RF and Microwave Circuit and Component Design for Wireless Systems

CHAPTER NINE

Mixers

9.1 INTRODUCTION

Mixer is one of the fundamental blocks of a wireless communication system. Many portable communications systems have at least two mixers, one in the receiving chain and another in the transmitting chain. The primary function of a mixer in a communication system is to translate signal from one frequency (RF frequency, for example) to another frequency (intermediate frequency, IF). This chapter deals with the mixer device characteristics and mixer designs. A system consisting of low-noise amplifier and a mixer, sometimes referred to as down-converter, is also discussed. Figure 9.1 shows the block diagram of a wireless communication system wherein several mixers are utilized.

9.2 MIXER FUNDAMENTALS

Mixer is fundamentally a frequency translating device. Mixer faithfully preserves the amplitude and phase properties of the RF signal at the input. Signals can therefore be translated into frequency without affecting their modulation properties. An ideal mixer multiplies the input signal (RF signal) by a sinusoidal signal (generated by a local oscillator). This results in a mixed product that consists of higher and lower frequency components. A schematic representation of a mixer is shown in Figure 9.2(a) and the mixer output spectrum consisting of the LO, RF, and some of the higher order products is shown in Figure 9.2(b).

Devices that exhibit nonlinear or rectifying characteristics are good candidates for designing mixers. Any device used as a mixer ...