MRI and NMR Spectroscopy in the Earth's Field: Building a Low-Cost NMR Spectrometer for Hobby Science and Teaching by Trevelyan

Author:Trevelyan [Trevelyan]
Language: eng
Format: epub
Published: 2019-04-15T16:00:00+00:00

The wide column with the blizzard of digits in it contains the output of the FFT algorithm: two 15-digit values, the second ending in i for imaginary, representing the complex numbers.

Fourier Analysis is an option in the Data Analysis menu under the Data tab in Excel (you may need to download the Analysis Tool Pack if it is not already installed). A dialog box comes up, requiring the input and output data ranges. Select only the column of voltage readings, not the time . The algorithm assumes the samples – whether real or complex numbers – are equally spaced in time.

The FFT algorithm requires the count of input data to be a power of 2. The built-in FFT facility of the DSO processes only 2,048 (2 to the 11th power) of the 2,500 values it saves. We can do better than this by padding out our Excel spreadsheet with zeroes to fill 4,096 cells. It is quite laborious to set up a 4,100-row spreadsheet, but you only need to do it once. After that, simply paste in new data copied from the CSV file that the DSO saves (use the Paste Special , Values option in Excel).

The complex numbers of the FFT output in column G need to be converted into single real values to be plotted as the spectrum. Excel provides the IMABS function, which does the trick.

Plotting Magnitude (column H) versus FFT frequency (column F), using an Excel X-Y (scatter) chart, yields a peculiar-looking graph consisting of the signal’s actual spectrum and its mirror image.

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