http://en.wikipedia.org/wiki/Electronic_oscillator
Electronic oscillator
Oscillators are often characterized by the frequency of their output signal:An electronic oscillator is an electronic circuit
that produces a repetitive, oscillatingelectronic signal, often a sine wave or a square wave.[1][2] Oscillators
convert direct current(DC) from a power supply to an alternating current 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 andquartz clocks, and the sounds produced
by electronic beepers and video games.[1]
- An audio oscillator produces 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 called inverters.
There are two main types of electronic oscillator: the linear or harmonic oscillator and
the nonlinear or relaxation oscillator.[2][3]
the nonlinear or relaxation oscillator.[2][3]
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[edit]Linear oscillator
Feedback oscillatorThe harmonic, or linear, oscillator produces a sinusoidal output.[2][3] There are two types:
[edit]
The most common form of linear oscillator is an electronic amplifier such as a transistor or op amp connected
in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide
positive feedback. When the power supply to the amplifier is first switched on, electronic noise in the circuit
rovides a signal to get oscillations started. The noise travels around the loop and is amplified and filtered until very quickly
it becomes asine wave at a single frequency.
in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide
positive feedback. When the power supply to the amplifier is first switched on, electronic noise in the circuit
rovides a signal to get oscillations started. The noise travels around the loop and is amplified and filtered until very quickly
it becomes asine wave at a single frequency.
Feedback oscillator circuits can be classified according to the type of frequency selective filter they use
in the feedback loop:[2][3]
in the feedback loop:[2][3]
In an RC oscillator circuit, the filter is a network of resistors and capacitors.[2][3] RC oscillators are mostly
used to generate lower frequencies, for example in the audio range. Common types of RC oscillator circuits
are the phase shift oscillator and the Wien bridge oscillator.
In an LC oscillator circuit, the filter is a tuned circuit (often called a tank circuit) consisting of an inductor (L)
and capacitor (C) connected together.[2][3] Charge flows back and forth between the capacitor's plates through the
inductor, losses in the tank circuit, but the amplifier compensates for those losses and supplies the power for
the output signal.
LC oscillators are often used at radio frequencies,[2] 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[2] and Clapp circuits.
In a crystal oscillator circuit the filter is a piezoelectric crystal (commonly a quartz crystal).[2][3]
The crystal mechanically vibrates as a resonator, 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 oscillator
circuit 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.
used to generate lower frequencies, for example in the audio range. Common types of RC oscillator circuits
are the phase shift oscillator and the Wien bridge oscillator.
In an LC oscillator circuit, the filter is a tuned circuit (often called a tank circuit) consisting of an inductor (L)
and capacitor (C) connected together.[2][3] Charge flows back and forth between the capacitor's plates through the
inductor, losses in the tank circuit, but the amplifier compensates for those losses and supplies the power for
the output signal.
LC oscillators are often used at radio frequencies,[2] 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[2] and Clapp circuits.
In a crystal oscillator circuit the filter is a piezoelectric crystal (commonly a quartz crystal).[2][3]
The crystal mechanically vibrates as a resonator, 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 oscillator
circuit 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.
[edit]Negative resistance oscillator
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 (two terminal) devices with
negative resistance,[2][3] such as magnetron tubes, tunnel diodesand Gunn diodes. Negative resistance oscillators
are often used at high frequencies in themicrowave range and above, since at these frequencies feedback
oscillators perform poorly due to excessive phase shift in the feedback path.
In negative resistance oscillators, a resonant circuit, such as an LC circuit, crystal, or cavity resonator, is connected across a device with
negative differential resistance, and a DC bias voltage is applied to supply energy. A resonant circuit by itself is almost an oscillator;
it can store energy in the form of electronic oscillations if excited, but because it has some internal resistance or other losses the oscillations
are damped and decline to zero. The negative resistance of the active device cancels the (positive) internal loss resistance in the resonator,
in effect creating a resonator with no damping, which generates spontaneous continuous oscillations at its resonant frequency.
negative differential resistance, and a DC bias voltage is applied to supply energy. A resonant circuit by itself is almost an oscillator;
it can store energy in the form of electronic oscillations if excited, but because it has some internal resistance or other losses the oscillations
are damped and decline to zero. The negative resistance of the active device cancels the (positive) internal loss resistance in the resonator,
in effect creating a resonator with no damping, which generates spontaneous continuous oscillations at its resonant frequency.
[edit]Circuits
These are some of the many linear oscillator circuits:
- 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.
- Tri-tet oscillator
- Robinson oscillator
[edit]Relaxation oscillator
Main article: relaxation oscillator
A nonlinear or relaxation oscillator produces a non-sinusoidal output, such as a square, sawtooth or triangle wave.[3] It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a nonlinear switching 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:
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