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FT EPR Manual

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 :

 

Filenamename of the imported file
Clipboard #Clipboard number used to load the data
YSthe 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:

 

ImportLoad a data file.
ExportWrite a data file.
ParametersDisplay the current spectral parameters.
FFT +1Calculate a forward FFT.
FFT -1Calculate a reverse FFT.
FilterFilter the FT spectrum by zero-fill(s).
ZeroZero either the Real or Imaginary data arrays.
ZoomZoom the display to either all Real or all Imaginary.
X-Y-scaleChange the X and Y scale display parameters.
CombineCombines the Real and Imaginary data sets to one array for simulation.
EnhanceFT based spectral enhancements.
PowerCalculate a power spectrum from the Real and Imaginary components of FT data.
AutocorCalculate an FT-generated autocorrelation of a spectrum using original CW data.
ExitExits this program

 

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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.

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