Electronic oscillator :
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. There are a few types of harmonic oscillators.
The basic form of a harmonic oscillator is an electronic amplifier with an electronic filter connected in the 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.
Capacitive-inductive oscillators also known as LC oscillators are built by a tank circuit, which oscillates by charging and discharging a capacitor through an inductor and an active negative resistance circuit that compensates the internal LC losses. 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.
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 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.[citation needed] 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
- Vackar oscillator
- Opto-Electronic Oscillator.
Relaxation oscillator :
Main article: 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 trigger circuit (a latch, Schmitt trigger, negative resistor, etc.) that periodically charges/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.
. In a practical circuit, the actual oscillation frequency will be slightly different from the value provided by this formula because of stray capacitance and inductance, internal losses (resistance), and the loading of the tank circuit by the tickler coil.
