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Tín hiệu trong các mạng viễn thông P3
The vast majority of the customers of telecommunication networks are subscribers who are attached to their local exchanges by analog subscriber lines. The signaling between subscriber and local exchange is known as subscriber application of subscriber signaling [l-4]. The original, and still predominant, signaling is plain old telephony service (POTS) calling. However, subscriber signaling today also supports supplementary services such as call waiting, call forwarding, caller identification, etc....
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Tín hiệu trong các mạng viễn thông P4
Early CAS systems were developed independently by individual equipment manufacturers, and exist in many varieties. Later CAS systems, notably those developed after the Second World War, show the increasing influence of national and international standards. This section describes three important CAS systems, and their use on frequency-division multiplexed (FDM) analog trunks and time-division multiplexed (TDM) digital trunks (1.4.5 and 1.5.2).
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Tín hiệu trong các mạng viễn thông P5
COMMON-CHANNEL SIGNALING In channel-associated signaling (CAS) systems, the signaling information for a trunk is carried by the trunk itself. In common-channel signaling (CCS), a common signaling link (SL) carries signaling messages for a number of trunks. Just as multi-frequency (MF) signaling became feasible with the introduction of the second-generation (common-control) switching systems, CCS was developed for the third-generation (stored program controlled, SPC) exchanges that were introduced in the 1960s. ...
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Tín hiệu trong các mạng viễn thông P6
This chapter describes signaling system No.6, the first-generation commonchannel signaling system. There are two versions of this system, both of which were first deployed in the mid-1970s. CCITT No.6 [l, 23 is still in use in the international network, on a number of transatlantic and transpacific trunk interofice signaling (CCIS), defined by the Bell groups. Common-channel System, has been used in the U.S. toll network [3,4], but has now been replaced by the North American version of signaling system No.7....
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RF và mạch lạc lò vi sóng P4
A communication circuit designer frequently requires means to select (or reject) a band of frequencies from a wide signal spectrum. Resonant circuits provide such ®ltering. There are well-developed, sophisticated methodologies to meet virtually any speci®cation. However, a simple circuit suf®ces in many cases. Further, resonant circuits are an integral part of the frequency-selective ampli®er as well as of the oscillator designs. These networks are also used for impedance transformation and matching. This chapter describes the analysis and design of these simple frequency-selective circuits, and presents the characteristic behaviors of series and parallel resonant circuits. Related parameters, such as quality factor,...
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RF và mạch lạc lò vi sóng P5
IMPEDANCE MATCHING NETWORKS One of the most critical requirements in the design of high-frequency electronic circuits is that the maximum possible signal energy is transferred at each point. In other words, the signal should propagate in a forward direction with a negligible echo (ideally, zero). Echo signal not only reduces the power available but also deteriorates the signal quality due to the presence of multiple re¯ections. As noted in the preceding chapter, impedance can be transformed to a new value by adjusting the turn ratio of a transformer that couples it with the circuit. However, it has several limitations....
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RF và mạch lạc lò vi sóng P6
IMPEDANCE TRANSFORMERS In the preceding chapter, several techniques were considered to match a given load impedance at a ®xed frequency. These techniques included transmission line stubs as well as lumped elements. Note that lumped-element circuits may not be practical at higher frequencies. Further, it may be necessary in certain cases to keep the re¯ection coef®cient below a speci®ed value over a given frequency band. This chapter presents transmission line impedance transformers that can meet such requirements....
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RF và mạch lạc lò vi sóng P7
TWO-PORT NETWORKS Electronic circuits are frequently needed for processing a given electrical signal to extract the desired information or characteristics. This includes boosting the strength of a weak signal or ®ltering out certain frequency bands and so forth. Most of these circuits can be modeled as a black box that contains a linear network comprising resistors, inductors, capacitors, and dependent sources. Thus, it may include electronic devices but not the independent sources. Further, it has four terminals, two for input and the other two for output of the signal....
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RF và mạch lạc lò vi sóng P8
FILTER DESIGN A circuit designer frequently requires ®lters to extract the desired frequency spectrum from a wide variety of electrical signals. If a circuit passes all signals from dc through a frequency oc but stops the rest of the spectrum, then it is known as a low-pass ®lter. The frequency oc is called its cutoff frequency. Conversely, a high-pass ®lter stops all signals up to oc and passes those at higher frequencies. If a circuit passes only a ®nite frequency band that does not include zero (dc) and in®nite frequency, then it is called a band-pass ®lter. Similarly, a band-stop ®lter...
