Overview

Up to this point, the circuits we have studied have been linear in nature. In amplifiers, for example, the input and output frequency are the same (assuming no harmonics are generated), only the amplitude has increased (and the phase changed). Even oscillators, while considered nonlinear in nature, are "linear" in the sense that they ideally output a single frequency sinusoid. Linear circuits, however, do not have the ability to translate or convert a signal to a higher frequency ( upconversion) or to a lower frequency ( downconversion); another circuit is required. The design of such a frequency converter or mixer should consider the following:

• The conversion should be done efficiently and with low loss of signal integrity;

• The conversion should suppress undesired signals and noise;

• The converting circuits should exhibit efficient power consumption.

This chapter describes a number of circuit types that perform these frequency conversion functions. Emphasis is placed on those circuit architectures widely used in CMOS RFIC design, and we also look at circuit enhancements to improve such issues as signal linearity and noise performance. The chapter starts off with a review of frequency conversion concepts and terminology. A number of simple frequency conversion circuits are covered to show both circuit possibilities but to also introduce more advanced concepts. The chapter finishes with several sections on high-performance frequency conversion circuits.

7.1 General Mixer Concepts

This section presents the concept behind frequency conversion and introduces both simple passive and active frequency conversion circuits as well...