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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.
Types of electronic oscillator
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 with the output attached to an electronic filter, and the output of the filter attached to the input of the amplifier, in a feedback loop. 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, being filtered and re-amplified until it increasingly resembles the desired signal.
A piezoelectric crystal (commonly quartz) may take the place of the filter to stabilise the frequency of oscillation, this is called a crystal oscillator.
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
Main article: relaxation oscillatorA relaxation oscillator produces a non-sinusoidal output, such as a square wave or sawtooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, 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.
Types of relaxation oscillator circuits include:
Comparison between relaxation and LC oscillators
Although the two types of circuits above produce oscillations, there are significant differences between them; they are revealed in the text below.
Similarities:
- Both the oscillating circuits contain (at least one) accumulating element that acts either as a source or as an integrator.
Differences:
- A relaxation oscillator consists of only one accumulating element while an LC oscillator consists of two accumulating elements.
- The relaxation accumulator is either flow or pressure like; the LC accumulators are heterogeneous (the one is flow-like and the other is pressure-like).
- A relaxation oscillator stores only one kind of energy (usually potential) in the accumulator while an LC oscillator stores two opposite kinds of energy (kinetic and potential) in the two accumulators.
- In a relaxation oscillator the energy is wasted while in an LC oscillator it is treasured temporarily in an additional accumulator with the purpose of future usage. That is why LC oscillators are more economical than relaxation ones.
- Resonance phenomenon does not exist in a relaxation circuit; it can be observed only in an LC tank.
- In a relaxation oscillator the energy moves only in one direction (source -> accumulator -> load) while in an LC oscillator the energy changes periodically its direction (it circulates between the two accumulating elements).
- The shape of a relaxation oscillation is peaked, angular while the shape of an LC oscillation is rounded (sinusoidal). The reason of that is that at the peaks the source of a relaxation oscillator changes its output quantity with a maximum rate while the "source" of an LC oscillator (charged accumulator) does not change its output quantity.
- The shape of the relaxation oscillation can be asymmetrical (the increase and the decrease can have different durations) while the shape of the LC oscillation is precisely symmetrical.
Oscillator implementations
Inductive oscillators also known as LC oscillators are built of an tank circuit, which oscillates by charging and discharging a capacitor through an inductor. These oscillators are typically used when a tunable precision frequency source is necessary, such as with radio transmitters and receivers. Most LC oscillators use off-chip inductors. On-chip inductors suffer large resistive losses, so that the Q-factor of the resulting tank circuit is generally less than 10. As processes have made larger numbers of metal layers available (allowing designers to distance the inductor metal layer from the resistive substrate), on-chip inductors have become more useful.
Crystal oscillators are piezoelectric quartz crystals that mechanically vibrate between two slightly different shapes. Crystals have very high Q-factor, and can only be tuned within a very small range of frequencies. 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.
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.
See also
- Electronic circuit
- Voltage-controlled oscillator
- Injection locked oscillator
- Numerically-controlled oscillator
- Opto-Electronic Oscillator
- Phase-locked loop
References
- E. Rubiola, Phase Noise and Frequency Stability in Oscillators Cambridge University Press, 2008. ISBN 978-0-521-88677-2.