PURPOSE: To demonstrate Gibbs' phenomenon.

DESCRIPTION: Putting a square wave signal from the Wavetek through a low-pass filter removes higher harmonics and causes ripples on the leading edge only of the square pulse, as seen on the photograph (left). That the trailing edge remains unaffected (center) seems to indicate that this effect is due to causality; if it were the Gibbs phenomenon, both the leading and trailing edges (in fact, the entire flat top) would be affected. Use the Fourier synthesizer to demonstrate the wave shape with a limited number of harmonics (photograph at right).

SUGGESTIONS: See Question of the Week #229 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for additional background information.

EQUIPMENT: Scope cart with T and dual-trace plug-ins, Fourier synthesizer, Wavetek generator, and Krohn-Hite active filter.

SETUP TIME: 10 min.