Introduction to Modulation

Cuthbert Nyack

Naturally occuring or artificial signals occur at frequencies which are controlled mostly by the physical properties of the source of the signal. The physical properties of the media used to transmit signals from one place to another is such that it only transmits signals in frequency bands that are usually different from the frequency band of the signal to be transmitted. In order to transmit a signal across such a medium it is necessary to shift the spectrum of the signal to a frequency band which is not absorbed by the medium or to an "allocated" portion of that frequency band. This process is called modulation.
Many types of modulation are possible and the choice of which one to use depends on efficiency, power requirements, bandwidth and S/N ratio. Generally S/N ratio can be improved by increasing bandwidth. Other practical problems like coupling of the signal to the medium may also play a part.
In the case of digital communications the modulating+coding system is chosen to transmit the maximum amount of information within the given bandwidth subject to meeting the specified error rates.

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