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RF và mạch lạc lò vi sóng P9
SIGNAL-FLOW GRAPHS AND APPLICATIONS A signal-¯ow graph is a graphical means of portraying the relationship among the variables of a set of linear algebraic equations. S. J. Mason originally introduced it to represent the cause-and-effect of linear systems. Associated terms are de®ned in this chapter along with the procedure to draw the signal-¯ow graph for a given set of algebraic equations. Further, signal-¯ow graphs of microwave networks are obtained in terms of their S-parameters and associated re¯ection coef®cients. The manipulation of signal-¯ow graphs is summarized to ®nd the desired transfer functions....
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RF và mạch lạc lò vi sóng P10
TRANSISTOR AMPLIFIER DESIGN Ampli®ers are among the basic building blocks of an electronic system. While vacuum tube devices are still used in high-power microwave circuits, transistorsÐ silicon bipolar junction devices, GaAs MESFET, heterojunction bipolar transistors (HBT), and high-electron mobility transistors (HEMT)Ðare common in many RF and microwave designs. This chapter begins with the stability considerations for a two-port network and the formulation of relevant conditions in terms of its scattering parameters....
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RF và mạch lạc lò vi sóng P11
OSCILLATOR DESIGN Oscillator circuits are used for generating the periodic signals that are needed in various applications. These circuits convert a part of dc power into the periodic output and do not require a periodic signal as input. This chapter begins with the basic principle of sinusoidal oscillator circuits. Several transistor circuits are subsequently analyzed in order to establish their design procedures. Ceramic resonant circuits are frequently used to generate reference signals while the voltage-controlled oscillators are important in modern frequency synthesizer design using the phase-lock loop. ...
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Lò vi sóng RF và hệ thống không dây P1
BRIEF HISTORY OF RF AND MICROWAVE WIRELESS SYSTEMS The wireless era was started by two European scientists, James Clerk Maxwell and Heinrich Rudolf Hertz. In 1864, Maxwell presented Maxwell's equations by unifying the works of Lorentz, Faraday, Ampere, and Gauss. He predicted the propagation of electromagnetic waves in free space at the speed of light. He postulated that light was an electromagnetic phenomenon of a particular wavelength and predicted that radiation would occur at other wavelengths as well. His theory was not well accepted until 20 years later, after Hertz validated the electromagnetic wave (wireless) propagation....
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Lò vi sóng RF và hệ thống không dây P2
Review of Waves and Transmission Lines At low RF, a wire or a line on a printed circuit board can be used to connect two electronic components. At higher frequencies, the current tends to concentrate on the surface of the wire due to the skin effect.
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Lò vi sóng RF và hệ thống không dây P3
Antenna Systems The study of antennas is very extensive and would need several texts to cover adequately. In this chapter, however, a brief description of relevant performances and design parameters will be given for introductory purposes. An antenna is a component that radiates and receives the RF or microwave power. It is a reciprocal device, and the same antenna can serve as a receiving or transmitting device. Antennas are structures that provide transitions between guided and free-space waves. ...
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Lò vi sóng RF và hệ thống không dây P4
Various Components and Their System Parameters An RF and microwave system consists of many different components connected by transmission lines. In general, the components are classified as passive components and active (or solid-state) components. The passive components include resistors, capacitors, inductors, connectors, transitions, transformers, tapers, tuners, matching networks, couplers, hybrids, power dividers=combiners, baluns, resonators, filters, multiplexers, isolators, circulators, delay lines, and antennas. ...
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Lò vi sóng RF và hệ thống không dây P5
Receiver System Parameters A receiver picks up the modulated carrier signal from its antenna. The carrier signal is downconverted, and the modulating signal (information) is recovered. Figure 5.1 shows a diagram of typical radio receivers using a double-conversion scheme. The receiver consists of a monopole antenna, an RF amplifier, a synthesizer for LO signals, an audio amplifier, and various mixers, IF amplifiers, and filters.
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Lò vi sóng RF và hệ thống không dây P6
Transmitter and Oscillator Systems A transmitter is an important subsystem in a wireless system. In any active wireless system, a signal will be generated and transmitted through an antenna. The signal’s generating system is called a transmitter. The specifications for a transmitter depend on the applications. For long-distance transmission, high power and low noise are important. For space or battery operating systems, high efficiency is essential. For communication systems, low noise and good stability are required....
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Lò vi sóng RF và hệ thống không dây P7
Radar and Sensor Systems Radar stands for radio detection and ranging. It operates by radiating electromagnetic waves and detecting the echo returned from the targets. The nature of an echo signal provides information about the target—range, direction, and velocity. Although radar cannot reorganize the color of the object and resolve the detailed features of the target like the human eye, it can see through darkness, fog and rain, and over a much longer range.
