enter image description hereI have written a script to plot the frequency spectrum of a sinusoidal signal with c++. Here are the steps 1-Applying Hanning window 2-Apply FFT using fftw3 library I have three graphs: Signal, Signal when is multiplied to Hanning function, and the frequency spectrum. The frequency spectrum looks wrong. It should have a pick at 50 Hz.Any suggestion would be appreciated. Here is the code:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <fftw3.h>
#include <iostream>
#include <cmath>
#include <fstream>
using namespace std;

int main()
int i;
double y;
int N=50;
double Fs=1000;//sampling frequency
double  T=1/Fs;//sample time 
double f=50;//frequency
double *in;
fftw_complex *out;
double t[N];//time vector 
double ff[N];
fftw_plan plan_forward;

in = (double*) fftw_malloc(sizeof(double) * N);
out = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);

 for (int i=0; i< N;i++)
    in[i] =0.7 *sin(2*M_PI*f*t[i]);// generate sine waveform
    double multiplier = 0.5 * (1 - cos(2*M_PI*i/(N-1)));//Hanning Window
    in[i] = multiplier * in[i];

  plan_forward = fftw_plan_dft_r2c_1d ( N, in, out, FFTW_ESTIMATE );

  fftw_execute ( plan_forward );

  double v[N];

  for (int i = 0; i < N; i++)

    v[i]=20*log(sqrt(out[i][0]*out[i][0]+ out[i][1]*out[i][1])/N/2);//Here I have calculated the y axis of the spectrum in dB


   fstream myfile;


   myfile << "plot '-' using 1:2" << std::endl;

   for(i = 0; i < N; ++i)


      myfile << ff[i]<< " " << v[i]<< std::endl;



 fftw_destroy_plan ( plan_forward );
 fftw_free ( in );
 fftw_free ( out );
 return 0;

I have to add that I have plotted the graphs using gnuplot after inserting the results into example2.txt. So ff[i] vs v[i] should give me the frequency spectrum.

Here are the plots: enter image description here Frequency Spectrum and Sinusoidal time Window respectively: enter image description here

Edit: After applying the changes to frequency sampling, this is the graph I got:enter image description here Edit: The first graph is the plot for frequency spectrum vs time

  • $\begingroup$ You probably don't want to plot it on a logarithmic scale. Values very close to zero (as you have around zero frequency) will completely obliterate the actual useful signal. $\endgroup$
    – Peter K.
    Commented Aug 28, 2015 at 18:04
  • $\begingroup$ Even when I change v[i] to v[i]=sqrt(out[i][0]*out[i][0]+ out[i][1]*out[i][1]); the result looks wrong. $\endgroup$
    – Jack
    Commented Aug 28, 2015 at 18:07

1 Answer 1


You seem to be doing something odd with your frequency axis. This:


should probably read:


The frequency axis you have now is nonlinear. The FFT generally generates linearly spaced frequency samples.

  • $\begingroup$ Only Fs * i/N works..Fs * i does not render any good result. $\endgroup$
    – Jack
    Commented Aug 28, 2015 at 18:28
  • $\begingroup$ I have added the new plot above. $\endgroup$
    – Jack
    Commented Aug 28, 2015 at 18:33
  • $\begingroup$ Yes, thats much more like it. $\endgroup$
    – Peter K.
    Commented Aug 28, 2015 at 18:39
  • $\begingroup$ But there is something wrong because now I try to plot time vs v[i] to get the spectrogram and the graph looks funny. I will ass it to my question above. $\endgroup$
    – Jack
    Commented Aug 28, 2015 at 18:40
  • $\begingroup$ v[i] is a frequency-domain variable. It makes little sense to try to plot it against time. $\endgroup$
    – Peter K.
    Commented Aug 28, 2015 at 18:42

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