ChapPowerSeries

ChapPowerSeries - 1[LE,RO[LO RE[LO c L Larson[LE c L...

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Unformatted text preview: 1 [LE,RO] [LO, RE] [LO] c L. Larson [LE] c L. Larson May 29, 2003 [C] 1 2 Wireless Communications Circuits Lawrence E. Larson May 29, 2003 2 Chapter 1 Linearity Characterization of Communications Systems So far we have dealt with characterization of the smallest signals in commu- nications systems, and how background noise affects the minimum detectable signal. The concept of Noise Figure was found to be a useful means for deter- mining the smallest signal that can be detected for a desired signal-to-noise ratio. We are now going to change our focus, and deal with issues that arise because the received signal is too large , rather than too small . At first, this notion seems to be absurd: how can a signal be too large? The answer to this question lies in an understanding of the nonlinear behavior of electronic circuits under large signal conditions, which is the subject of this and the following Chapter. 1.1 Characterization of Circuit Behavior All experienced engineers have an intuitive understanding that the behavior of electronic circuits that contain transistors, diodes, resistors, capacitors, and inductors, is incredibly complicated. Computer-aided design techniques can provide superbly detailed simulations of the response of these circuits, but analytical tools are still needed to gain “insight” into their behavior. In order to aid the analysis of the complex time and frequency behavior of electronic circuits, various simplifying “regimes” of behavior have been developed over the years in order to ease their analysis in certain special cases. In order to understand the analysis of the nonlinear behavior of communications circuits, 3 4 CHAPTER 1. LINEARITY CHARACTERIZATION OF COMMUNICATIONS SYSTEMS we need to develop an appreciation for these categories, and under what situations they apply. As it turns out, most practical electronic circuits can be safely analyzed under a rather narrow set of conditions, which has allowed engineers to develop powerful analytical tools to accurately characterize their operation. Linear vs. Nonlinear Circuit Operation Most engineers begin their stud- ies with a thorough grounding in linear circuit theory . Linear circuits obey the principle of superposition in their responses. This implies that the response of a circuit to any sum of a set of scaled inputs is also summed and scaled by the same amount, i.e. Y out = a 1 Y out 1 + a 2 Y out 2 = F ( a 1 X in 1 + a 2 X in 2 ) (1.1) where Y out 1 is the response to excitation X in 1 ( Y out 1 = F ( X in 1 )) and Y out 2 is the response to excitation X in 2 , and Y out is the total output response of the circuit. An amplifier operating at the lowest possible signal levels — a radio astronomy low-noise amplifier as an example — can usually be safely analyzed using linear circuit theory, and all of the low-noise amplifier analysis in Chapter XX assumed the circuit was operating under linear conditions. This condition of linearity is shown in Figure ??...
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This note was uploaded on 04/02/2011 for the course ECE 264 taught by Professor Song during the Spring '11 term at UCSB.

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ChapPowerSeries - 1[LE,RO[LO RE[LO c L Larson[LE c L...

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