Lò vi sóng rf

hoặc bằng cách sử dụng một chất nền giá trị hằng số điện môi cao. Q-giá trị và chế tạo đạt được photoetching giới hạn về bề rộng đường tối thiểu và tách ra lệnh kích thước của tụ điện. Gộp yếu tố cho RF và lò vi sóng mạch

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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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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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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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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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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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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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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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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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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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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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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.

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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.

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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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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.

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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.

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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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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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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.

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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.

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