Version 1.1

FFT is a Fast Fourier Transform for Excel.  It works on both real
and imaginary data.  The length of the data must be a power
of 2. (2,4,8,16...).  The width of the data may be 1 column
(for a real to complex forward transform) or 2 columns (for a complex to
complex forward or inverse transform or a complex to real inverse
transform).

To use this function, you may either put it in the 'Excel
Startup Folder' in your system folder (which loads it
in automatically when you start Excel), or use 'Open...'
from within Excel.

A Fourier transform gives the frequency components of a series of data,
or vice-versa.  

The output is an array with the same number of elements as the input.
Output is two columns wide, except for the complex to real inverse
transform, which has 1 column of output.

To use, select a region on the spreadsheet which is the right size
and shape for the output.  Type '=FFT(reference, direction)', then
hold down the clover key and hit return.  (This is the general way you
tell excel that a function gives an array output.)

The "direction" argument gives the mode,
		negative is inverse, positive is forward,
		2 is complex, 1 is real.
Examples:
     =FFT(A1:B32, -2)
       Takes the 32x2 complex spectrum A1:B32 and gives a 32x2
       complex result.
   =FFT(A1:B32, -1)
       Takes the 32x2 complex spectrum and gives a 64x1 real result
   =FFT(A1:A64, 1)
       Takes the 64x1 real numbers and gives a 32x2 spectrum
   =FFT(A1:B32, 2)
       Takes the 32x2 complex numbers gives 32x2 spectrum

This particular implementation uses the code from
'Numerical Recipes'.  If somebody has a more general C code (i.e.
not restricted to powers of 2) that they can donate, I'll plug it in.

If you are interested in writing your own Excel code resources,
there is a 'Software Developer's Kit' for Excel available from
Microsoft Developer Services 1 (800) 227-4679.

Changes from Version 1.0:
Now works properly when placed in Excel Startup Folder.

©1992, 1993 by David Palmer (palmer@tgrs.gsfc.nasa.gov),
All rights reserved.
This is Freeware.  If you use it, strike a blow against tyranny.