On 13 Okt, 00:22, "Peter123" <p...@[EMAIL PROTECTED]
> wrote:
> I have implemented Jens Joergen Nielsen FFT code in my windows program
> that converts V(t) to V(f). =A0I am using 2500 Hz sampling with N=3D8192
=
point
> FFT.
> I am plotting the FFT amplitude as sqrt(Re^2 + Im^2)/N. Im getting the
> correct peak frequency after FFT, however, I am getting only about 1/2
of
> the peak =A0amplitude (of the input sine wave input amplitude) with
> rectangular window. (If my input wave is 20.0*sin(omega*t) the FFT gives
> ~10 for peak height at omega).
> Using various windows (Hamming, Bartlett, etc) the V(f) peak amplitude
> becomes even smaller.
>
> Any suggestion why do I get 1/2 peak heights?
It's because of Euler's equations:
cos(x) =3D 1/2 (exp(jx)+exp(-jx))
sin(x) =3D 1/j2 (exp(jx)-exp(-jx))
> What correction factors should I use for the spectrum height for various
> window types?
Don't bother. You will not see the exact numbers you use
for the amplitudes unless the frequency of the sinusoidal
is an integer fraction of the sampling frequency,
f =3D k/N
wher k and N integers and k < N/2.
Rune
|
