In amplitude modulation, the amplitude of a transmission represents the signal. The figure to the right, taken from Signals by John Pierce (an interesting book that is unfortunately out of print), shows how such a signal is created. The carrier wave has a frequency in the kiloHertz range (600 on the AM dial is 600 kHz), so the antenna need only be 125 m long. The initial signal is raised until it is completely positive, then this positive version of the signal is used as the envelope that determines the amplitude of the transmitted wave. The transmitted wave can be broken down into its Fourier components, and that information sent along with the wave to help the decoding process. Since the transmitted wave is non-periodic, a Fourier transform must be used. The collection of amplitudes for the different component frequencies is called a spectrum. This spectrum contains two groups of frequencies, called sidebands. Sometimes one sideband is sent, and sometimes both are sent. The information in the sidebands is in theory redundant to the information in the carrier signal, but the two can be compared to check for losses and interference.
The bandwidth for an AM signal will depend on the bandwidth of the original signal, but it will be larger than this original bandwidth. A typical signal has an original bandwidth of 5 kHz and a transmitted bandwidth of 10 kHz. This increase in bandwidth might seem like a reason not to modulate, but the advantages of modulation far outweigh this bandwidth increase. As already mentioned, antennae can be of a reasonable length, and several signals containing information at similar frequencies can be sent without risk of interference. | |
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