As already mentioned, 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. No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.
Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors.
Size of the antenna or aerial
For transmitting a signal, we need an antenna π‘ or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least δΊΊ/4 in dimension) so that the antenna π‘ properly senses the time variation of the signal. For an electromagnetic wave of frequency 20 kHz, the wavelength δΊΊ is 15 I'm.
Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such baseband signals is not practical. We can obtain transmission with reasonable antenna π‘ lengths if transmission frequency is high (for example, if v is 1MHz, then ⋌ is 300 m).
Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.
Important Notes:-
- Electronic communication refers to the faithful transfer of information or massage (available in the from of electrical voltage and current) from one point to another point.
- Transmitter, transmission channel and receiver π are three basic units of a communication system.
- Two important form of communication system are: Analog and Digital. The information to be transmitted is generally in continuous waveform for the former while for the latter it has only it has only discrete or quantised levels.
- Every message signal occupies a range of frequencies. The bandwidth of a message signal refers to the band of frequencies, which are necessary for satisfactory transmission of the information contained in the signal. Similarly, any practical communication system permits transmission of a range of frequencies only, which is referred to as the bandwidth of known as modulation.
