Chapter07 digital comm tech

1

Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.

© 2008 The McGraw-Hill Companies

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Chapter 7

Digital Communication Techniques


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Topics Covered in Chapter 7
 7-1: Digital Transmission of Data
 7-2: Parallel and Serial Transmission
 7-3: Data Conversion
 7-4: Pulse Modulation
 7-5: Digital Signal Processing


4

7-1: Digital Transmission of Data
 Since the mid-1970s, digital methods of transmitting

data have slowly replaced analog.
 Radio communication has remained primarily analog
because the type of information to be conveyed 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.


5

 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 or LANs) are
formed to support communication between computers.


6



There are three primary reasons for the growth of
digital communication systems:
1. Increased use of computers has made it necessary

to find a way for computers to communicate and
exchange data.
2. Digital transmission methods offer some major
benefits over analog communication techniques.
3. The telephone system, the largest and most widely
used communication system, has been converting
from analog to digital over the years.


7

Proliferation of Computers
 Some common examples of computer data

communication include:
 File transfer
 Electronic mail (e-mail)
 Computer-peripheral links
 Internet access
 Local area networks (LANs)


8

Noncomputer Uses of Digital Communication
 Among the non-computer applications of digital

techniques:
 TV remote control
 Garage door opener
 Carrier current controls
 Radio control of models
 Remote keyless entry


9

Benefits of Digital Communication
 Noise Immunity: Digital signals, which are usually

binary, are more immune to noise than analog signals.
 Error Detection and Correction: With digital
communication, transmission errors can usually be
detected and corrected.
 Compatibility with Time-Division Multiplexing: 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.


10

Benefits of Digital Communication
 Digital ICs: Digital ICs are smaller and easier to make

than linear ICs, so therefore can be more complex and
provide greater processing capability.
 Digital Signal Processing (DSP): DSP is the processing
of analog signals by digital methods. This involves
converting an analog signal to digital and then
processing with a fast digital computer. Processing
means filtering, equalization, phase shifting, mixing, and
other traditionally analog methods.


11

Disadvantages of Digital Communication
 Considerable bandwidth size is required by a digital

signal.
 Digital communication circuits are usually more complex

than analog circuits.


12

7-2: Parallel and Serial Transmission


There are two ways to move binary bits from one
place to another:
1. Transmit all bits of a word simultaneously (parallel

transfer).
2. Send only 1 bit at a time (serial transfer).


13

Parallel Transfer
 Parallel data transmission is extremely fast because all

the bits of the data word are transferred simultaneously.
 Parallel data transmission is impractical for longdistance communication because of:
 cost.
 signal attenuation.


14


Figure 7-2: Parallel data transmission.

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Serial Transfer
 Data transfers in communication systems are made

serially; each bit of a word is transmitted one after
another.
 The least significant bit (LSB) is transmitted first, and
the most significant bit (MSB) last.
 Each bit is transmitted for a fixed interval of time t.


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Figure 7-3: Serial data transmission.

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Serial-Parallel Conversion
 Because both parallel and serial transmission occur in

computers and other equipment, there must be
techniques for converting between parallel and serial
and vice versa.
 Such data conversions are usually taken care of by
shift registers, sequential logic circuits made up of a
number of flip-flops connected in cascade.


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 The flip-flops in a shift register can store a multibit

binary word, usually loaded in parallel into the
transmitting register.
 When a clock pulse (CP) is applied to the flip-flops, the
bits of the word are shifted from one flip-flop to another
in sequence.
 The last (right-hand) flip-flop in the transmitting register
stores each bit in sequence as it is shifted out.
 The serial data word is transmitted over the
communication link and is received by another shift
register.

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 Serial data can typically be transmitted faster over

longer distances than parallel data.
 Serial buses are now replacing parallel buses in
computers, storage systems, and telecommunication
equipment where very high speeds are required.
 Serial-to-parallel and parallel-to-serial data conversion
circuits are also referred to as serializer-deserializers
(serdes).


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Figure 7-4: Parallel-to-serial and serial-to-parallel data transfers with shift registers.

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Delta Modulation
 Delta modulation is a special form of A/D conversion

that results in a continuous serial data signal being
transmitted.
 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.


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7-3: Data Conversion
 The key to digital communication is to convert data in

analog form into digital form.
 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.


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Basic Principles of Data Conversion
 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.


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Basic Principles of Data Conversion: A/D Conversion
 An analog signal is a smooth or continuous voltage or

current variation.
 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.


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 To retain the high-frequency information in the analog

signal, a sufficient number of samples must be taken to
adequately represent the waveform.
 The minimum sampling frequency is twice the highest
analog frequency content of the signal.
 This minimum sampling frequency is known as the
Nyquist frequency.
 In practice the sampling rate is much higher (typically
2.5 to 3 times more) than the Nyquist minimum.


