Overview

Oscillator design is perhaps one of the least understood practices of wireless engineering in general, and is definitely considered to be the most complex. Indeed, until Randall W. Rhea released his groundbreaking book Oscillator Design and Computer Simulation in 1990, oscillator design was strictly a hit-or-miss affair for many engineers. As anyone in RF is well aware, it is quite easy to design an oscillator just design a poor amplifier and turn on the power, and it will probably begin to oscillate. But the problem is to design an oscillator that will oscillate at the desired frequency and amplitude, that will start reliably and not wander, that will not be plagued with spurious responses and harmonics, that will not be excessively affected by normal changes in temperature, and that will be consistent in operation when built over a long production run.

This chapter will concentrate on the design, simulation, and verification of voltage-controlled oscillators (VCOs), LC oscillators, and crystal oscillators over a wide range of frequencies. But first, a memory refresher on basic oscillator theory.

Sine-wave oscillators. When a pulse is applied to a tank circuit, it will ring at the tank s resonant frequency, creating a decaying sinusoidal wave (Fig. 4.1). But if amplification from an active device, such as a transistor, is used to amplify and sustain this output, then an oscillator can be formed. The natural resonant frequency of the tank circuit is established by the tank s L and C components, or: