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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 | Radio-Frequency and Microwave Communication Circuits Analysis and Design Devendrá K. Misra Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-41253-8 Hardback 0-471-22435-9 Electronic 5 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 reflections. 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. This chapter presents a few techniques to design other impedance transforming networks. These circuits include transmission line stubs and resistive and reactive networks. Further the techniques introduced are needed in active circuit design at RF and microwave frequencies. As shown in Figure 5.1 impedance matching networks are employed at the input and the output of an amplifier circuit. These networks may also be needed to perform some other tasks such as filtering the signal and blocking or passing the de bias voltages. This chapter begins with a section on the impedance matching techniques that use a single reactive element or stub connected in series or in shunt. Theoretical Figure 5.1 Block diagram of an amplifier circuit. 146 SINGLE REACTIVE ELEMENT OR STUB MATCHING 147 principles behind the technique are explained and the graphical procedure to design these circuits using the Smith chart is presented. Principles and procedures of the double-stub matching are discussed in the following section. The chapter ends with sections on resistive and reactive L-section matching networks. Both analytical as well as graphical procedures to design these networks using ZY-charts .