Principles of ElectronicCommunication Systems

Second Edition

Louis Frenzel

© 2002 The McGraw-Hill Companies

Principles of Electronic Communication SystemsSecond EditionChapter 8

Digital Communication Techniques

©2003 The McGraw-Hill Companies

Communication TechniquesSince the mid-1970s, digital methods of transmitting

data have slowly but surely replaced analog.Radio communication has remained primarily analogmainly 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 8Digital Transmission of DataData ConversionParallel and Serial TransmissionPulse-Code ModulationPulse ModulationDigital Signal Processing

Digital Transmission of DataData refers to information to be communicated.

Data is in digital form if it comes from a computerIf 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 SystemsThere 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 CommunicationSome common examples of computer data

communication are as follows:

File transferElectronic mail (E-mail)Computer-peripheral linksInternet accessLocal area networks (LANs)

Non-Computer Uses of Digital CommunicationAmong the non-computer applications of digital

techniques is remote control, for example:

TV remote controlGarage door openerCarrier current controlsRadio control of modelsRemote keyless entry

Benefits of Digital CommunicationDigital 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

Considerable bandwidth size is required by a digital signal.

Digital communication circuits are usually more complex than analog circuits.

Data ConversionThe 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.

By Definition…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.

Analog-to-Digital ConversionAn 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 ConversionIn 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 ConverterA D/A converter consists of four major sections. They

are:The precise reference voltage regulator, a zenerdiode, 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)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 ConverterA/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 CircuitA 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 ConverterA 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 TransmissionThere 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 simultaneouslyData transfers in communication systems are made serially; each bit of a word is transmitted one after another.

Pulse-Code ModulationThe 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 PCMIn 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 ModulationDelta modulation is a special form of A/D conversion that results is 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.

Delta Modulator

Delta Modulator OperationThe 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 ConverterA variation of the delta converter is the sigma-delta (ΣΔ) 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…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 ModulationPulse 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 ModulationSampling 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 ModulatorAn 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 andcloses 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 ModulationPulse-width modulation (PWM), also known as pulse-

duration modulation (PDM), is perhaps the most widely used of the pulse-modulation techniques.

PWM ModulatorThe 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 ModulationA 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 ProcessingDigital 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 DSPAn 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 ApplicationsThe 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.