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# Long description

This is another screenshot from Interactive 3, showing just the bottom wave. As usual, the horizontal axis shows time from 0 to 20 milliseconds and the vertical axis shows voltage from minus 1 to 1 volt. Figures 27 to 29 show how the ‘rectangular’ wave is made by adding higher-frequency sine waves.

Here there are two sine waves. The second, higher-frequency wave causes depressions in the tops and bottoms of the lower-frequency wave, effectively ‘chopping’ a bit off the bottom and top to make it more rectangle-like.