4.0- FTEPR - Fourier Transform program
The Fourier transform program is intended to compute the FFT of experimental
data and perform some basic manipulations of data as well as serve as a
platform for more extensive computations not yet ported to the PC. The
Import/Export utility provides for file loading and saving.
4.0.1 The data screen
message area
The top line of the screen is reserved for status messages and user prompts.
display box
The two large boxed areas are reserved for spectral display. The upper box is
for Real data display and the lower box is for Imaginary data display. The
display can be zoomed to either the Real of Imaginary display occupying the
entire display area. A scale is printed along the X-axis in data point units.
The top line of the display box is used for :
Filename: name of the imported file
Clipboard # Clipboard number used to load the data
YS: the y-scale multiplier applied to the data, constant for Real
and Imaginary data.
menu area
The menu options are displayed on the left of the screen with the current choice
printed in a different color. The up and down arrows move the current choice,
the enter key will initiate that action. You must choose EXIT to leave the
program.
4.0.2 Function keys
No function keys are active in this program.
4.1.0 The FTEPR menu
The main FTEPR menu has the following choices:
Import Load a data file.
Export Write a data file.
Parameters Display the current spectral parameters.
FFT +1 Calculate a forward FFT.
FFT -1 Calculate a reverse FFT.
Filter Filter the FT spectrum by zero-fill(s).
Zero Zero either the Real or Imaginary data arrays.
Zoom Zoom the display to either all Real or all Imaginary.
X-Y-scale Change the X and Y scale display parameters
Combine Combines the Real and Imaginary data sets to one
array for simulation.
Enhance FT based spectral enhancements.
Power Calculate a power spectrum from the Real and Imaginary
components of FT data.
Autocor Calculate an FT-generated autocorrelation of a spectrum
using original CW data
Exit Exits this program
Each function is described in more detail.
Import
Loads an exported data file. You will be prompted for R or I for
Real and Imaginary data. Next, enter the clipboard page number
( 1-99 ) to transfer experimental data into the Simepr.exe program.
Export
Saves data to a file. You will be prompted for R or I if the display is
Real and Imaginary. After that, you will be prompted for the clipboard
page number ( 1-99 ).
Parameters
Displays the current spectral parameters on screen.
FFT +1
Compute a forward Fourier transform of the current data.
FFT -1
Compute a inverse Fourier transform of the current data.
After each FFT operation, the display shows both Real and Imaginary data. If
the number of forward and inverse transforms is equal, the Y-scale is set to be
the original value when the data was imported into the program.; otherwise, the
Y-scale is set to produce a display with the data set (R or I) having the most
intensity to be nearly full screen in its display window. The FT calculations are
performed in double precision for speed and accuracy.
Filter
Filter the data by zeroing a portion of the Fourier transformed data. You are
prompted to zero either a Low or High frequency region. You are next
prompted for n, the highest or lowest filtered frequency accordingly. The low
frequency filter is particularly useful for removing non-linear baseline effects
and the typical value of n is 2 or 3. The high frequency filter is useful for
'noisy' spectra, the value of n should then be something just above the region of
visible spectral information.
Zero
Zero's either the Real or Imaginary data. This option is especially useful when
the Real and Imaginary data do not belong together. When you import CW
data into the Real display and the Imaginary data is non-zero, you should Null
the Imaginary display
Zoom
Zooms the display. You will be prompted to zoom either the Real or Imaginary
displays. A subsequent zoom will revert to a split screen display.
X-Y-scale
Use the arrow keys to change the X and Y display scale parameters. Press
ESC or Backspace to return to the menu.
Combine
Combines the Real and Imaginary data sets to one array for exporting to the
Simepr.exe program. You must use this feature to Optimize or Tune an FT
spectrum. This feature has no meaning for a CW spectrum. Only a symmetric
spectrum which is centered in the data array will produce a Fourier transform
which is purely either Real or Imaginary depending on the harmonic with the
exception of noise. A first derivative CW spectrum will produce a purely
Imaginary display after an FFT+1. To simulate the FT of non-symmetric
and/or non-centered spectra, this feature has been added. Because the spectral
information is compressed into the low frequency region of an FT by the
exponential linewidth function, the high frequency region may be discarded.
This now empty space of the Imaginary array is used to hold the low frequency
information of the Real array. You will be prompted for n, the frequency value
(data point) for combination. This number should be above the region of
significant low frequency spectral information. The left n values will now
contain Imaginary array values and the n+1 to 2n+1 values will now contain
the Real array values. The display is now scaled to show 2n frequency values
which will be in the Imaginary display and the Real display will be null. This
new Imaginary array may then be exported to the Simepr.exe program for
efficient simulation of FT data. This is an unrecoverable process.
Enhance
This feature is reserved for future FT based apodization style applications.
Power
Computes the Power spectrum from Real and Imaginary FT data; do not use
CW data. The power spectrum is defined as the square of the complex
coefficients. P= (a+bi)(a+bi)= aa + 2abi + bb = aa + bb + 2abi . This is often a
useful product in FT calculations.
Autocor
Computes the Autocorrelation spectrum of the experimental spectrum stored in
the Real display. The starting data should be a CW spectrum. First, a forward
FFT is computed, then a Power spectrum, then an inverse FFT. Data with
value below zero is eliminated. The result is comparable to the output of the
Autocor.exe program, however this result is a new spectral map of the splitting
constants. The Gauss units remain constant and splittings are measured by the
distance from the low field limit to a peak.