TABLE OF CONTENTS An oscillator is a circuit that generates a periodic waveform. While oscillators have numerous applications, from serving as reference tone generators for receivers to clocks for digital circuits, the application of most importance for this text is their use as the central component in a frequency synthesizer. In this application, important design considerations include tunability, spectral purity (low phase noise), and, in the fully integrated context, low power. Where the lowest possible phase noise with low power dissipation is the driving consideration, designers may decide to use LC-based oscillators. However, in many situations, ring oscillators are used; since they can potentially provide wider tuning range, they are simpler to design in a standard digital process as, typically, no inductors or varactors are needed. Without inductors, their layout area is also significantly lower. In addition, it is possible to design ring oscillators in such a way that they automatically have quadrature outputs. This chapter considers crystal oscillators, in addition to LC and ring oscillators. Crystal oscillators are important since they are commonly used as reference signals for frequency synthesizers.