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Figure 7-7: Sampling an analog signal

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 The analog signal represents an infinite number of

actual voltage values.
 The A/D converter can represent only a finite number of
voltage values over a specific range.


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 The samples are converted to a binary number whose

value is close to the actual sample value.
 An A/D converter divides a voltage range into discrete
increments, each of which is represented by a binary
number.
 The analog voltage measured during the sampling
process is assigned to the increment of voltage closest
to it.
 Errors associated with this process are known as
quantizing errors.

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Figure 7-8: The A/D converter divides the input voltage range into discrete voltage
increments.

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Basic Principles of Data Conversion: D/A Conversion
 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.
 The samples can then 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.

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Figure 7-9: A D/A converter produces a stepped approximation of the original signal.

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Basic Principles of Data Conversion: Aliasing
 If the sampling frequency is not high enough, aliasing

occurs.
 Aliasing causes a new signal near the original to be
created.
 This signal has a frequency of fs− fm.
 When the sampled signal is converted back to analog
by a D/A converter, the output will be the alias, not the
original signal.


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Basic Principles of Data Conversion: Aliasing
 To eliminate this problem, a low-pass filter called an

antialiasing filter is usually placed between the
modulating signal source and the A/D converter input.
 The antialiasing filter ensures that no signal with a
frequency greater than one-half the sampling frequency
is passed.
 This filter must have extremely good selectivity.


34

D/A Converters
 There are many ways to convert digital codes to

proportional analog voltages.
 The most popular methods are
 R-2R
 string
 weighted current source converters.


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D/A Converters
 An R-2R converter consists of four major sections:
 Reference Regulator: The reference voltage regulator, a
zener diode, receives the DC supply voltage as an input
and translates it into a highly precise reference voltage.
 Resistor Networks: The voltage from the reference is
applied to this resistor network, which converts it into a
current proportional to the binary input.


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D/A Converters
 Output Amplifiers: 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.
 Electronic Switches: The resistor network is modified by a
set of electronic switches that can be either current or
voltage switches. They are usually implemented with
diodes or transistors.


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Figure 7-13: Major components of a D/A converter.

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D/A Converters: String DAC
 The string DAC is made up of a series string of equal-

value resistors forming a voltage divider.
 This voltage divider divides the input reference voltage
into equal steps of voltage proportional to the binary
input.
 The output voltage is determined by a set of
enhancement mode MOSFET switches controlled by a
standard binary decoder.


39


Figure 7-15: A string DAC.

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D/A Converters: Weighted Current Source DAC
 A popular configuration for very high-speed DACs is the

weighted current source DAC.
 The current sources supply a fixed current that is determined
by the external reference voltage.
 Each current source supplies a binary weighted value of I,
I/2, I/4, I/8, etc.
 The current sources are made up of some combination of
resistors, MOSFETs, or in some cases bipolar transistors.


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D/A Converters: Weighted Current Source DAC
 The switches are usually fast enhancement mode

MOSFETs, but bipolar transistors are used in some models.
 The parallel binary input is usually stored in an input register,
and the register outputs turn the switches off and on as
dictated by the binary value.
 The current source outputs are added at the summing
junction of an op amp.
 The output voltage Vo = It X Rf.


42


Figure 7-16: Weighted current source DAC.

43

D/A Converter Specifications
 Three important specifications are associated with D/A

converters:
 Resolution is the smallest increment of voltage that the

D/A converter produces over its output voltage range.
 Error is expressed as a percentage of the maximum, or
full-scale, output voltage, which is the reference voltage
value.
 Settling time is the amount of time it takes for the output
voltage of a D/A converter to stabilize to within a specific
voltage range after a change in binary input.

44

A/D Converters
 A/D conversion begins with 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.


45

A/D Converters: S/H Circuits
 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.


46


Figure 7-18: An S/H amplifier

47

A/D Converters: S/H Circuits
 The primary benefit of an S/H amplifier is that it stores

the analog voltage during the sampling interval.
 In some high-frequency signals, the analog voltage may
change during the sampling interval.
 This is undesirable because it introduces aperture
error.
 The S/H amplifier stores the voltage on the capacitor.
With the voltage constant during the sampling interval,
quantizing is accurate.


48

 Common ways to translate an analog voltage to a

binary number include:
 Successive-Approximations Converters:
 This converter contains an 8-bit successiveapproximations register (SAR).
 Special logic in the register causes each bit to be turned
on one at a time from MSB to LSB until the closest binary
value is stored in the register.
 The clock input signal sets the rate of turning the bits off
and on.
 Successive-approximations converters are fast and
consistent.

49

 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.
 The flash converter produces an output as fast as the
comparators can switch and the signals can be translated
to binary levels by the logic circuits.
 Flash converters are the fastest type of A/D converter.
 Flash A/D converters are complicated and expensive but
are the best choice for high-speed conversions.

