Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

• We modulate a signal for transmission because baseband transmission has many limitations which can be overcome using modulation. It is explained below .
• In the process of modulation, the baseband signal is translated i.e., shifted from low frequency to high frequency.This frequency shift is proportional to the frequency of carrier.

We modulate the signal for transmission because of the following advantages:

1. Reduction in the height of antenna:

• For the transmission of radio signals, the antenna height must be multiple of λ/4 ,where λ is the wavelength . λ = c /f

where c: is the velocity of light and f: is the frequency of the signal to be transmitted

• The minimum antenna height required to transmit a baseband signal of f = 10 kHz is calculated as follows :

• The antenna of this height is practically impossible to install .
• Now, let us consider a modulated signal at f = 1 MHz . The minimum antenna height is given by,

• This antenna can be easily installed practically . Thus, modulation reduces the height of the antenna .

2. Avoids mixing of signals:

• If the baseband sound signals are transmitted without using the modulation by more than one transmitter, then all the signals will be in the same frequency range i.e. 0 to 20 kHz . Therefore, all the signals get mixed together and a receiver can not separate them from each other .
• Hence, if each baseband sound signal is used to modulate a different carrier then they will occupy different slots in the frequency domain (different channels). Thus, modulation avoids mixing of signals .

3.Increase the Range of Communication:

• The frequency of baseband signal is low, and the low frequency signals can not travel long distance when they are transmitted . They get heavily attenuated .
• The attenuation reduces with increase in frequency of the transmitted signal, and they travel longer distance .
• The modulation process increases the frequency of the signal to be transmitted . Therefore, it increases the range of communication.

4.Multiplexing is possible:

• Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously .
• This is possible only with modulation.
• The multiplexing allows the same channel to be used by many signals . Hence, many TV channels can use the same frequency range, without getting mixed with each other or different frequency signals can be transmitted at the same time .

Improves Quality of Reception:

• With frequency modulation (FM) and the digital communication techniques such as PCM, the effect of noise is reduced to a great extent . This improves quality of reception .