Principles of Electronic Communication Systems

Principles of ElectronicCommunication SystemsThird Edition

Louis E. Frenzel, Jr.

2008 The McGraw-Hill Companies#

2008 The McGraw-Hill Companies#Chapter 7Digital Communication Techniques

2008 The McGraw-Hill Companies#

2008 The McGraw-Hill Companies#Topics Covered in Chapter 77-1: Digital Transmission of Data7-2: Parallel and Serial Transmission7-3: Data Conversion7-4: Pulse Modulation7-5: Digital Signal Processing

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataSince 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.

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataData 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.

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataThere are three primary reasons for the growth of digital communication systems: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.

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataProliferation of ComputersSome common examples of computer data communication include:File transferElectronic mail (e-mail)Computer-peripheral linksInternet accessLocal area networks (LANs)

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataNoncomputer Uses of Digital CommunicationAmong the non-computer applications of digital techniques:TV remote controlGarage door openerCarrier current controlsRadio control of modelsRemote keyless entry

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataBenefits of Digital CommunicationNoise 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.

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataBenefits of Digital CommunicationDigital 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.

2008 The McGraw-Hill Companies#7-1: Digital Transmission of DataDisadvantages of Digital Communication Considerable bandwidth size is required by a digital signal.

Digital communication circuits are usually more complex than analog circuits.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionThere are two ways to move binary bits from one place to another: Transmit all bits of a word simultaneously (parallel transfer). Send only 1 bit at a time (serial transfer).

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionParallel TransferParallel 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.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionFigure 7-2: Parallel data transmission.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionSerial TransferData 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.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionFigure 7-3: Serial data transmission.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionSerial-Parallel ConversionBecause 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.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionSerial-Parallel ConversionThe 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.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionSerial-Parallel ConversionSerial 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).

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionFigure 7-4: Parallel-to-serial and serial-to-parallel data transfers with shift registers.

2008 The McGraw-Hill Companies#7-2: Parallel and Serial TransmissionDelta ModulationDelta 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.

2008 The McGraw-Hill Companies#7-3: 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data ConversionTranslating 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: A/D ConversionAn 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: A/D ConversionTo 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-7: Sampling an analog signal

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: A/D ConversionThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: A/D ConversionThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-8: The A/D converter divides the input voltage range into discrete voltage increments.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: D/A ConversionTo 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-9: A D/A converter produces a stepped approximation of the original signal.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: AliasingIf 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionBasic Principles of Data Conversion: AliasingTo 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A ConvertersThere are many ways to convert digital codes to proportional analog voltages. The most popular methods areR-2Rstringweighted current source converters.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/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.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A ConvertersOutput 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-13: Major components of a D/A converter.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A Converters: String DACThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-15: A string DAC.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A Converters: Weighted Current Source DACA 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A Converters: Weighted Current Source DACThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-16: Weighted current source DAC.

2008 The McGraw-Hill Companies#7-3: Data ConversionD/A Converter SpecificationsThree 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionA/D ConvertersA/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.

2008 The McGraw-Hill Companies#7-3: Data ConversionA/D Converters: S/H CircuitsA 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-18: An S/H amplifier

2008 The McGraw-Hill Companies#7-3: Data ConversionA/D Converters: S/H CircuitsThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionCommon 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFlash 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionPipelined 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionADC SpecificationsThe key ADC specifications areResolutionDynamic rangeSignal-to-noise ratioEffective number of bitsSpurious free dynamic range.

2008 The McGraw-Hill Companies#7-3: Data ConversionADC SpecificationsResolution 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionADC SpecificationsSpurious 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).

2008 The McGraw-Hill Companies#7-3: Data ConversionFigure 7-26: Delta modulator

2008 The McGraw-Hill Companies#7-3: Data ConversionThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionSigma-Delta ConverterA variation of the delta converter is the sigma-delta ( ) 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).

2008 The McGraw-Hill Companies#7-3: Data ConversionSigma-Delta ConverterThe 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.

2008 The McGraw-Hill Companies#7-3: Data ConversionFig. 7-29: A sigma-delta () converter.

2008 The McGraw-Hill Companies#7-4: 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 areNoise toleranceAbility to regenerate a degraded signal.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationThere are four basic forms of pulse modulation: Pulse-amplitude modulation (PAM) Pulse-width modulation (PWM)Pulse-position modulation (PPM)Pulse-code modulation (PCM).

2008 The McGraw-Hill Companies#7-4: Pulse ModulationComparing Pulse-Modulation MethodsThe 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.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationFigure 7-30: Types of pulse modulation.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationComparing Pulse-Modulation MethodsThe 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.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationPulse-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 delta modulator.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationPulse-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.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationFigure 7-31: Basic PCM system.

2008 The McGraw-Hill Companies#7-4: Pulse ModulationPulse-Code Modulation: Companding and Codecs and VocodersCompanding 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingThe Basis of DSPDigital 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingFigure 7-36: Concept of DSP

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingBasis 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingDSP ProcessorsMost 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingDSP ProcessorsDSP 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingDSP 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.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingFigure 7-38: A block diagram showing the processing algorithm of a nonrecursive FIR filter.

2008 The McGraw-Hill Companies#7-5: Digital Signal ProcessingFigure 7-39: The fast Fourier transform decimation in time.

2008 The McGraw-Hill Companies#