Play with an FFT
This applet lets you edit real or imaginary values in the time domain or the frequency domain. As you edit the values in one domain, the other domain is automatically recalculated using an FFT. The magnitude and phase of the frequency domain are also shown.
(We would like to thank Dave Hale for posting FFTLab to the web. It was the starting point for developing this applet.)
To do:
 Click the Clear button to zero out all the data.
 Drag the mouse straight across the topleft box labeled "Time Domain," "Real." Notice that a peak appears in the first bin of the frequency domain. This is because a constant value has a frequency of zero*fundamental.
 Draw a single cycle of a sine wave in the same box. Notice that the peak now appears in the second bin, representing the 1*fundamental frequency of the FFT.
 Draw two sine waves within the same box and notice that the frequency domain peak moves to the third bin, representing 2*fundamental.
 Hit Clear and then click once in the box so that you get a single peak. Notice that the frequency domain now shows a constant magnitude. This is because an "impulse" peak has a spectrum that contains all frequencies equally.
 Hit Clear and then click on the second bin. You should see a sine wave in the frequency domain. This shows that the time domain and the frequency domain are complementary.
 Poke around in the frequency domain boxes, drawing various signals and watching the results, until everything suddenly becomes clear.
