An FFT Example - 20% Pulse Signal

        In this note we are going to analyze a pulse signal using the FFT. Here is the signal.  (NOTE:  If you are used to Matlab, the indices here start at zero.)

Now, we need to examine what happens when we FFT this signal.  We will use the Mathcad file this link takes you to.  (You may have to change the file name for the data file that is loaded in this workspace.)  That file assumes that the data file has two columns (Time in the first column, data in the second column.  Now, if we look at the plot of the absolute value of the SigFFT array, we get a plot like the one below.

        Now, the fundamental frequency of the data record is 500 Hz.  You need to be able to get from that to the actual frequency components of the signal.  Here is what you need to use.

Harmonic of the fundamental frequency of the data record
 The actual frequency
0 Hz (DC)
500 Hz
 1000 Hz
(1 KHz)
 1500 Hz
2000 Hz
2500 Hz
3000 Hz

        Now, we can get at the frequencies in the FFT plot.  Notice the following.