title: fftshift/ifftshift
body: <p>Please, see the the description of both <code>fftshift</code> and <code>ifftshift</code>. 
  I would like to understand how to call the above two functions in relationship with <code>fft</code> and <code>fftn</code> in Matlab.</p>
  
  <p>Let say that my signal has a certain frequency content; now, the frequency array can generally be stored as:</p>
  
  <pre><code>f = (-N/2:N/2-1)*df;
  
  f = (1:N)*(df/2);
  
  f = [(0:N/2-1) (-N/2:-1)];
  </code></pre>
  
  <p>What is the best way to call <code>fft</code>, coupled with <code>fftshift</code> and <code>ifftshift</code>, for the 3 study cases early mentioned?</p>
  
  <p>What is the effect on the standard deviation of the signal of calling the sequence of commands or the wrong one?</p>