Tuesday 19 June 2012

Electronic oscillator


An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave.[1][2] They are widely used in many electronic devices. Common examples of signals generated by oscillators include signals broadcast by radioand television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.[1]
Oscillators are often characterized by the frequency of their output signal: an audio oscillatorproduces frequencies in the audio range, about 16 Hz to 20 kHz.[2] An RF oscillator produces signals in the radio frequency (RF) range of about 100 kHz to 100 GHz.[2] A low-frequency oscillator (LFO) is an electronic oscillator that generates 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 calledinverters.
There are two main types of electronic oscillator: the linear or harmonic oscillator and the nonlinear or relaxation oscillator.[2]

Contents

  [hide

[edit]Linear oscillator

Block diagram of a linear oscillator; an amplifier A with its output vo fed back into its input vf through a filterβ(jω).
The harmonic, or linear, oscillator produces a sinusoidal output.[2]
The basic form of a linear oscillator is an electronic amplifier connected in a feedback loopwith its output fed back into its input through a frequency selective electronic filter to providepositive feedback. 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.
Linear oscillator circuits can be classified according to the type of frequency selective filter they use in the feedback loop:[2]
  • In a crystal oscillator circuit the filter is a piezoelectric crystal (commonly a quartz crystal).[2] The crystal mechanically vibrates as aresonator, and its frequency of 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 and quartz clocks. Crystal oscillators often use the same circuits as LC oscillators, with the crystal replacing the tuned circuit;[2] the Pierce oscillatorcircuit is commonly used. Quartz crystals are generally limited to frequencies of 30 MHz or below.[2] Surface acoustic wave (SAW) devices are another kind of piezoelectric resonator used in crystal oscillators, which can achieve much higher frequencies. They are used in specialized applications which require a high frequency reference, for example, in cellular telephones.
In addition to the feedback oscillators described above, which use two-port amplifying active elements such as transistors and op amps, linear oscillators can also be built using one-port devices with negative resistance,[2] such as magnetron tubes, tunnel diodes and Gunn diodes. In these oscillators, a resonator, such as an LC circuit, crystal, or cavity resonator, is connected across the negative resistance device, and a DC bias voltage is applied to supply energy. The negative resistance of the active device can be thought of as cancelling the (positive) effective loss resistance of the resonator and permitting a sustained oscillation. These circuits are frequently used for oscillators at microwave frequencies.
These are some of the many linear oscillator circuits:

[edit]Relaxation oscillator

A nonlinear or relaxation oscillator produces a non-sinusoidal output, such as a squaresawtooth or triangle wave. It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a nonlinear switching circuit (a latchSchmitt 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:

No comments:

Post a Comment