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Further

## Communication Systems : Revision Test

One mark questions

1. What is the meant by the term attenuation ?
2. Why ground wave transmission is restricted to 1500 kHz?
3. Define critical

## IMPORTANT POINTS TO REMEMBER

• Communication system is a system that transmits information from one point to another. The three basic components of a communication system are :
1. Transmitter
2. Communication channel
• Transmitter is a device that transmits the receiving end through a communication channel.It basically transforms a message signal into a suitable form for transmission to the receiver. A transmitter consists of :
1. A signal generator
2. A modulator
3. An antenna
• Message signal is a time varying electrical signal produced by a transducer,a device that converts energy in one form to another. This signal carries information. There are two types of message signals:
1. Analog, and
2. Digital
• A signal has two important characteristics-(a) amplitude ,and (b) frequency or phase.Analog signal is a continuous function of time.It may take any value in a given range. Digital signal is a discrete signal discontinuous in time.They are sampled with time.
• Bandwidth is the range over which frequencies in a signal vary
• Base band is the range of frequency representing the original signal;the one delivered by source.
• Modulation is the modification of a message signal by varying some of the characteristics of a wave to transmit information. The radio frequency carrier wave is superimposed by an audio frequency signal.
• There are three types of modulation:
•     (a)Amplitude modulation
•     (b)Frequency modulation
•     (c) Pulse modulation,

(a) Amplitude modulation: The amplitude of the carrier wave is varied in proportion to amplitude of modulating signal.

(b) Frequency modulation: The frequency of a carrier