# Amplitude modulation

• B
1) I am studying modulation of carrier waves, but I still can't understand something: knowing that in AM the carrier wave has only one frequency and just only its amplitudes vary according to variations in modulating signal amplitudes, why AM carrier waves have bandwidth if the frequency is supposed to be unique? Does it have any point with variations in the modulating signal frequencies? If it does, what about simple signals with only just one frequency? Thank you all for the answers.

phyzguy
The only way it can have one frequency is to have a constant amplitude. Once you start varying the amplitude, it necessarily consists of a range of frequencies. Are you familiar with the Fourier transform? If you take the Fourier transform of a sine wave of constant amplitude, it is a delta function, meaning it has only a single frequency. But if you take the Fourier transform of a sine wave of varying amplitude, it will consist of a range of frequencies. If you have a program that can take the Fourier transform of different functions (Mathematica, Matlab, ...) try putting in sine waves with different modulations and you will see the result.

davenn
Gold Member
2021 Award
1) I am studying modulation of carrier waves, but I still can't understand something: knowing that in AM the carrier wave has only one frequency and just only its amplitudes vary according to variations in modulating signal amplitudes, why AM carrier waves have bandwidth if the frequency is supposed to be unique? Does it have any point with variations in the modulating signal frequencies? If it does, what about simple signals with only just one frequency? Thank you all for the answers.