The applet demonstrates the use of the Fast Fourier Transform (FFT) to determine the frequency spectrum of a discrete signal (time series).
The sampled signal is generated by the applet according to the choices you make from the control panel. After making your choices, click the "Plot signal" button. The resulting sampled signal will be plotted in the upper graph.
Next, click the "Spectrum" button to start the FFT calculations. Once the FFT has been performed (which may take several seconds, depending on the speed of your computer and how many samples you specify), the frequency spectrum of the signal should be plotted as a graph (bar plot) of magnitude against frequency.
The signal amplitude is fixed at 1 V and the sampling rate is 8000 samples/s. The frequency scale on the spectrum plot is from 0 to 4000 Hz (half the sampling frequency).
More detailed guidance notes on how the applet can be used to illustrate various aspects of signals and spectra are available.