Modulation is simply a widely used process in communication systems in which a very high-frequency carrier wave is used to transmit the low-frequency message signal so that the transmitted signal continues to have all the information contained in the original message signal.
What is the need for modulation in communication systems?
The message signals have a very low frequency due to which these signals cannot be transmitted over long distances. Hence such low-frequency message signals are modulated over the high-frequency carrier signal due to the following reasons:
1. Practical Length of Antenna:
For the effective transmission of a signal, the height h of the antenna should be comparable to the wavelength λ of the signal at least the height of the antenna h should be λ / 4 in length so that the antenna can sense the variations of the signal properly.
The low-frequency message signal has a very high value of λ which will require a very high antenna (practically not possible).
For example: If we have to transmit a signal of 20 kHz then λ = C / f and height of the antenna h ≈ λ where C is the wave velocity, here C = 3 × 108 m/s.
h ≈ λ = (3 × 108) / (20 × 103)
h = 15 km.
Hence, we need to modulate the message signal over the high-frequency carrier signal so that we can have a practical value for the height h of the antenna.
2. Narrow Banding of Signal
An audio signal usually has a frequency range (20 Hz to 20 kHz), if it is directly transmitted then the ratio of highest to the lowest frequency becomes (20 kHz / 20 Hz) = 1000. But if this audio signal is modulated over a carrier signal of frequency 1000 kHz then the ratio of highest to the lowest frequency becomes:
(1000 kHz + 20 kHz) / (1000 kHz + 20 Hz) ≅ 1.2
Hence, we need modulation to convert a wideband signal into a narrow band signal.
3. Frequency Multiplexing
It is practically not possible to distinguish between the different audio signals when transmitted simultaneously through a single antenna as all of them lie in the same spectral range. Hence, each of these signals is translated to a low-frequency range before transmission which makes it quite easier to recover them and distinguish each of them from one another at the receiver’s end.
4. Effective Power Radiated By Antenna
Power radiated by an antenna ∝ (l / λ)2 where l is the length of the antenna and λ is the wavelength of the signal which is to be transferred through the antenna. This relation clearly shows that when signals having a low frequency and high wavelength is transmitted directly the power radiated by the antenna is very low and the signal will vanish after traveling some distance.
Hence, to transmit such signals over long distances, we superimpose these low-frequency signals over the carrier signal having a high frequency and short wavelength so that the power radiated by the antenna of the same length will be very large.
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