the end of this email.
index z x = FFT(z)
0 z(t = 0) x(f = 0)
1 z(t = 1) x(f = 1/(N Delta))
2 z(t = 2) x(f = 2/(N Delta))
. ........ ..................
N/2 z(t = N/2) x(f = +1/(2 Delta),
-1/(2 Delta))
. ........ ..................
N-3 z(t = N-3) x(f = -3/(N Delta))
N-2 z(t = N-2) x(f = -2/(N Delta))
N-1 z(t = N-1) x(f = -1/(N Delta))
Example:
N = 10 / Delta = 2
0 z(t = 0) x(f = 0) 0Hz
1 z(t = 1) x(f = 1/(10 * 2)) 1/20Hz
2 z(t = 2) x(f = 2/(10 * 2)) 1/10Hz
3 z(t = 3) x(f = 3/(10 * 2)) 3/20Hz
4 z(t = 4) x(f = 4/(10 * 2)) 2/10Hz
5 z(t = 5) x(f = 1/(2 * 2)) 1/4Hz
6 z(t = 6) x(f = -4/(10 * 2)) -2/10Hz
7 z(t = 7) x(f = -3/(10 * 2)) -3/20Hz
8 z(t = 8) x(f = -2/(10 * 2)) -1/10Hz
9 z(t = 9) x(f = -1/(10 * 2)) -1/20Hz
Tanks for your help,
Philip
--
*Deutsches Zentrum für Luft- und Raumfahrt* e.V. (DLR)
Institut für Aerodynamik und Strömungstechnik | Abteilung Raumfahrzeuge
| Bunsenstr. 10 | 37073 Göttingen
*Philip Poloczek* | Dualer Student (DHBW-Mannheim
<http://www.dhbw-mannheim.de/>)
Telefon 0551 709-2391 | [email protected]
<mailto:[email protected]>
www.DLR.de <http://www.DLR.de>
