Why does AM have sidebands? Isn't the whole point of AM to broadcast on ONE frequency and ONLY change the amplitude?

AM wave.

In the picture there is only ONE frequency and the amplitude changes, so why got AM sidebands? And why can't AM work without the sidebands? After all it's only the amplitude we need.


In the picture there is only ONE frequency and the amplitude changes

Ah, but this actually causes a second frequency component, the one that you wish to transmit:

Modulated wave

(Please excuse my rudimentary paint skills.)

It is this second, implicit frequency which combines with the AM carrier to then form a strong carrier signal in the centre of the spectrum, and a couple of sidebands that are mirrored in either direction.

Intuitively it can be quite difficult to understand, however there is a relatively simple mathematical explanation (if you understand basic trigonometry) which shows this is the case.

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  • $\begingroup$ Two questions: -So you mean that if you change the amplitude it can make a sideband? -But why cant you just send the carrier-wave you ONLY need the amplitude. $\endgroup$ – Axel H. Aug 8 '15 at 19:27
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    $\begingroup$ @AxelH. Yes, exactly :) And you can just send the carrier wave, but then by definition it wouldn't change (a carrier wave has a constant amplitude) so no data would be transferred. $\endgroup$ – berry120 Aug 9 '15 at 1:02
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    $\begingroup$ Thanks for your answer and I also find some good images that I understand better. clas.mq.edu.au/speech/acoustics/waveforms/waveadd06.gif --- danalee.ca/ttt/images/16/SIDEBAND.gif $\endgroup$ – Axel H. Aug 9 '15 at 17:07

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