Why is all the information contained only in the sidebands in AM? (And what is this supposed to mean? I read it off a textbook.)

Also, why does an increase in bandwidth lead to an increase in quality of the signal?


1 Answer 1


Here is why.

Imagine a radio frequency transmitter, broadcasting a pure sine wave carrier signal with no modulation at the AM radio frequency of 1000 kilohertz. This signal shows up in the radio spectrum as an extremely sharp spike centered right at 1 megahertz with essentially no width to that spike.

Now we modulate that 1 megahertz sine wave by mixing it with an audio signal that varies in frequency from 20 Hz to 20kHz. This produces a set of sum and difference frequencies that vary from (1 MHz-20 kHz) on the difference side to (1 MHz+20kHz) on the sum side. This means that the slice of the radio frequency band occupied by the modulated signal starts at 980 kHz and extends up to 1,020 kHz: it is 40kHz wide.

If we restrict the audio signal to no more than 3kHz, the bandwidth occupied by the modulated signal is only 6kHz wide but you have lost all the treble content in the signal: If you were to watch the modulated RF transmission on an oscilloscope you would see the width of the signal widen out whenever high frequencies were present in the audio signal and shrink down when no highs were present in the audio signal.

Now imagine that your receiver has a signal bandpass width of only 6kHz, and it is receiving a modulated signal with 40kHz bandwidth. The receiver cuts off the extremes- the upper and lower sidebands- and what you hear in the speaker is limited to 3kHz.

If you increased the bandpass width to 40 kHz, then you would hear the treble frequencies contained in the transmitted signal.

In this sense, the audio signal is encoded in the sidebands and the wider the frequency range in the audio signal, the broader the sidebands become and the more bandwidth the radio frequency signal occupies in the AM radio band.

  • $\begingroup$ But doesn't the variations in amplitude in the modulated carrier wave already carry the data for the differing frequencies of the audio signal? $\endgroup$ Commented Jun 18, 2020 at 6:04
  • $\begingroup$ @RichieHarvy They did, and you can think of it that way. However, its much more effective to think in frequency terms, especially when you have access to lots of linear components like capacitors and inductors which are each to work with in frequency space. Software defined radios, which just slurp up time histories of RF signals, can think more in the way you are thinking. $\endgroup$
    – Cort Ammon
    Commented Jun 18, 2020 at 6:18

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