12th physics. NCERT notes

Modulation is the process of variation of some characteristics of a carrier wave in accordance with the instantaneous value of a modulating signal.

Types of modulations:

  1. Amplitude modulation
  2. Frequency modulation
  3. Phase modulation
  4. Pulse modulation

Need for modulation: It is due to the fact that low-frequency signal

  • needs antenna of very large length (≈ 5 km):

For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) so that the antenna properly senses the time variation of the signal.

  • mixes up of the signal transmitted from different stations:

There is a need for translating the original low-frequency baseband message or information signal into high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal. In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in Fig.

Sinusoidal and pulse shaped signals

A sinusoidal carrier wave can be represented as

  • get attenuated significantly. As power radiated by antenna is given by

This implies that for the same antenna length, the power radiated increases with decreasing λ, i.e., increasing frequency. Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using
high-frequency transmission.

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