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Modulation is a technique for incorporating information onto a carrier wave, that varies the characteristics of the wave in accordance with the information signal.
We can find three kind of modulation: amplitude modulation, frequency modulation and pulse-coded modulation.
In amplitude modulation, the carrier wave varies its amplitude following the modification of the signal’s amplitude and we can extract the information signal just using a capacitor.

This kind of modulation has advantages and disadvantages; in fact the signal is easily recovered in the receiver, but it’s the oldest method of modulation. AM is used in radio broadcasting at low and middle frequencies.
In frequency modulation, the amplitude of carrier wave is kept constant, and its frequency varies in accordance with information signal frequency.
This form of modulation was developed in 1930 to overcome interference and noise of radio reception, even if the FM is susceptible of interference.
On the contrary of AM, FM gives a more accurate reproduction of the signal and a better quality of the information signal.
To recover the information signal we have to use a demodulator FM, for example the integrated circuit PLL (Phase Locked Loop), that is able to follow all the modification of the input signal frequency.
In digital transmissions, the pulse-coded modulation is used. In this kind of modulation, the intelligent signal converts the carrier wave into a serial of constant-amplitude pulses; these pulses are continuous signals, witch are transformed into patterns of “on” and “off” pulses.
PCM eliminates noise and interference, and the pulses can be regenerated by repeater stations along the transmission roule.
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