Amplitude Modulation

General Instructions

This spectral simulation is an interactive Java applet. You can change parameters by clicking on the vertical arrow keys. The five control buttons at the lower right are used to start (triangle) and pause (square) the simulation, to skip forward or back one section at a time (double triangles), and to change speed (+ and -).

After the simulation is complete, the start button takes you back to the beginning of the simulation. You may experience a delay at this point.

Amplitude modulation was the original method used for transmitting audio signals over radio frequency waves. It is still used today in AM radio. This applet demonstrates the modulation process.

In an AM signal, the idea is to modulate the carrier waveform without changing phase - only the amplitude is supposed to change. that leads us to the model of a single carrier and two oppositely-rotating vectors that sum to change the amplitude of the carrier, but not its phase. If we look at the math:

e(t)=A(1+m*cos(Wm*t))*cos(Wc*t)

Where Wc is the carrier frequency, Wm is the modulation frequency and m is the modulation index. A modulation index of 1 means that the modulation is just as big as the carrier, ie., 100% modulation.

The above expression can be broken into a carrier part plus two modulation parts:

e(t)=A*cosWc*t    +   [(m/2)*cos(Wc+Wm)   +   (m/2)*cos(Wc-Wm)]
      Carrier                  modulation component 1       modulation component 2

The common example of AM is a radio that is tuned to an "AM Band". For instance, an AM radio station at 850 KHZ is actually broadcasting a carrier at 850 KHZ all the time, and the modulating that carrier with the modulation components shown above. It is the modulation components that carry the information.