Need Of Modulation In Communication Systems

Modulation is a fundamental process in communication systems where the characteristics of a carrier signal, such as amplitude, frequency, or phase, are varied in accordance with a message signal. This process is essential to efficiently transmit data over various distances and through different mediums. By converting the information into a form that can be easily and effectively transmitted, modulation plays a critical role in enhancing the reliability and quality of communication systems, making it possible for data to be transmitted over long distances without significant loss or degradation.

Consider the example of FM radio broadcasting. When you tune into a radio station, you are actually tuning into a specific frequency that carries the audio signal. Frequency modulation (FM) is used here to encode the audio signal onto a carrier wave. This allows the music or speech to be transmitted over large distances with minimal interference and high quality.

Modulation and Its Necessity

The purpose of a communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.

Digital and analogue signals to be transmitted are usually of low frequency and hence cannot be transmitted as such. These signals require some carrier to be transported. These carriers are known as carrier waves or high-frequency signals. The process of placement of a low-frequency (LF) signal over a high-frequency (HF) signal is known as modulation.

Need for Modulation

Modulation is a technique used in communication systems to transmit a signal over a long distance. Here are some key reasons for the need for modulation: The sound wave (20 Hz to 20 kHz) cannot be transmitted directly from one place to another for the following reasons :

1. Size of the antenna or aerial: For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter of the wavelength of the frequency used. For 15 kHz it is 5000 m (too large) and for 1 MHz it is 75 m.
2. Effective power radiated by an antenna: The power radiated is proportional to $\left(\frac{l}{\lambda}\right)^2$. It shows that for the same antenna length, the power radiated increases with decreasing $\lambda$ 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 the need to use the high-frequency transmission.
3. Detecting signals: All audible signals are in the range of 20 Hz to 20 kHz so the signals from all sources remain heavily mixed up in the air. It will be very difficult to differentiate or detect the broadcast signal at the receiving station. Thus modulation is necessary for a low-frequency signal. When it is to be sent to a distant place so that the information may not get erased in the way itself as well as for the proper identification of a signal and to keep the height of the antenna small too.

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Solved Example Based On Need Of Modulation In Communication Systems

Example 1: For efficient transmission of signal, the height of the antenna should be at least equal to (1/4) th of signal wavelength. In case an EM wave of frequency 1 M Hz is to be transmitted, What shall be the minimum height (in meters) of the antenna-

1)75

2)150

3)300

4)600

Solution:

For efficient transmission, the height of the antenna should be comparable to a quarter of the wavelength.

Given the frequency $=1 \mathrm{MHz}=10^6 \mathrm{~Hz}$
The wavelength of an electromagnetic wave $=c / f$
$
\begin{aligned}
& \Rightarrow d=\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{10^6 \mathrm{~Hz}} \\
& \Rightarrow d=300 \mathrm{~m}
\end{aligned}
$

Minimum length of antenna $=\mathrm{d} / 4$
$
=75 \mathrm{~m}
$

Hence, the correct answer is the option (1).

Example 2: A 25 m long antenna is mounted on an antenna tower. The height of the antenna tower is 75 m. The wavelength (in meters) of the signal transmitted by this antenna would be :

1) 100

2)400

3)300

4)200

Solution:

The height of the antenna tower, $\mathrm{H}=25 \mathrm{~m}$
The wavelength of the signal transmitted by this antenna, $\lambda=4 \times H$

On putting the value we get,

The wavelength of the transmitted signal = 100

Hence, the correct answer is the option (1).

Example 3: We do not transmit low-frequency signals to long distances because-
(a) The size of the antenna should be comparable to the signal wavelength which is the unreal solution for a signal of longer wavelength.
(b) Effective power radiated by a long wavelength baseband signal would be high.
(c) We want to avoid mixing up signals transmitted by different transmitters simultaneously.
(d) Low-frequency signals can be sent to long distances by superimposing with a high-frequency wave as well.
Therefore, the most suitable option will be :

1)All statements are true

2) (a), (b) and (c) are true only
3) (a), (c) and $(\mathrm{d})$ are true only
4) (b), (c) and (d) are true only

Solution:

We do not transmit low-frequency signals to long distances because the size of the antenna should be comparable to the signal wavelength, which is impractical for a signal of longer wavelength. Additionally, we want to avoid mixing up signals transmitted by different transmitters simultaneously. Low-frequency signals can be sent to long distances by superimposing them with a high-frequency wave. Therefore, the most suitable option is that statements (a), (c), and (d) are true only.

Example 4: A signal of frequency 20 kHz and peak voltage of 5 Volt is used to modulate a carrier wave of frequency 1.2 MHz and peak voltage 25 Volts. Choose the correct statement.

1) Modulation index=5, side frequency bands are at 1400 kHz and 1000 kHz

2)Modulation index=5, side frequency bands are at 21.2 kHz and 18.8 kHz

3)Modulation index=0.8, side frequency bands are at 1180 kHz and 1220 kHz

4)Modulation index=0.2, side frequency bands are at 1220 kHz and 1180 kHz

Solution:

Frequency of signal $=\mathbf{2 0 K H}$
Peak voltage $\mathrm{Am}=5 \mathrm{~V}$
Frequency of Carrier wavw $=1.2 \mathrm{MHz}$
Peak voltage $\mathrm{Ac}=25 \mathrm{~V}$
Modulation Index $=\mathrm{Am} / \mathrm{Ac}$
Modulate Index : $\frac{5}{25}=\frac{1}{5}=0.2$
Side frequency $(1200+20) \mathrm{Hz}=1220 k H z$ and $1200-20=1180 \mathrm{kHz}$

Hence, the correct answer is the option (2).

Example 5: To radiate EM signal of wavelength with high efficiency, the antennas should have a minimum size equal to:

1) $\lambda$
2) $\frac{\lambda}{2}$
3) $2 \lambda$
4) $\frac{\lambda}{4}$

Solution:

Minimum length of antenna
Should be $\frac{\lambda}{4}$

Hence, the correct answer is the option (4).

Summary

Modulation is the primary way how signals are transacted in communication systems. Much of the difficulty in this problem was solved by adding the information signal to a very high-frequency carrier wave that can travel much further without significant loss. The modulation function, through which the whole problem of a small bandwidth is solved and the antennas' dimensions are reduced, lessening the bandwidth requirements as well as the antenna size for the satellite team. Also, it allows for the mixing of various signals using the same channel and therefore an increase in the efficiency of the whole system. Furthermore, the modulation technology increases the signal-to-noise ratio by avoiding the distortion of a noisy channel and thus giving clearer and more reliable communication by the minimum of the noise and interference impact.