Message from discussion fft from numerical recipes
From: heiko liebhart <heiko.liebh...@googlemail.com>
Subject: fft from numerical recipes
Date: Tue, 17 Apr 2007 22:48:23 +0200
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i am quite as new to programming in general and to fortran in particular
as to programming ffts.
as i wanted to apply a fouriertransformation to one of my progs for
pulseshaping i started with the subroutine four1 provided in numerical
as a start i used a gaussian as input data expecting to get another
gaussian as output data.
so i generated the input values, filled in the zeros in the imag part in
order to use a real function, called the subroutine an off we go for the
after rearranging the outputarray in the way according to the book i had
to realize unfortunately the output i got was not really what i suspected.
what i got was something like a double gaussian. i.e. the sign was
alternating. like sign of every second value was inverted oszillating
from plus to minus.
in the next step i took a sin wave as input.
again what i got was not the suspected delta peak at the frequency of
the oszillation but two antsymmetric peaks in the imaginary part.
according to the symmetry mentioned in the book this should be right for
a real and odd funktion like sin but is not fitting to my
understanding of fouriertransformation.
i was wondering whats going wrong. is the trick in the rearranging of
the output or even deeper in the theory of the transformation so that
maybe i would have to use the product of H and H*.
can somebody please give me a hand to get me through this numerical
jungle even step by step.
i would appreciate every hint and assistance.