Types of Modulation
As you'll recall, modulation is the process of changing breadth or frequence or phase of a carrier surge in agreement with the intensity of the signal.
Consequently, there are three introductory types of modulation, videlicet:
(i) breadth modulation (ii) frequence modulation (iii) phase modulation
In India, breadth modulation is use in radio broadcasting.
Still, in TV transmission, frequence modulation is used for sound signal and breadth modulation for picture signal.
Thus, our attention in this chapter shall be confine to the first two most important types of modulation.
When the breadth of high frequence carrier surge is change in Agreement with the intensity of the signal, it's call breadth modulation.
In breadth modulation, only the breadth of the carrier surge is change in agreement with the intensity of the signal.
Still, the frequence of the modulated surge remains the same i.e. carrier
frequence. Fig-1 shows the principle of breadth modulation.
Fig-1(i) shows the audio electrical signal whereas Fig-1 (ii)
shows a carrier surge of constant breadth.
Fig-1(iii) shows the breadth modulated (AM) surge.
Note that the confines of both positive and negative half- cycles of carrier surge are changed in agreement with the signal.
For case, when the signal is adding in the positive sense, the breadth of carrier surge also increases.
On the other hand, during negative half- cycle of the signal, the breadth of carrier surge decreases. Breadth modulation is do by an electronic circuit called modulator.
The following points are worth noting in breadth modulation
(i) The breadth of the carrier surge changes according to the intensity of the signal.
( ii) The breadth variations of the carrier surge is at the signal frequence fs
(iii) The frequence of the breadth modulated surge remains the same i.e. carrier frequence fc.
Frequence Modulation (FM)
When the frequence of carrier surge is change in agreement with the intensity of the signal, it is call frequence modulation (FM).
In frequence modulation, only the frequence of the carrier surge is change in agreement with the signal.
Still, the breadth of the modulated surge remains the same i.e. carrier surge breadth.
The frequence variations of carrier surge depend upon the immediate breadth of the signal
as shown in Fig-2(iii).
When the signal voltage is zero as at A, C, E and G, the carrier frequence is unchange.
When the signal approaches its positive peaks as at B and F, the carrier frequence is increase to outside as shown by the nearly spaced cycles.
Still, during the negative peaks of signal as at D, the carrier frequence is reduce to minimum as shown by the extensively spaced cycles.
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