In amplitude modulation the amplitude of a slowly varying signal is impressed onto the amplitude of a rapidly varying signal. The mathematical description of the modulation of a sinusoidal carrier by another sinusoid is given in the formula below where wc is the carrier frequency, wm is the modulation frequency, and m is the degree of modulation. The time domain representation (amplitude versus time) of this modulated signal is shown in the interactive graph labeled Time Domain.
A simple expansion of the formula shows that the resulting signal consists of three sinusoids, or components, of different frequencies. One of these components has the same frequency as the unmodulated carrier. The second component has a frequency equal to the sum of the carrier frequency and the modulation frequency wc+ wm. This component is called the "upper sideband." Similarly, the third component has a frequency equal to the carrier frequency minus the modulation frequency wc- wm and is called the "lower sideband."
The two sideband components have equal amplitudes which are proportional to the amplitude of the modulating signal. The figure labeled Frequency Domain shows the carrier and sideband components as a plot of amplitude versus frequency.
These graphs are interactive. The parameters of the mathematical model can be adjusted by dragging the sliders with your mouse. You may also click and drag the graphical objects inside the Time and Frequency Domain graphs. For example, the figure above shows how the carrier frequency can be adjusted by dragging the center arrow in the Frequency Domain graph. Activating the checkbox marked "overmodulate" allows an investigation of signal properties for values of m greater than 1. A complete mathematical definition and analysis of Amplitude Modulation is available in the application note listed below.
View AM Interactive Demo
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