chapter07 digital comm tech

© 2008 The McGraw-Hill Companies

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Principles of ElectronicCommunication Systems

Third Edition

Louis E. Frenzel, Jr.


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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


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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.


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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.


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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.


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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)


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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


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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.


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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.


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

signal.

Digital communication circuits are usually more complex than analog circuits.


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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).


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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 long-

distance communication because of: cost. signal attenuation.


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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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Serial-Parallel Conversion 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-Parallel Conversion 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-to-

analog (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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Basic Principles of Data Conversion: A/D Conversion 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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Basic Principles of Data Conversion: A/D Conversion 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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Basic Principles of Data Conversion: A/D Conversion 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.


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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.


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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.


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Figure 7-16: Weighted current source DAC.


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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.


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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.


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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.


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Figure 7-18: An S/H amplifier


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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.


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Common ways to translate an analog voltage to a binary number include: Successive-Approximations Converters:

This converter contains an 8-bit successive-approximations 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.


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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 low-

resolution flash converters to achieve higher speed and higher resolution than successive-approximations converters but less than a full flash converter.


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ADC Specifications The key ADC specifications are

Resolution Dynamic range Signal-to-noise ratio Effective number of bits Spurious free dynamic range.


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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.


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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).


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Figure 7-26: Delta modulator


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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.


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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).


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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.


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


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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.


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There are four basic forms of pulse modulation: 1. Pulse-amplitude modulation (PAM) 2. Pulse-width modulation (PWM)3. Pulse-position modulation (PPM)4. Pulse-code modulation (PCM).


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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.


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Figure 7-30: Types of pulse modulation.


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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.


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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.


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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.


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Figure 7-31: Basic PCM system.


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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 large-scale IC chip known as a codec or vocoder.


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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.


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Figure 7-36: Concept of DSP


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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.


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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.


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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.


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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.


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


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

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