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Introduction to oscillators#

Pendulums#

Lucas Vieira / Public domain

Two pendulums with the same period fixed on a string act as pair of coupled oscillators. The oscillation alternates between the two.

This page has both theory and practical content

This page has both theory and practical content. The theory is useful in order to understand how oscillators work, while the practical content provides hands-on directions on how to build test and use oscillators.

Definition of an oscillator

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.

Oscillators categories#

Broadly, oscillators in the context of low power radio fall into two main categories:

  1. Sign wave oscillators
  2. Square wave oscillators

Both of these can be the starting place for QRP receivers or transmitters. However, normally, we want to end up with a sign wave. Luckily, we can obtain a sign wave from other harmonic rich waves, such as square waves, using filtering, so this is not a problem.

Sign wave oscillators#

How do they work? The harmonic, or linear, oscillator produces a sinusoidal output. There are two main types:

  1. Feedback oscillator
  2. Negative-resistance oscillator

Feedback oscillators#

For now we will look at the most commonly used oscillator circuits. There are a number of types of these that are of interest. Some of these are listed below:

List of commonly used variaents of these oscillators

The oscillators of most interest to us are highlighted in bold.

  • Armstrong oscillator, a.k.a. Meissner oscillator
  • Clapp oscillator
  • Colpitts oscillator
  • Cross-coupled oscillator
  • Dynatron oscillator
  • Hartley oscillator
  • Opto-electronic oscillator
  • Pierce oscillator
  • Phase-shift oscillator
  • Robinson oscillator
  • Tri-tet oscillator
  • Vackář oscillator
  • Wien bridge oscillator

All of these oscillator types employ the same principal.#

All of these types employ the same principal. An amplifier with feedback is used. In order to obtain the desired frequency, within the feedback path there is a filter. The job this filter performs is simply to select only the desired frequency and reject all others.

Chetvorno / Public domain

Fourier analysis#

The following shows how square waves are comprised of sign waves.

René Schwarz / CC BY-SA

Sine wave#

Omegatron / CC BY-SA

Square wave#

Omegatron / CC BY-SA

A nice explanation of the Colpitts oscillator#

This is a nice explanation of the Colpitts oscillator by Craig. His blog is worth checking out: Analogue Zoo

References and additional reading:#

Description Reference
Quantitative analysis of Colpitts crystal oscillators Crystal Oscillator Design and Temperature Compensation, Marvin E. Frerking, Chapter 7.3, Appendix F
Oscillator phase vs frequency, and crystal loaded Q analysis Crystal Oscillator Circuits, Robert J. Matthys, Chapters 6, 7
Colpitts crystal oscillator phase vs gain analysis Oscillator Design and Computer Simulation, 2nd Edition, Randall W. Rhea, Chapter 11.2
Crystal drive level equations Intel application note AP-155 (Oscillators for Microcontrollers), Appendix A
Common collector gain equations Common collector, Wikipedia
Transistor biasing RF Circuit Design, Chris Bowick, Chapter 6

This is a Vackar oscillator with a frequency range 6-12 MHz#

It is worth noting that the Vackar oscillator is highly stable and although free running, is suitable for a VFO for a radio transmitter or receiver, when the correct parts are selected

M0OOZ - YooFab / CC BY-SA

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