Communication Systems
All electronic communication systems have a transmitter, a communication channel or
medium, and a receiver. These basic components are shown in Fig. 1-2. The process of
communication begins when a human being generates some kind of message, data, or
other intelligence that must be received by others. A message may also be generated by
a computer or electronic current. In electronic communication systems, the message is
referred to as information, or an intelligence signal. This message, in the form of an
electronic signal, is fed to the transmitter, which then transmits the message over the
communication channel. The message is picked up by the receiver and relayed to another
human. Along the way, noise is added in the communication channel and in the receiver.
Noise is the general term applied to any phenomenon that degrades or interferes with
the transmitted information
Figure 1-2 A general model of all communication systems
Transmitter
The i rst step in sending a message is to convert it into electronic form suitable for
transmission. For voice messages, a microphone is used to translate the sound into an
electronic audio signal. For TV, a camera converts the light information in the scene to
a video signal. In computer systems, the message is typed on a keyboard and converted
to binary codes that can be stored in memory or transmitted serially. Transducers convert
physical characteristics (temperature, pressure, light intensity, and so on) into electrical
signals.
The transmitter itself is a collection of electronic components and circuits designed
to convert the electrical signal to a signal suitable for transmission over a given communication medium. Transmitters are made up of oscillators, amplii ers, tuned circuits
and i lters, modulators, frequency mixers, frequency synthesizers, and other circuits. The
original intelligence signal usually modulates a higher-frequency carrier sine wave
generated by the transmitter, and the combination is raised in amplitude by power amplii ers, resulting in a signal that is compatible with the selected transmission medium.
Communication Channel
The communication channel is the medium by which the electronic signal is sent from
one place to another. Many different types of media are used in communication systems,
including wire conductors, i ber-optic cable, and free space.
Electrical Conductors. In its simplest form, the medium may simply be a pair of
wires that carry a voice signal from a microphone to a headset. It may be a coaxial cable
such as that used to carry cable TV signals. Or it may be a twisted-pair cable used in a
local-area network (LAN).
Optical Media. The communication medium may also be a fiber-optic cable or “light
pipe” that carries the message on a light wave. These are widely used today to carry
long-distance calls and all Internet communications. The information is converted to digital form that can be used to turn a laser diode off and on at high speeds. Alternatively,
audio or video analog signals can be used to vary the amplitude of the light.
Free Space. When free space is the medium, the resulting system is known as radio.
Also known as wireless, radio is the broad general term applied to any form of wireless
communication from one point to another. Radio makes use of the electromagnetic spectrum. Intelligence signals are converted to electric and magnetic fields that propagate
nearly instantaneously through space over long distances. Communication by visible or
infrared light also occurs in free space.
Other Types of Media. Although the most widely used media are conducting cables
and free space (radio), other types of media are used in special communication systems.
For example, in sonar, water is used as the medium. Passive sonar “listens” for underwater sounds with sensitive hydrophones. Active sonar uses an echo-reflecting technique
similar to that used in radar for determining how far away objects under water are and
in what direction they are moving
The earth itself can be used as a communication medium, because it conducts electricity and can also carry low-frequency sound waves.
Alternating-current (ac) power lines, the electrical conductors that carry the power
to operate virtually all our electrical and electronic devices, can also be used as
communication channels. The signals to be transmitted are simply superimposed on or
added to the power line voltage. This is known as carrier current transmission or power
line communications (PLC). It is used for some types of remote control of electrical
equipment and in some LANs.
Receivers
A receiver is a collection of electronic components and circuits that accepts the
transmitted message from the channel and converts it back to a form understandable by
humans. Receivers contain amplii ers, oscillators, mixers, tuned circuits and i lters, and
a demodulator or detector that recovers the original intelligence signal from the modulated carrier. The output is the original signal, which is then read out or displayed. It
may be a voice signal sent to a speaker, a video signal that is fed to an LCD screen for
display, or binary data that is received by a computer and then printed out or displayed
on a video monitor.
Transceivers
Most electronic communication is two-way, and so both parties must have both a
transmitter and a receiver. As a result, most communication equipment incorporates
circuits that both send and receive. These units are commonly referred to as transceivers.
All the transmitter and receiver circuits are packaged within a single housing and
usually share some common circuits such as the power supply. Telephones, handheld
radios, cellular telephones, and computer modems are examples of transceivers.
Attenuation
Signal attenuation, or degradation, is inevitable no matter what the medium of transmission. Attenuation is proportional to the square of the distance between the transmitter and
receiver. Media are also frequency-selective, in that a given medium will act as a low-pass
i lter to a transmitted signal, distorting digital pulses in addition to greatly reducing
signal amplitude over long distances. Thus considerable signal amplii cation, in both the
transmitter and the receiver, is required for successful transmission. Any medium also
slows signal propagation to a speed slower than the speed of light
Noise
Noise is mentioned here because it is the bane of all electronic communications. Its effect
is experienced in the receiver part of any communications system. For that reason, we
cover noise at that more appropriate time in Chapter 9. While some noise can be i ltered
out, the general way to minimize noise is to use components that contribute less noise
and to lower their temperatures. The measure of noise is usually expressed in terms of
the signal-to-noise (S/N) ratio (SNR), which is the signal power divided by the noise
power and can be stated numerically or in terms of decibels (dB). Obviously, a very high
SNR is preferred for best performance.
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