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Lò vi sóng RF và hệ thống không dây P8
Wireless Communication Systems The RF and microwave wireless communication systems include radiolinks, troposcatter=diffraction, satellite systems, cellular=cordless=personal communication systems (PCSs)=personal communication networks (PCNs), and wireless localarea networks (WLANs). The microwave line-of-sight (LOS) point-to-point radiolinks were widely used during and after World War II. The LOS means the signals travel in a straight line. The LOS link (or hop) typically covers a range up to 40 miles....
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Lò vi sóng RF và hệ thống không dây P9
Modulation and Demodulation Modulation is a technique of imposing information (analog or digital) contained in a lower frequency signal onto a higher frequency signal. The lower frequency is called the modulating signal, the higher frequency signal is called the carrier, and the output signal is called the modulated signal. The benefits of the modulation process are many, such as enabling communication systems to transmit many baseband channels simultaneously at different carrier frequencies without their interfering with each other....
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Lò vi sóng RF và hệ thống không dây P10
Multiple-Access Techniques Three commonly used techniques for accommodating multiple users in wireless communications are frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). Frequency division multiple access and TDMA are old technologies and have been used for quite a while. Code division multiple access is the emerging technology for many new cellular phone systems. This chapter will brie¯y discuss these techniques.
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Lò vi sóng RF và hệ thống không dây P11
Other Wireless Systems The two major applications of RF and microwave technologies are in communications and radar=sensor systems. Radar and communication systems have been discussed in Chapters 7 and 8, respectively. There are many other applications such as navigation and global positioning systems, automobile and highway applications, direct broadcast systems, remote sensing, RF identi®cation, surveillance systems, industrial sensors, heating, environmental, and medical applications. Some of these systems will be discussed brie¯y in this chapter....
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Sổ tay RFID (P1)
In recent years automatic identification procedures (Auto-ID) have become very popular in many service industries, purchasing and distribution logistics, industry, manufacturing companies and material flow systems. Automatic identification procedures exist to provide information about people, animals, goods and products in transit. The omnipresent barcode labels that triggered a revolution in identification systems some considerable time ago, are being found to be inadequate in an increasing number of cases....
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Sổ tay RFID (P2)
Differentiation Features of RFID Systems Fundamental Differentiation Features RFID systems exist in countless variants, produced by an almost equally high number of manufacturers. If we are to maintain an overview of RFID systems we must seek out features that can be used to differentiate one RFID system from another (Figure 2.1). RFID systems operate according to one of two basic procedures: full duplex (FDX)/ half duplex (HDX) systems, and sequential systems (SEQ). In full and half duplex systems the transponder’s response is broadcast when the reader’s RF field is switched on....
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Sổ tay RFID (P3)
Fundamental Operating Principles This chapter describes the basic interaction between transponder and reader, in particular the power supply to the transponder and the data transfer between transponder and reader (Figure 3.1). For a more in-depth description of the physical interactions and mathematical models relating to inductive coupling or backscatter systems please refer to Chapter 4.
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Sổ tay RFID (P4)
Physical Principles of RFID Systems The vast majority of RFID systems operate according to the principle of inductive coupling. Therefore, understanding of the procedures of power and data transfer requires a thorough grounding in the physical principles of magnetic phenomena. This chapter therefore contains a particularly intensive study of the theory of magnetic fields from the point of view of RFID.
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Sổ tay RFID (P5)
Frequency Ranges and Radio Licensing Regulations 5.1 Frequency Ranges Used Because RFID systems generate and radiate electromagnetic waves, they are legally classified as radio systems. The function of other radio services must under no circumstances be disrupted or impaired by the operation of RFID systems.
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Sổ tay RFID (P6)
Coding and Modulation The block diagram in Figure 6.1 describes a digital communication system. Similarly, data transfer between reader and transponder in an RFID system requires three main functional blocks. From the reader to the transponder — the direction of data transfer — these are: signal coding (signal processing) and the modulator (carrier circuit) in the reader (transmitter), the transmission medium (channel ), and the demodulator (carrier circuit) and signal decoding (signal processing) in the transponder (receiver)....
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Sổ tay RFID (P7)
Data Integrity The Checksum Procedure When transmitting data using contactless technology it is very likely that interference will be encountered, causing undesired changes to the transmitted data and thus leading to transmission errors (Figure 7.1). A checksum can be used to recognise transmission errors and initiate corrective measures, for example the retransmission of the erroneous data blocks. The most common checksum procedures are parity checks, XOR sum and CRC.
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