14.1 INTRODUCTION

Message or information in any and every form such as words, codes, symbols, sound or picture needs to be propagated from a source to a destination. This communication by all general means can be defined as the process of establishing a link of information exchange between two points: the transmitter (source) and the receiver (destination). Ever since the term communication received its technical significance, typical examples of modern communication systems like line telephony, line telegraphy, ratio-broadcasting, point-to-point communication, radar communication, satellite communication, optical fibre communication and many more have come into existence. Modern communication uses different frequency bands of the entire electromagnetic spectrum from 30Hz to 300GHz, as shown in Table 14.1. These bands are popularly known as Extremely Low Frequency (ELF), Very Low Frequency(VLF), Low Frequency (LF),Medium Frequency (MF), Very High Frequency (VHF), Ultra High Frequency (UHF) and Super High frequency (SHF), indicated sequentially in Table 14.1, along with applications of various bands.

Apart from the radio frequency bands extending up to 300GHz, the electromagnetic spectrum comprise of millimeter wave bands too with wavelengths varying from 10-3–10-4m, followed by Infrared signals (∼ 0.7μm–100μm) used for astronomy for detection of heavenly bodies, weapon guidance and TV remote control. In between the microwaves (3GHz– 300GHz) and infrared waves (3THz–380THz) lies the Terahertz radiation (300GHz–3THz) having wavelengths from 1mm to 100μm. Thus, the entire electromagnetic spectrum can be classified as shown in Figure 14.1.

14.2 ELEMENTS OF COMMUNICATION SYSTEM

Any communication system designed to operate over a particular band of frequency, as shown in Figure 14.1, for a specialized purpose either military or civilian or commercial, needs a certain set of electronic stages assembled together, as shown in Figure 14.2. These stages collectively form a communication link between the transmitter end and the receiver end.

The information source provides the desired message which may or may not be electrical in nature. The input transducer converts the given message into electrical signal; e.g., for radio broadcasting a microphone converts incoming sound waves into electrical signals, making it suitable for transmission.