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接收机增益自动控制电路的设计,接收机,增益,自动,控制电路,设计
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Radar Radar is an electromagnetic system for detection and location of objects. It operates by transmitting a particular type of waveform, a pulse-modulated sine wave for example, and detects the nature of the echo signal. Radar is used to extend the capability of ones senses for observing the environment, especially the senses of vision. The value of radar lies not in being a substitute for the eye, but in doing what the eye cannot do. Radar cannot resolve details as well as the eye, nor is it capable of recognizing the color of objects to the degree of sophistication of which the eye is capable. However, radar can be designed to see through those conditions impervious to normal human vision, such as darkness, haze, fog, rain, and snow. In addition, radar has the advantage of being able to measure the distance or range to the objects. This is probably its most important attribute. An elementary form of radar consists of a transmitting antenna emitting electromagnetic radiation generated by an oscillator of some sort, a receiving antenna, and an energy-detecting device , or receiver .A portion of the transmitted signal is intercepted by a reflecting object(target)and is reradiated in all directions. It is the energy reradiated in the back direction that is of prime interest to the radar .The receiving antenna collects the returned energy and delivers it to a receiver, where it is processed to detect the presence of the target and to extract its location and relative velocity. The distance to the target is determined by measuring the time taken for the radar signal to travel to the target and back. The direction, or angular position, of the target may be determined from the direction of arrival of the reflected wave-front. The usual method of measuring the direction of arrival is with narrow antenna beams. If relative motion exists between target and radar, the shift in the carrier frequency of the reflected wave ( doppler effect) is a measure of the targets relative(radial)velocity and may be used to distinguish moving targets from stationary objects. In radars which continuously track the movement of a target, a continuous indication of the rate change of target position is also available. ( 摘自：M.I.Skolnik,introduction to Radar system,P1) Radar Block Diagram and Operation The operation of a typical pulse radar may be described with the aid of the block diagram shown in Fig.1.2. The transmitter may be an oscillator, such as a magnetron, that is pulsed(turned on and off) by the modulator to generate a repetitive train of pulses. The magnetron has probably been the most widely used of the various microwave generators for radar. A typical radar for the detection of aircraft at ranges of 100 or 200 nmi might employ a peak power of the order of a megawatt, an average power of several kilowatts, a pulse width of severalmicroseconds, and a pulse repetition frequency of several hundred pulses per second. The waveform generated by the transmitter travels via a transmission line to the antenna, where it is radiated into space. A single antenna is generally used for both transmitting and receiving. The receiver must be protected from damage caused by the high power of the transmitter. This is the function of the duplexer. The duplexer also serves to channel the returned echo signals to the receiver and not to the transmitter. The duplexer might consist of two gas-discharge devices, one known as a TR (transmit-receive) and the other an ATR(anti-transmit-receive).The TR protects the receiver during transmission and the ATR directs the echo signal to the receiver during reception.The receiver is usually of the superheterodyne type. The first stage might be low-noise RF amplifier, such as a parametric amplifier or a low-noise transistor.The mixer and local oscillator (LO) convert the RF signal to an intermediate frequency(IF).A typical IF amplifier for an air-surveillance radar might have a center frequency of 30 or 60 MHz and a bandwidth of the order of one megahertz. The IF amplifier should be designed as a match filter; i.e. , its frequency-response function H(f)should be maximize the peak-signal-to-mean-noise-power ratio at the output.After maximizing the signal-to-noise ratio in the IF amplifier, the pulse modulation is extracted by the second detector and amplified by the video amplifier to a level where it can be properly displayed, usually on a cathode-ray tube (CRT).Timing signals are also supplied to the indicator to provide the range zero. Angle information is obtained from the pointing direction of the antenna. The most common form of cathode-ray tube display is the plan position indicator, or PPI, which maps in polar coordinates the location of the target in azimuth and range. ( 摘自：M.I.Skolnik,introduction to Radar system,P5)Radar Receivers The function of a radar receiver is to amplify the echoes of the radar transmission and to filter them in a manner that will provide the maximum discrimination between desired echoes and undesired interference. The interference comprises not only the noise generated in the receiver but also energy received from galactic sources, neighboring radars and communication equipment, and possibly jammers. The portion