Generate Frequency for FFT

mod_fft.f90

  function fftfreq(st, block)
    implicit none

    real*8, intent(in)  :: st(:) ! sample time
    integer, intent(in) :: block
    real*8              :: fftfreq(size(st))

    integer*4 :: n, i
    real*8    :: t

    n = size(st)
    t = (st(n) - st(1))*block/(n - 1)

    if (mod(block, 2) == 0) then
      fftfreq = (/ (i, i=0, block/2-1), (i, i=-block/2, -1) /) / t
    else
      fftfreq = (/ (i, i=0, (block-1)/2), (i, i=(-block-1)/2, -1) /) / t
    end if
  endfunction