Download Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 10
Multiplexing and Demultiplexing
©2003 The McGraw-Hill Companies
Multiplexing and Demultiplexing
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Transmitting two or more signals simultaneously can
be accomplished by running multiple cables or setting
up one transmitter-receiver pair for each channel, but
this is an expensive approach.
A single cable or radio link can handle multiple
signals simultaneously using a technique known as
multiplexing.
Multiplexing permits hundreds or even thousands of
signals to be combined and transmitted over a single
medium.
Topics Covered in Chapter 10
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Multiplexing Principles
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Frequency Division Multiplexing
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Time Division Multiplexing
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Pulse-Code Modulation
Multiplexing Principles
Multiplexing is the process of simultaneously
transmitting two or more individual signals over a
single communication channel.
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It increases the number of communication channels
so more information can be transmitted.
An application may require multiple signals.
Cost savings can be gained by using a single channel
to send multiple information signals.
Multiplexing Applications
Three communication applications that would be
prohibitively expensive or impossible without
multiplexing are:
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Telephone systems
Telemetry
Broadcasting (Radio and TV)
Concept of Multiplexing
Frequency Division Multiplexing
In frequency division multiplexing (FDM) multiple
signals share the bandwidth of a common
communication channel.
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All channels have specific bandwidths
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A wide bandwidth can be shared for the purpose of
transmitting many signals at the same time.
Transmitter-Multiplexers
In an FDM system each signal to be transmitted feeds a
modulator circuit.
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The carrier for each modulator is on a different
frequency.
The carriers are equally spaced from one another.
These carriers are referred to as subcarriers.
Each input signal is given a portion of the bandwidth.
Transmitter-Multiplexers
(Continued)
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The modulator outputs containing the sideband
information are added algebraically in a linear mixer.
The resulting output signal is a composite of all the
modulated subcarriers.
This signal can be used to modulate a radio
transmitter, or can itself be transmitted over a single
channel
The composite signal can alternatively become one
input to another multiplexed system.
Receiver-Demultiplexer
In an FDM system a receiver picks up the signal and
demodulates it, recovering the composite signal.
 The composite signal is sent to a group of bandpass
filters, each centered on one of the carrier
frequencies.
 Each filter passes only its channel and rejects all
others.
 A channel demodulator then recovers each original
input signal.
Telemetry
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Sensors in telemetry systems generate electrical
signals which change in some way in response to
changes in physical characteristics.
An example of a sensor is a thermistor, a device used
to measure temperature.
A thermistor’s resistance varies inversely with
temperature.
Telemetry (Continued)
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The thermistor is usually connected into a resistive
network, such as a voltage divider or bridge.
The thermistor is also connected to a DC voltage
source.
The result is a DC output voltage which varies in
accordance with temperature.
This voltage is transmitted to a remote receiver for
measurement, readout, and recording.
The thermistor becomes one channel of an FDM
system.
By Definition…
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The varying direct or alternating current changes the
frequency of an oscillator operating at the carrier
frequency. Such a circuit is called a voltagecontrolled oscillator (VCO) or subcarrier oscillator
(SCO).
Most VCOs are astable multivibrators whose
frequency is controlled by the input from the signal
conditioning circuits.
A system that uses FM of the VCO subcarriers as
well as FM of the final carrier is called FM/FM.
FM/FM Telemetry Receiver
Telephone System
For decades, telephone companies used FDM to send multiple
telephone conversations over a minimum number of cables.
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The original voice signal, in the 300- to 3000-Hz range is used
to modulate a subcarrier.
Lower sideband (LSB) SSB AM was used.
Each subcarrier is on a different frequency, and those
subcarriers are then added together to form a single channel.
The FDM system has been replaced by an all-digital time
multiplexing (TDM) system.
Cable TV
In a cable TV system, TV signals, each in its own 6MHz channel, are multiplexed on a common coaxial
or fiber-optic cable and sent to homes.
 Each 6-MHz channel carries the video and voice of
the TV signal.
 Coaxial and fiber-optic cables have an enormous
bandwidth and can carry more than one hundred TV
channels.
 Many cable TV companies also use their cable system
for Internet access.
FM Stereo Broadcasting
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In recording original stereo, two microphones are
used to generate two separate audio signals.
Two microphones pick up sound from a common
source, such as voice, but from different directions.
The separation of the two microphones provides
sufficient differences in the two audio signals to
provide a realistic reproduction of the original sound.
FDM techniques are used to transmit these
independent signals by a single transmitter.
Time Division Multiplexing
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In FDM, multiple signals are transmitted over a
single channel, each signal being allocated a portion
of the spectrum within that bandwidth.