of the radars own radiated energy that is scattered by undesired targets (such as rain , snow, birds ,insects, atmospheric perturbations, and chaff)may also be classed as interference. Where airborne radars are used for altimeters or mapping, other aircraft are undesired targets, and the ground is the desired target. More commonly, radars are intended for detection of aircraft, ships, surface vehicles, or personnel, and the reflection from sea or ground is classified as clutter interference. Receiver types include the superregenerative , crystal video ,and tuned radio frequency(TRF)and superheterodyne .The superregenerative receiver is sometimes employed in radar-beacon applications because a single tube may function as both transmitter and receiver and because simplicity and compactness are more important than superior sensitivity. The crystal video receiver also is simple but of poor sensitivity. The TRF receiver uses only RF and video amplification;although its noise temperature may be low, its sensitivity is poor because optimum-bandwidth filtering of the usual radar echo spectrum is generally impractical to achieve. Only for radars that radiate a relatively wide-percentage-bandwidth signal is filtering practical. Virtually all radar receivers operate on the superheterodyne principle shown in Fig.3.1. The input signal is derived from the duplexer, which permits a single antenna to be shared between transmitter and receiver. The echo, after modest amplification, is shifted to an intermediate frequency by mixing with a local-oscillator (LO) frequency. More than one conversion step may be necessary to reach the final IF, generally between 0.1 and 100 MHz, without encountering serious image or spurious-frequency problem in the mixing process .Not only is amplification at IF less costly and more stable than at microwave frequency, but the wider percentage bandwidth occupied by the desired echo simplifies the filtering operation. In addition, the superheterodyne receiver can vary the LO frequency to follow any desired turning variation of the transmitter without disturbing the filtering at IF. These advantages have been sufficiently powerful that competitive forms of receiver have virtually disappeared. ( 摘自：M.I.Skolnik,Radar Handbook,P3.1)Principles of Automatic Control The automatic control disciplines in recent decades, developed a very important subject. Its very rapid development, especially the rapid development of the computer, but also accelerated its development, especially in recent years, the development of the industrial automation technology.The scope of the study of automation disciplines is very broad, important and positive role in the modernization of industry, agriculture,national defense and science and technology, the rapid increase of Chinas comprehensive national strength.Automatic control (automatic control) means that no one directly involved in the case, the use of additional equipment or devices, machinery, equipment or production process of a state or parameters to automatically run according to a predetermined law.The automatic control is relatively manual control concept.Is no one involved in the case, the use of the control device so that the charged object or process to run automatically according to predetermined rule.Study of automatic control technology in favor of humanity from the complex, dangerous, tedious labor freed and greatly improve the efficiency of control.The automatic control is a branch of engineering science.It involves the automatic impact of the feedback principle of a dynamic system, the output value close to the value we want.From a methodological viewpoint, it is based on mathematical systems theory.Today, we called the automatic control is a control on a branch of the mid-twentieth century.The basis of the conclusions raised from Norbert Wiener, Rudolf Kalman.Example: adjust the indoor temperature of the indoor temperature regulation is a straightforward example.Is the indoor temperature is kept at a constant value , window and other factors make the indoor heat from the outdoor (interference d).For this purpose, the heating must be the right impact.Regulation through the valve, the temperature will remain constant.In addition, before people feel the radiator hot water temperature will be affected by the interference of outside temperature.The rest of the examples of three-barrel system.Developments in the field of automatic control process150 years ago the first generation of process control system is based on the standard 5-13psi pneumatic signal (pneumatic control system PCS, - In Pneumatic Control System).Simple local mode of operation, control theory has been initially formed, yet the concept of the control room.The second-generation process control system (analog or the ACS, Analog Control System) is based on the current 0-10mA or 4-20mA analog signals, the obvious progress in the whole 25 years, firmly ruled the entire field of automatic control.It characterized the era of electrical automatic control.Control theory have been significant developments, the three Kybernetika establishing laid the foundation of the modern control; the establishment of the control room, the control function of the separation model has been in use ever since.Third-generation process control system (CCS, Computer Control System) .70, began the application of digital computers, had a huge technological advantages, the first to use in the field of measurement, simulation, and logic control, resulting in a third-generation process control system (CCS, Computer Control System).