Amplitude modulation sidebands. Power/frequency plot.
<a href="//commons.wikimedia.org/wiki/User:%D0%94.%D0%98%D0%BB%D1%8C%D0%B8%D0%BD" title="User:Д.Ильин">Д.Ильин</a>
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<a href="//commons.wikimedia.org/wiki/File:Am-sidebands.png" title="File:Am-sidebands.png">File:Am-sidebands.png</a> by <a href="//commons.wikimedia.org/w/index.php?title=U:Heron2&amp;action=edit&amp;redlink=1" class="new" title="U:Heron2 (page does not exist)">Heron2</a>