1.4 Overview

Chapter 1.4 Overview

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • . Overview LNA amplifies the input signal without adding much noise. The input signal can be very weak, so the first thing to do is strengthen the signal without corrupting it. As a result, noise added in later stages will be of less importance. The image filter that follows the LNA removes out-of-band signals and noise before the signal enters the mixer. The mixer translates the input radio frequency signal down to the intermedi-ate frequency, since filtering, as well as circuit design, become much easier at lower frequencies for a multitude of reasons. The other input to the mixer is the local oscillator (LO) signal provided by a voltage-controlled oscillator inside a frequency synthesizer. The desired output of the mixer will be the difference between the LO frequency and the RF frequency.At the input of the radio, there may be many different channels or frequency bands. The LO frequency is adjusted so that the desired RF channel or frequency band is mixed down to the same intermediate frequency (IF) frequency in all cases. The IF stage then provides channel filtering at this frequency to remove the un-wanted channels. The IF stage provides further amplification and automatic gain control (AGC) to bring the signal to a specific amplitude level before the signal is passed on to the back end of the receiver. It will ultimately be converted into bits (most modern communications systems use digital modulation schemes) that could represent voice, video, data, and so forth, through the use of an A/D converter. On the transmit side, the back-end digital signal is used to modulate the car-rier in the IF stage. A mixer converts the modulated signal and IF carrier up to the desired RF frequency. A frequency synthesizer provides the other mixer input. Since the RF carrier and associated modulated data may have to be transmitted over large distances through lossy media (e.g., air, cable, or fiber), a power amplifier (PA) must be used to increase the signal power. Typically, the power level is increased from the milliwatt range to a level in the range of hundreds of milliwatts, to watts, depending on the particular application. A low pass filter after the PA removes any harmonics produced by the PA to prevent them from also being transmitted. Radio architec-tures will be studied in much more detail in Chapter 3. 1.4  OverviewWe will spend the rest of this book trying to convey the various design constraints of the RF building blocks mentioned in the previous sections. Components are designed with consideration to the main problems of frequency response, gain, sta-bility, noise, distortion (nonlinearity), impedance matching, and power dissipation. Dealing with design constraints is what keeps the RFIC designer employed.The focus of this book will be how to design and build the major circuit blocks that make up the RF portion of a radio using an IC technology. To that end, block level performance specifications are described in Chapter 2 and architecture con-siderations are discussed in Chapter 3. A brief overview of IC technologies and transistor performance is given in Chapter 4. Various methods of impedance match-ing, which is very important at chip boundaries and for some interconnections of circuits on chip, will be discussed in Chapter 5. The realization and limitations of passive circuit components in an IC technology will be discussed in Chapter 6. Chapters 7, 8, 9, and 11 will be devoted to individual circuit blocks such as LNAs