In time division multiplexing (TDM), each signal can
occupy the entire bandwidth of the channel.
Each signal, however, is transmitted for only a brief
period of time.
TDM can be used with both digital and analog
signals.
TDM Concept
PAM Multiplexer
The simplest time multiplexer operates like a singlepole multiple-position mechanical or electronic
switch that sequentially samples the multiple analog
inputs at a high rate of speed.
 The switch arm dwells momentarily on each contact.
 This allows the input signal to be passed to the
output.
 It then switches quickly to the next channel, allowing
that channel to pass for a fixed duration.
 The remaining channels are sampled in the same way.
Simple Rotary-Switch Multiplexer
Four Channel PAM Sampling
Four different analog signals can be sampled by a PAM
multiplexer. Signals such as:
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Signals A and C are continuously varying analog
signals.
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Signal B is a positive-going linear ramp.
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Signal D is a constant DC voltage.
Four-Channel PAM Time Division
Multiplexer
Electronic Multiplexer
In practical TDM/PAM systems, electronic circuits are
used instead of mechanical switches or commutators.
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The multiplexer itself is usually implemented with
FETs.
FETs are nearly ideal ON-OFF switches and can turn
off and on at very high speeds.
Time Division Multiplexer Used to
Produce PAM
PAM Demultiplexer
Once the composite signal is received, it must be
demodulated and demultiplexed.
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The signal is picked up by the receiver.
The signal is sent to an FM demodulator that recovers
the original PAM data.
The PAM signal is then demultiplexed into the
original analog signals.
Demultiplexing PAM Signals
Demultiplexer Circuit
Once the composite PAM signal is recovered, it is applied to a
demultiplexer (DEMUX).
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The demultiplexer is the reverse of a multiplexer.
It has a single input and multiple outputs.
Most demultiplexers use FETs driven by a counter-decoder.
Individual PAM signals are sent to op amps, where they are
buffered and amplified.
They are then sent to low-pass filters, where they are
smoothed into the original analog signals.
The main problem encountered in demultiplexing is
synchronization.
By Definition…
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Clock recovery circuits used to generate the
demultiplexer clock pulses are used to remedy the
synchronization problem.
After clock pulses of the proper frequency have been
obtained, it is necessary to synchronize the
multiplexed channels.
This synchronization is achieved by using a special
synchronizing (sync) pulse applied to one of the input
channels at the transmitter.
Pulse-Code Modulation
The most popular form of TDM uses pulse-code
modulation (PCM).
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With pulse-code modulation, multiple channels of
digital data are transmitted in serial form.
Each channel is assigned a time slot in which to
transmit one binary word of data.
The data streams from the various channels are
interleaved and transmitted sequentially.
PCM Multiplexer
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When PCM is used to transmit analog signals, the
signals are sampled with a multiplexer.
The signals are then converted by an A/D converter
into a series of binary numbers.
Each number is proportional to the amplitude of the
analog signal at various sampling points.
These binary words are converted from parallel to
serial format and then transmitted.
PCM System
PCM Demultiplexer
At the receiving end of the communication link, the PCM signal
is demultiplexed and converted back into the original data.
 The PCM baseband signal may come over a cable.
 If the PCM signal has modulated a carrier and is being
transmitted by radio, the RF signal will be picked up by a
receiver and then demodulated.
 The original serial PCM binary waveform is recovered and fed
to a shaping circuit to clean up and rejuvenate the binary
pulses.
 The original signal is then demultiplexed by means of a digital
demultiplexer using AND or NAND gates.
PCM Receiver-Demultiplexer
Benefits of PCM
PCM is reliable, inexpensive, and highly resistant to
noise.
 In PCM, the transmitted binary pulses all have the
same amplitude and can be clipped to reduce noise.
 Even when signals have been degraded because of
noise, attenuation, or distortion, all the receiver has to
do is determine whether a pulse was transmitted or
not.
 PCM signals are easily recovered and rejuvenated.
Digital Carrier Systems
The most widespread use of TDM is in the telephone
system.
 Years ago, the telephone companies developed a
complete digital transmission system called the Tcarrier system.
 The T-carrier system defines the range of PCM TDM
systems with progressively faster data rates.
 The physical implementations of these systems are
referred to as T-1, T-2, T-3, and T-4.
T-Carrier Systems
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T-1 systems transmit each voice signal at a 64-kbp/s
rate. They are also used to transmit fewer than 24
inputs at a faster rate.
T-2 systems are not widely used except as a
steppingstone to form DS3 signals.
T-1 and T-3 lines are widely used by business and
industry for telephone service as well as for digital
data transmission.
T-2 and T-4 lines are rarely used by subscribers, but
they are used within the telephone system itself.