RF and Microwave Circuit Design

11Oscillators

11.1 Introduction

Oscillators, for the generation of power, are fundamental devices in RF and microwave transmitters and receivers. The most common type of semiconductor oscillator in modern high-frequency systems is that using transistors with positive feedback, and this type of oscillator will be the main focus of this chapter. Other types of oscillators exist, such as those using microwave diodes, but they are normally restricted to relatively specialized applications. For completeness, a brief account of the two most common types of microwave diode oscillator, namely those using Gunn and Impatt diodes, has been included.

The chapter commences with a discussion of the basic conditions for oscillation in feedback circuits. Particular emphasis is given to the Colpitts oscillator, which is the most common form of oscillator for RF and low microwave frequency applications. The use of crystals and varactor diodes will be included in the discussion as they have particular relevance for oscillator stability and tuning. In addition to oscillators using discrete components, the application of feedback in distributed circuits, such as the dielectric resonator oscillator (DRO), often used in hybrid microwave circuits, will be discussed.

Many communication circuits, and much high-frequency instrumentation, derive the required oscillation frequency from very stable low-frequency (LF) sources through the use of frequency synthesizers, and a review of basic synthesizer ...