Principles of ElectronicCommunication Systems Third Edition Louis Frenzel
Chapter 7 Digital Communication Techniques
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
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.
7-1: 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 or LANs) are formed to support communication between computers.
7-1: Digital Transmission of Data • There 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.
7-1: Digital Transmission of Data 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)
7-1: Digital Transmission of Data 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
7-1: Digital Transmission of Data 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.
7-1: Digital Transmission of Data 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.
7-1: Digital Transmission of Data Disadvantages of Digital Communication • Considerable bandwidth size is required by a digital signal. • Digital communication circuits are usually more complex than analog circuits.
7-2: Parallel and Serial Transmission • There 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).
7-2: Parallel and Serial Transmission 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.
7-2: Parallel and Serial Transmission Figure 7-2: Parallel data transmission.
7-2: Parallel and Serial Transmission 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.
7-2: Parallel and Serial Transmission Figure 7-3: Serial data transmission.
7-2: Parallel and Serial Transmission 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.
7-2: Parallel and Serial Transmission Serial-Parallel Conversion • The flip-flops in a shift registercan 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.
7-2: Parallel and Serial Transmission 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).
7-2: Parallel and Serial Transmission Figure 7-4: Parallel-to-serial and serial-to-parallel data transfers with shift registers.
7-2: Parallel and Serial Transmission 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.
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.
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-7: Sampling an analog signal
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-8: The A/D converter divides the input voltage range into discrete voltage increments.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-9: A D/A converter produces a stepped approximation of the original signal.
7-3: Data Conversion 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.
7-3: Data Conversion Basic Principles of Data Conversion: Aliasing • To eliminate this problem, a low-pass filter called an antialiasing filteris 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.
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-13: Major components of a D/A converter.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-15: A string DAC.
7-3: Data Conversion 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.
7-3: Data Conversion 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 = ItXRf.
7-3: Data Conversion Figure 7-16: Weighted current source DAC.
7-3: Data Conversion D/A Converter Specifications • Three important specifications are associated with D/A converters: • Resolutionis 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 timeis 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.
7-3: Data Conversion 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.
7-3: Data Conversion 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.
7-3: Data Conversion Figure 7-18: An S/H amplifier
7-3: Data Conversion 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.
7-3: Data Conversion • 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.
7-3: Data Conversion • 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.
7-3: Data Conversion • 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.