MODULATION

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To modulate is to mix a data signal onto a carrier frequency signal and modify its characteristics for transmission in a communication network. A carrier frequency signal is an electromagnetic wave that vibrates at a fixed frequency.

Due to the fact that both attenuation and propagation speed are dependent on frequency of the signal, it is undesirable to have a wide range of frequencies in the signal. But square waves in digital data have a very wide range of frequencies. Thus, square waves are subjected to strong attenuation and also delay distortion. Each square wave consists of series of Fourier components. Each component is attenuated by a different amount which results in a different Fourier spectrum at the receiver and hence a different signal. These adverse effects make baseband (DC) signalling unsuitable except at slow speeds and over short distances.

            To overcome the problems associated with DC signalling, especially on telephone lines, AC signalling is used. A continuous tone in the range of 1000 to 2000 Hz, called a sine wave carrier, is used.

            The data can modulate the carrier by various methods. These methods are:

1.      Amplitude Modulation (AM)

2.      Frequency Modulation (FM)

3.      Phase Modulation (PM)