Transcript 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 8
Digital Communication Techniques
©2003 The McGraw-Hill Companies
Communication Techniques
Since the mid-1970s, digital methods of transmitting
data have slowly but surely replaced analog.
 Radio communication has remained primarily analog
mainly because the type of information to be
conveyed (e.g. voice and video) is analog and
because of the high frequencies involved.
 Today, digital circuits are fast enough to handle the
processing of radio signals.
 Digital processing is more cost-effective and
practical.
Topics Covered in Chapter 8
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Digital Transmission of Data
Data Conversion
Parallel and Serial Transmission
Pulse-Code Modulation
Pulse Modulation
Digital Signal Processing
Digital Transmission of Data
Data refers to information to be communicated.
 Data is in digital form if it comes from a computer
 If analog (e.g. voice), it can be converted into digital
form before it is transmitted.
 Digital communication was initially limited to the
transmission of data between computers.
 Networks (e.g. Local Area Networks) have been
formed to support communication between
computers.
Digital Communication Systems
There are three primary reasons for the growth of digital
communication systems. They are:
 Increased use of computers has made it necessary to
find a way for computers to communicate and
exchange data.
 Digital transmission methods offer some major
benefits over analog communication techniques.
 The telephone system, the largest and most widely
used communication system has been converting
from analog to digital over the years.
Computer Data Communication
Some common examples of computer data
communication are as follows:
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File transfer
Electronic mail (E-mail)
Computer-peripheral links
Internet access
Local area networks (LANs)
Non-Computer Uses of Digital
Communication
Among the non-computer applications of digital
techniques is remote control, for example:
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TV remote control
Garage door opener
Carrier current controls
Radio control of models
Remote keyless entry
Benefits of Digital Communication
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Digital signals, which are usually binary, are more
immune to noise because the noise amplitude must be
much higher than the signal amplitude to make a
binary 1 look like a binary 0 or vice versa.
With digital communication, transmission errors can
usually be detected and even corrected.
Digital data communication is adaptable to time
division multiplexing schemes. Multiplexing is the
process of transmitting two or more signals
simultaneously on a single channel.
Disadvantages of Digital
Communication
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Considerable bandwidth size is required by a digital
signal.
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Digital communication circuits are usually more
complex than analog circuits.
Data Conversion
The key to digital communication is to convert data in
analog form into digital form.
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Once in digital form, the data can be processed or
stored.
Data must usually be reconverted to analog form for
final consumption by the user.
By Definition…
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Translating an analog signal into a digital signal is
called analog-to-digital (A/D) conversion, digitizing a
signal, or encoding.
The device used to perform this translation is known
as an analog-to-digital converter or ADC.
Translating a digital signal into an analog signal is
called digital-to-analog (D/A) conversion.
The circuit used to perform this is called a digital-toanalog (D/A) converter or DAC or a decoder.
Analog-to-Digital Conversion
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An analog signal is a smooth or continuous voltage or
current variation.
It could be a voice signal, a video waveform, or a
voltage representing a variation of some other
physical characteristic such as temperature.
Through A/D conversion these continuously variable
signals are changed into a series of binary numbers.
A/D conversion is a process of sampling or
measuring the analog signal at regular time intervals.
Sampling an Analog Signal
Digital-to-Analog Conversion
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In order to retain an analog signal converted to digital, some
form of binary memory must be used.
The multiple binary numbers representing each of the samples
can be stored in random access memory (RAM), on disk, or on
magnetic tape.
Once in this form, the samples can be processed and used as
data by a microcomputer which can perform mathematical and
logical manipulations.
The D/A converter receives the binary numbers sequentially
and produces a proportional analog voltage at the output.
D/A Converter
A D/A converter consists of four major sections. They
are:
 The precise reference voltage regulator, a zener
diode, receives the DC supply voltage as an input and
translates it into a highly precise reference voltage.
 The precision resistor network is connected in a
unique configuration. The voltage from the reference
is applied to this resistor network, which converts it
into a current proportional to the binary input.
D/A Converter (Continued)
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The output of the resistive network is connected to
the summing junction of the op amp. The output of
the op amp is equal to the output current of the
resistor network multiplied by the feedback resistor
value.
The resistor network is modified by a set of electronic
switches that can be either current or voltage switches
and are usually implemented with diodes or
transistors.
Components of a D/A Converter
A/D Converter
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A/D conversion begins with the process of sampling,
which is carried out by a sample-and-hold (S/H)
circuit.
The S/H circuit takes a precise measurement of the
analog voltage at specified intervals.
The A/D converter then converts this instantaneous
value of voltage and translates it to a binary number.
Sample-And-Hold Circuit
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A sample-and-hold (S/H) circuit, also called a
track/store circuit, accepts the analog input signal and
passes it through, unchanged, during its sampling
mode.
In the hold mode, the amplifier remembers or
memorizes a particular voltage level at the instant of
sampling.
The output of the S/H amplifier is a fixed DC level
whose amplitude is the value at the sampling time.
S/H Amplifier
Flash Converter
A flash converter uses a large resistive voltage divider
and multiple analog comparators. The number of
comparators is equal to 2N – 1, where N is the number
of desired output bits.
Parallel and Serial Transmission
There are two ways to move binary bits from one place
to another: transmit all bits of a word simultaneously
or send only 1 bit at a time. These methods are
referred to as parallel and serial transfer.
 In parallel data transfers, all the bits of a code word
are transferred simultaneously
 Data transfers in communication systems are made
serially; each bit of a word is transmitted one after
another.
Pulse-Code Modulation
The most widely used technique for digitizing
information signals for electronic data transmission is
pulse-code modulation (PCM). PCM signals are
serial digital data. There are two ways to generate:
 Use an S/H circuit and traditional A/D converter to
sample and convert the analog signal into a sequence
of binary words, convert the parallel binary words
into serial form, and transmit the data serially.
 Use a special method of A/D conversion that
generates a serial data signal directly.
Traditional PCM
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In traditional PCM, the analog signal is sampled and converted
into a sequence of parallel binary words. A successive
approximations A/D converter is the most common method.
The parallel binary output word is converted into a serial
signal by a shift register.
Each time a sample is taken, a 8-bit word is generated by the
A/D converter.
This word must be transmitted serially before another sample
is taken and another word is generated.
The clock and start conversion signals are synchronized so that
the resulting output signal is a continuous train of binary
words.
Delta Modulation
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Delta modulation is a special form of A/D conversion
that results is a continuous serial data signal being
transmitted.
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The delta modulator looks at a sample of the analog
input signal, compares it to a previous sample, and
then transmits a 0 or a 1 if the sample is less than or
more than the previous sample.
Delta Modulator
Delta Modulator Operation
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The analog signal is sampled by an S/H circuit.
The sample is also applied to a comparator.
The other input to the comparator comes from a D/A
converter driven by an up-down counter.
The counter counts up (increments) or down
(decrements) depending on the output state of the
comparator.
The comparator output is also the serial data signal
representing the analog value.
Sigma-Delta Converter
A variation of the delta converter is the sigma-delta (Σ
Δ) converter.
 It is also known as a delta-sigma or charge balance
converter.
 This circuit provides extreme precision, wide
dynamic range, and low noise.
 It is available with word output lengths of 18, 20, 22,
and 24 bits.
 These converters are widely used in digital audio
applications (e.g. CD and MP3 players).
By Definition…
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Companding is a process of signal compression and
expansion that is used to overcome problems of
distortion and noise in the transmission of audio
signals.
All A/D and D/A conversion and related functions
such as serial-to-parallel and parallel-to-serial
conversion as well as companding are taken care of
by a single large scale IC chip known as a codec or
vocoder.
Pulse Modulation
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Pulse modulation is the process of changing a binary
pulse signal to represent the information to be
transmitted.
The primary benefits of transmitting information by
binary techniques are the great noise tolerance and
the ability to regenerate a degraded signal.
There are three basic forms of pulse modulation:
pulse-amplitude modulation (PAM), pulse-width
modulation (PWM), and pulse-position modulation
(PPM).
Pulse-Amplitude Modulation
Sampling is the process of “looking at” an analog signal
for a brief time.
 During this short sampling interval, the amplitude of
the analog signal is allowed to be passed or stored.
 If multiple samples of the analog signal are taken at a
periodic rate, most of the information contained in the
analog signal is passed.
 The resulting signal is a series of samples or pulses
that vary in amplitude according to the variation of
the analog signal.
PAM Modulator
PAM Modulator
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An astable clock oscillator drives a one-shot multivibrator that
generates a narrow, fixed-width pulse.
This pulse is applied to a gate circuit, a switch that opens and
closes in accordance with the one-shot signal.
When the one-shot signal is OFF, the gate is closed and the
analog signal applied to it will not pass.
When the clock triggers the one shot once per cycle, the gate
opens for a short time, allowing the analog signal to pass
through.
Pulse-Width Modulation
Pulse-width modulation (PWM), also known as pulseduration modulation (PDM), is perhaps the most
widely used of the pulse-modulation techniques.
PWM Modulator
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The constant-frequency clock oscillator drives the
PWM modulator.
The other input to the modulator is the analog
information signal to be transmitted.
The modulator modifies the width or duration of the
clock pulses in accordance with the modulating
signal.
The output is a varying pulse-width signal.
Pulse-Position Modulation
A pulse-position modulation (PPM) signal is easily
derived from a PWM signal by adding an RC
differentiator and a half-wave rectifier.
PPM Modulator
Digital Signal Processing
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Digital signal processing (DSP) is the use of a fast
digital computer to perform processing on digital
signals.
Any digital computer with sufficient speed and
memory can be used for DSP.
The superfast 32-bit reduced instruction set
computing (RISC) processors are especially adept at
DSP.
Concept of DSP
Basis of DSP
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An analog signal to be processed is fed to an A/D converter,
where it is converted into a series of binary numbers which are
stored in a read-write random-access memory (RAM).
A program, usually stored in a read-only memory (ROM),
performs mathematical and other manipulations on the data.
Most digital processing involves complex mathematical
algorithms that are executed in real time.
The processing results in another set of data words which are
also stored in RAM.
They can be used in digital form or fed to a D/A converter.
DSP Applications
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The most common DSP application is filtering. A
DSP processor can perform bandpass, low-pass, highpass, and band-reject filter operation.
Data compression is a process that reduces the
number of binary words needed to represent a given
analog signal.
Spectrum analysis is the process of examining a
signal to determine its frequency content.