RF and Microwave Circuit and Component Design for Wireless Systems

CHAPTER TEN

Oscillators and Modulation

10.1 INTRODUCTION

The oscillator is a very important component in any RF/microwave system. It serves as the signal generator in a transmitter. In a receiver, the local oscillator is used together with a mixer to convert the received RF signal to an IF signal. The oscillator can be modulated by a low-frequency analog or digital signal. In this case, the oscillator signal serves as a carrier, and the modulating low-frequency signal is the information to be transmitted. In a system's design, one would like to have an oscillator with some or all of these characteristics: good power output, high dc-to-RF efficiency, low noise, good stability, good frequency tunability, etc.

The key to an oscillator is a negative-resistance solid-state device. Due to the device physics, several solid-state devices exhibit a negative resistance under certain dc biases. These include the Gunn device, impact ionization avalanche transit time (IMPATT) devices, resonant tunneling diodes (RTDs), and transistors [1]. Gunn and IMPATT are two-terminal devices used only in microwave and millimeter-wave frequencies. Transistors have many different variations used from low RF frequencies to microwave and millimeter-wave frequencies. In the RF frequencies, silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) are commonly used. In the microwave frequencies, bipolar transistors, metal-semiconductor field-effect transistors (MESFETs), high electron-mobility transistors ...