Agilent has done a marvelous job of animating the spectra of a square wave
with various numbers of harmonics included at this site.
http://www.educatorscorner.com/experiments/spectral/SpecAn2.shtml You can
look at the waveform, alter the number of harmonics included and see the
You can also vary the duty cyle of the wave and view the resulting frequency
spectra as you would see on an analyzer. If the wave shown is used to
modulate a carrier, as in CW, the sidebands will be 6 dB lower in amplitude
and will show up on both sides. i.e. The bandwidth is twice as wide.
This spectra is not as wide as a that of a keyed carrier where the shaping
is done with an RC time constant. Many higher order harmonics are present
in the keyed waveform, but the general idea is shown.
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