-Called third-generation process control system is a revolution in the field of automatic control, which give full play to the strengths of the computer, so people generally believe that the computer can do all things naturally produced known as the centralized control central controlcomputer systems, should be noted that the signal transmission system of the system is still the majority of follow the 4-20mA analog signal, but long after it was found that with the concentration of control and reliability issues, the risk of runaway concentratedthe slightest mistake will make the whole system paralyzed.So it will soon be developed into a distributed control system (DCS).Fourth-generation process control system (DCS, Distributed Control System Distributed Control System): With the rapid development of semiconductor manufacturing technology, the widespread use of microprocessors, computer technology, the reliability of a substantial increase in current widespread use of the fourthgeneration of process control system (DCS, or distributed digital control system), which is characterized by the whole control system is no longer just a computer, but by a few computers and a number of smart meter and smart components to form a control system.So become the most important features of distributed control.Except Another important development is the signal to pass between them is not only dependent on the 4-20mA analog signal, gradually replaced by the digital signal to analog signal.The fifth-generation process control system (FCS, Fieldbus Control System fieldbus control system): FCS from the DCS evolved, just like DCS come from CCS development, has been a qualitative leap.Decentralized control development site control; data transmission using the bus.FCS and DCS, the real difference is that FCS has a broader space for development.DCS technical level has been improving, but most low-end communications network only to a site controller, the link between field controllers and field instrumentation, actuators still use the one-to-one transmission 4-20mA analogsignal, the high cost, low efficiency, difficult to maintain, can not play the potential of intelligent field instrumentation, and comprehensive monitoring and deep-seated state of the field devices work.The so-called fieldbus is connected intelligent measurement and control equipment, all-digital, two-way transmission, the branch structure of the multi-node communication links.Traditional control is a loop, while the FCS technology is the various modules such as controllers, actuators, detector hung up on a bus communication, of course transmission is digital signal.The main bus Profibus, the LonWorks.The development of modern control theory: the classical control theory has great practical value, but also has obvious limitations.The performance of its limitations in the following two aspects: first, the classical control theory built on the basis of the transfer function and frequency characteristics, while passing the function and frequency characteristics belong to the system description of the external (only describe the input and outputrelations), can not fully reflect the internal state of the system; second, whether it is the root locus method or frequency method, in essence, is the frequency domain method (also known as Complex Act), go through integral transforms (Laplace transform,Fourier transform, Z transform), therefore, in principle, suitable to solve the problem of single input - single output linear constant system, multi-input - multiple output system should not use the classical control theory to solve, especially for nonlineartime-varying system is powerless.Modern control theory essentially is a time-domain method.It introduces the concept of state and state variable (within the system variables) and the equation of state to describe the system, and thus better reflect the inherent nature and characteristics of the system.From the mathematical point of view of view, the state variable method in the modern control theory, simply put, is the higher order differential equations describing the system motion, rewritten in the form of first-order simultaneous differential equations, or the motion of the system directly with a first-order differential equation group said.This first-order differential equations is called the equation of state. Using the equation of state, the main advantage is the system of equations of motion using vector and matrix form, therefore the form of simple, clear concepts, computing, especially for multi-variable, time-varying systems is even more evident.Theory, modern control theory to solve the system controllability, observability , stability, and many of the complex system control problems.However, with the rapid development of modern science and technology, the scale of the production system, the formation of a complex system, leading to a control object, the controller, and control tasks and the purpose of an increasingly complex, leading to the modern control theorythe outcome rarely in practice.Intelligent control theory: Intelligent Control refers to the