Module 13: Oscillators and Signal Sources

Almost every piece of electronic equipment contains at least one oscillator. The crystal next to a microcontroller sets the pace of every instruction it executes. The oscillator in a quartz watch counts the seconds; the one in a function generator produces test signals; the ones in a radio receiver, a Wi-Fi module or a GPS receiver select the frequency they work on. A switching power supply, a class D amplifier and an LED dimmer all contain an oscillator setting their switching rate. This module explains exactly how oscillators work — from the basic conditions for sustained oscillation all the way to the digital synthesis techniques inside modern signal generators and wireless chips.

Module Overview

Oscillators appear everywhere. The clock that drives a microcontroller or a computer is an oscillator. So is the timebase of a frequency counter or an oscilloscope, the local oscillator in a superheterodyne receiver, the carrier source in a wireless transmitter chip, the reference in a GPS receiver, and the heart of every signal generator. Understanding how oscillators work is therefore central to understanding how a great deal of electronic equipment functions.

This module begins with the universal conditions required for oscillation — the Barkhausen criterion — which applies to every oscillator regardless of its technology. It then covers the two classic LC oscillator circuits, the Colpitts and Hartley, which form the basis of most RF signal sources, from simple projects to commercial equipment. From there the module moves to crystal oscillators, which trade tunability for exceptional frequency stability, and to the drift and microphonics that affect all variable-frequency oscillators.

The second half of the module covers the modern approach to frequency generation. Phase-locked loops (PLLs) allow a single stable reference oscillator to control a variable-frequency oscillator, locking it to exact multiples of the reference frequency — the same technique that multiplies a 25 MHz crystal up to the gigahertz clocks of a computer processor. Frequency synthesizers extend this to generate thousands of precise frequencies from a single crystal reference, as every channelized radio and wireless chip does. Finally, direct digital synthesis (DDS) builds the output waveform mathematically in the digital domain and converts it to an analog signal — the technology inside most modern function and arbitrary waveform generators.

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