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The document discusses various radio communication systems, including broadcast radio, mobile phones, and satellite TV, and outlines the principles of modulation and demodulation. Modulation involves impressing a message signal onto a higher-frequency carrier wave, allowing for effective transmission. The chapter also highlights the importance of modulation for practical aerial dimensions and signal transmission efficiency.

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7 views1 page

First Page PDF

The document discusses various radio communication systems, including broadcast radio, mobile phones, and satellite TV, and outlines the principles of modulation and demodulation. Modulation involves impressing a message signal onto a higher-frequency carrier wave, allowing for effective transmission. The chapter also highlights the importance of modulation for practical aerial dimensions and signal transmission efficiency.

Uploaded by

gniwespinosawing
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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9

Radio communication
techniques

9.1 Radio communication systems


Broadcast radio and TV, including satellite TV, mobile radio and mobile phones, cordless
telephones, point-to-point microwave multi-channel telephony links and space probe `com'
links are all examples of radio communication systems. They are all represented by the block
diagram of Figure 9.1. In this chapter, we look at the signal spectra, the techniques and some
of the circuits used in modulation and demodulation, together with some of the other circuits
used in transmitters and receivers.

9.1.1 Modulation and demodulation


All radio systems use some form of modulation. In the process of modulation, the audio, video
or digital signal to be transmitted is impressed upon, or modulates, a higher-frequency sine
wave called the carrier. Modulation involves changing one or more of the properties of the
carrier sine wave, that is amplitude, frequency or phase. It is this modulated carrier which is
transmitted as a radio wave, sometimes called a modulated carrier wave. The audio, video or
digital signal to be transmitted is commonly called the message signal to distinguish it from the
carrier. The modulation process effectively shifts the message signal up to a band of frequen-
cies clustered around the frequency of the carrier. The band of frequencies of the original
message signal is sometimes called the baseband, and the message signal is called the baseband
signal. Why modulate? There are several reasons for this as follows.
1. Transmitting on a higher frequency allows one to use aerials of more practical dimensions,
since aerials are most effective when of the order of a wavelength or more in size, and the
wavelength is inversely proportional to the frequency
c ˆ lf

modulated radio waves

message
signal transmitter receiver message
source signal

Fig. 9.1 Radio communication system block diagram.

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