drive autonomous intelligent machines to achieve its objective, that is, the intelligent control intelligent machines can be driven independently of the direct intervention of a class without automatic control to achieve its goals.Intelligent control is based on the intersection of artificial intelligence, cybernetics, operations research, and information theory and other disciplines.The theory of intelligent control system has the following distinctive features: First, the analysis and design of intelligent control system, focusing not on the traditional analysis and design of the controller, while on the model of intelligent machines, which means thatnot the description of the mathematical formulas, calculating and processing (in fact, some of the complex system may simply can not use a precise mathematical model description), and must focus on the description of the non-mathematical model, symbolsabove and the recognition of the environment, the knowledge base and inference engine design and development.Second, the core of the intelligent control of high-level control, its mission is to organize the physical environment or process, decision-making and planning, the generalized problem solving.Third, the intelligent control is an edge cross-disciplinary, Professor Fu Jing Sun in 1971 first proposed the binary intersection theory of intelligent control (ie, artificial intelligence and automatic control of the cross) the United States GNSaridis in 1977, Fu Jing Sun binarystructure extended to the ternary structure (ie, artificial intelligence, automatic control and operations research, cross), and later, Central South University, Professor Cai Zixing turn ternary structure is extended to the quaternary structure (ie, artificial intelligence, automatic control, operations research and information theorycross), which further improved the structure theory of intelligent control.Fourth, the intelligent control is an emerging research and applications, has a very attractive development prospects.Prerequisite for the development of intelligent control, scholars proposed Hierarchical Intelligent Control: Hierarchical Intelligent Control (the Hierarchical Intelligent Control) is the learning control system based on early research, engineering cybernetics point of view summary of artificial intelligence and adaptive control,self-learning control and self-organizing control of the relationship gradually formed, it is one of the earliest theory of intelligent control.Experts in intelligent control: it reduce the scope of the research in a specific, relatively narrow area of expertise.Human experts have become experts, because he has a lot of expertise to solve the problems of their own areas of expertise, including a variety of useful know-how and experience, experts in the field of expert system is actually implemented on the computer that imitate matter.Fuzzy Intelligent Control the application of fuzzy logic control theory and fuzzy logic control system has developed rapidly, showing the fuzzy theory in the field of control has a very good development prospects.Fuzzy logic control has now become an important part of intelligent control.Neural network intelligent control intelligent control system based on neural networks as an emerging field, has been able to cause the broad interest of the automatic control community, the reason is: The neural network has the ability to approximate any nonlinear function; neural network is easy to use VLSIachieve, so that the neural network has the advantages of fast and high fault tolerance; neural network structure and its multi-input multi-output characteristics, making it easy for the control of multivariable systems, and compared with other approximation methods moreeconomy; (4) neural network with adaptive and self-learning features.These features of the neural network that it has a role in the transfer function of the linear system, but can be naturally extended to the nonlinear, time varying complexity, uncertainty system.Learning control system, and qualitative control theory.Above through the course of development of automatic control theory, a general overview of the basic principles of automatic control and its future development trends through the study of automatic control theory, we know the importance of the feedback system to control the stability of the system, after the systemthe calculation of various parameters, consider which method is more favorable to the stability of the system, learning design and calibration of the automatic control system, which is conducive to the future application of automatic control.AGC (automatic the gain control)Amplifier gain is automatically adjusted with the signal strength automatic control method.Circuit to achieve this functionality is referred to as the AGC loop.The AGC loop is a closed-loop electronic circuit is a negative feedback system, which is gain controlled amplifier circuit and the control voltage can be divided into two parts forming circuit.Gain controlled amplifier circuit is located in the path of positive amplification, the gain with the control voltage changes.The basic components of the control voltage forming circuit the AGC detector and low-pass smoothing filter, and sometimes also contains the parts of the gate circuit and the
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