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An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable 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.

A low-frequency oscillator (LFO) is an electronic oscillator that generates an AC waveform at a frequency below ≈20 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.

Oscillators designed to produce a high-power AC output from a DC supply are usually called inverters.

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation oscillator.

Harmonic oscillator

The harmonic, or linear, oscillator produces a sinusoidal output.

The basic form of a harmonic oscillator is an electronic amplifier connected in a positive feedback loop with its output fed back into its input through a filter. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop and is filtered and re-amplified until it increasingly resembles a sine wave at a single frequency.

In inductive-capacitive or LC oscillators, the filter is a tuned circuit (often called a tank circuit) consisting of an inductor (L) and capacitor (C) connected together. Charge flows back and forth between the capacitor's plates through the inductor, so the tuned circuit can store electrical energy oscillating at its resonant frequency. There are small losses in the tank circuit, but the amplifier compensates for those losses and supplies the power for the output signal. LC oscillators are typically used when a tunable frequency source is necessary, such as in signal generators, tunable radio transmitters and the local oscillators in radio receivers. Typical LC oscillator circuits are the Hartley, Colpitts and Clapp circuits. On-chip inductors usually don't have a high enough Q-factor to use in the tuned circuit.

A piezoelectric crystal (commonly quartz) may take the place of the filter to stabilise the frequency of oscillation, this is called a crystal oscillator. These kinds of oscillators contain quartz crystals that mechanically vibrate as resonators, and their vibration determines the oscillation frequency. Crystals have very high Q-factor and also better temperature stability than tuned circuits, so crystal oscillators have much better frequency stability than LC or RC oscillators. They are used to stabilize the frequency of most radio transmitters, and to generate the clock signal in computers. The Pierce oscillator circuit is often used for crystal oscillators. Because the crystal is an off-chip component, it adds some cost and complexity to the system design, but the crystal itself is generally quite inexpensive.

Surface acoustic wave (SAW) devices are a kind of crystal oscillator, but achieve much higher frequencies by establishing standing waves on the surface of the quartz crystal. These are more expensive than crystal oscillators, and are used in specialized applications which require a direct and very accurate high frequency reference, for example, in cellular telephones.

There are many ways to implement harmonic oscillators, because there are different ways to amplify and filter. Some of the different circuits are:

• Armstrong oscillator
• Hartley oscillator
• Colpitts oscillator
• Clapp oscillator
• Delay line oscillator
• Pierce oscillator (crystal)
• Phase-shift oscillator
• RC oscillator (Wien Bridge and "Twin-T")
• Cross-coupled LC oscillator
• Vackář oscillator
• Opto-Electronic Oscillator.

Relaxation oscillator

A relaxation oscillator produces a non-sinusoidal output, such as a square, sawtooth or triangle wave. It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a nonlinear trigger circuit (a latch, Schmitt trigger, or negative resistance element) that periodically charges and discharges the energy stored in the storage element thus causing abrupt changes in the output waveform.

Square-wave relaxation oscillators are used to provide the clock signal for sequential logic circuits such as timers and counters, although crystal oscillators are often preferred for their greater stability. Triangle wave or sawtooth oscillators are used in the timebase circuits that generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and television sets. In function generators, this triangle wave may then be further shaped into a close approximation of a sine wave.

Ring oscillators are built of a ring of active delay stages. Generally the ring has an odd number of inverting stages, so that there is no single stable state for the internal ring voltages. Instead, a single transition propagates endlessly around the ring.

Types of relaxation oscillator circuits include:

• multivibrator
• ring oscillator
• delay line oscillator
• rotary traveling wave oscillator.

See also

• Voltage-controlled oscillator
• Injection locked oscillator
• Numerically-controlled oscillator
• Opto-Electronic Oscillator
• Phase-locked loop
• Barkhausen stability criterion

References

• E. Rubiola, Phase Noise and Frequency Stability in Oscillators Cambridge University Press, 2008. ISBN 978-0-521-88677-2.

External links

• Howstuffworks: oscillator.
• Oscillator Oddities.
• Tutorial on Precision Frequency Generation.

• Oscillators