6 - Oscillators

Abstract

This chapter discusses radio frequency (RF) oscillators that can be built using a number of different types of frequency selective resonators. Common types include inductor-capacitor (L-C) networks and quartz crystal resonators. The crystal resonator has by far the best accuracy and stability, but can only be adjusted over a narrow range of frequency. There are several different configurations for RF oscillators, but the fundamental forms are Colpitts and Hartley. The distinguishing characteristic that identifies the Colpitts oscillator is that the feedback network consists of a tapped capacitive voltage divider. The output of this voltage divider is fed back to the input of the amplifier. A special variation on the Colpitts theme is the Clapp oscillator. The difference is that the Colpitts uses parallel resonant tuning, while the Clapp uses series resonant tuning. Otherwise, they are both identical (both use the capacitive voltage divider). There are variations on the Hartley theme that use a tapped coil as part of the feedback network, but a crystal to actually set the frequency of oscillation.