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 Pipelined Converters:
 A pipelined converter is one that uses two or more lowresolution flash converters to achieve higher speed and
higher resolution than successive-approximations
converters but less than a full flash converter.


51

ADC Specifications
 The key ADC specifications are
 Resolution

 Dynamic range
 Signal-to-noise ratio
 Effective number of bits

 Spurious free dynamic range.


52

ADC Specifications
 Resolution is related to the number of bits. Resolution

indicates the smallest input voltage recognized by the
converter. It is the reference voltage VREF divided by 2N,
where N is the number of output bits.
 Dynamic range is a measure of the range of input
voltages that can be converted.
 The signal-to-noise (S/N) ratio (SNR) is the ratio of
the actual input signal voltage to the total noise in the
system.


53

ADC Specifications
 Spurious free dynamic range (SFDR) is the ratio of

the rms signal voltage to the voltage value of the
highest “spur” expressed in decibels.
 A spur is any spurious or unwanted signal that may
result from intermodulation distortion.
 Noise, harmonics, or spurious signals all add together
and reduce the resolution of an ADC. This effect is
expressed by a measure called the effective number
of bits (ENOB).


54


Figure 7-26: Delta modulator

55

 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.


56

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).


57

Sigma-Delta Converter
 The converter is what is known as an oversampling

converter.
 It uses a clock or sampling frequency that is many times
the minimum Nyquist rate required for other types of
converters.
 The oversampling techniques used in the sigma-delta
converter translate the noise to a higher frequency that
can be easily filtered out by a low-pass filter.
 This technique also eliminates the problem of aliasing.


58


Fig. 7-29: A sigma-delta (ΣΔ) converter.

59

7-4: Pulse Modulation
 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
 Noise tolerance
 Ability to regenerate a degraded signal.


60



There are four basic forms of pulse modulation:
1.
2.
3.
4.

Pulse-amplitude modulation (PAM)
Pulse-width modulation (PWM)
Pulse-position modulation (PPM)
Pulse-code modulation (PCM).


61

Comparing Pulse-Modulation Methods
 The following slide shows an analog modulating signal

and the various waveforms produced by PAM, PWM,
and PPM modulators.
 In all three cases, the analog signal is sampled, as it
would be in A/D conversion.


62


Figure 7-30: Types of pulse modulation.

63

Comparing Pulse-Modulation Methods
 The PAM signal is a series of constant-width pulses

whose amplitudes vary in accordance with the analog
signal.
 The PWM signal is binary in amplitude (has only two
levels). The information signal varies the width or time
duration of the pulse.
 In PPM, the pulses change position according to the
amplitude of the analog signal.
 Of the four types of pulse modulation, PAM is the
simplest and least expensive to implement.

64

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:
1. 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.
2. Use a delta modulator.

65

Pulse-Code Modulation: Traditional PCM
 In traditional PCM, the analog signal is sampled and






converted into a sequence of parallel binary words by
an A/D converter.
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.

66


Figure 7-31: Basic PCM system.

67

Pulse-Code Modulation: Companding and Codecs and
Vocoders
 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.
 Companding is the most common means of overcoming
the problems of quantizing error and noise.
 All A/D and D/A conversion and related functions, as
well as companding, are taken care of by a single largescale IC chip known as a codec or vocoder.


68

7-5: Digital Signal Processing
The Basis of DSP
 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.


69


Figure 7-36: Concept of DSP

70

Basis of DSP
 An analog signal to be processed is fed to an A/D







converter, where it is converted into a series of binary
numbers and 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.

71

DSP Processors
 Most computers and microprocessors use an

organization known as the Von Neumann
architecture.
 Physicist John Von Neumann created the stored
program concept that is the basis of operation of all
digital computers.
 The key feature of the Von Neumann arrangement is
that both instructions and data are stored in a common
memory space.
 There is only one path between the memory and the
CPU, and therefore only one data or instruction word
can be accessed at a time.

72

DSP Processors
 DSP microprocessors work in a similar way, but they

use a variation called the Harvard architecture.
 In a Harvard architecture microprocessor, there are two
memories, a program or instruction memory, usually a
ROM, and a data memory, which is a RAM.
 There are two data paths into and out of the CPU
between the memories.
 Because both instructions and data can be accessed
simultaneously, very high-speed operation is possible.


73

DSP Applications
 The most common DSP application is filtering. A DSP

processor can perform bandpass, low-pass, high-pass,
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.
 Signal averaging is the process of sampling a
recurring analog signal transmitted in the presence of
noise.

74


Figure 7-38: A block diagram showing the processing algorithm of a nonrecursive FIR
filter.

75


Figure 7-39: The fast Fourier transform decimation in time.

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