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,. | Radio-Frequency and Microwave Communication Circuits Analysis and Design Devendra K. Misra Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-41253-8 Hardback 0-471-22435-9 Electronic 4 RESONANT CIRCUITS A communication circuit designer frequently requires means to select or reject a band of frequencies from a wide signal spectrum. Resonant circuits provide such filtering. There are well-developed sophisticated methodologies to meet virtually any specification. However a simple circuit suffices m many cases. Further resonant circuits are an integral part of the frequency-selective amplifier 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 ami presents the characteristic behaviors of series and parallel resonant circuits. Related parameters such as quality factor bandwidth and input impedance are introduced that will be used in several subsequent chapters. Transmission lines with an open or short circuit at their ends are considered next and their relationships with the resonant circuits are established. Transformer-coupled parallel resonant circuits are briefly discussed because of their significance in the radio frequency range. The final section summarizes the design procedure for rectangular and circular cylindrical cavities ami the dielectric resonator. SERIES RESONANT CIRCUITS Consider the series R-L-C circuit shown in Figure . Since the inductive reactance is directly proportional to signal frequency it tries to block the high-frequency contents of the signal. On the other hand capacitive reactance is inversely proportional to the frequency. Therefore it tries to stop its lower frequencies. Note that the voltage across an ideal inductor leads the current by 90 . the phase angle of an 105 106 RESONANT CIRCUITS Figure A series R-L-C circuit with input-output terminals. inductive reactance is 90 . In
