An oscillator is a free-running electronic circuit for generating sinusoidal frequencies. The oscillator principle is based on the principle of positive feedback, whereby part of the resonance energy is fed back to a resonant circuit via feedback in order to maintain resonance.
Such an oscillator circuit can oscillate only if the cascaded gain and damping in the amplifier and in the feedback is greater than "1", and if the phase of the feedback signal is in phase with the oscillating frequency
The Construction of OscillatorsThe resonant circuit of an oscillator may be a resonant circuit or a crystal circuit that is caused to oscillate by an active electronic component, an electron tube, or a transistor. The oscillation state is maintained by feedback.
In LC oscillators, the feedback can be inductive coupling or capacitive coupling. In this case, the feedback signal can be tapped via a transformer coupling between the oscillating circuit coil and a decoupling coil, but equally the oscillating circuit coil can be tapped or the feedback signal can be tapped between two capacitors. The feedback signal can be fed in the collector circuit, at the emitter, or at the base. The various feedback circuits are known as oscillating circuits after their inventors as Meissner oscillator, Colpitts oscillator and Hartley oscillator.
The frequency of the oscillator is determined by the resonant frequency of the oscillating circuit and can be changed by control voltages, adjustable variable capacitors or trim capacitors.
Oscillators with high frequency accuracy and stability
Ceramic resonators and quartz crystals can be used in oscillators instead of the resonant circuit. The frequency stabilities that can be achieved with these are several orders of magnitude higher than those using resonant circuits. To further increase frequency stability, the crystal oscillators can be temperature compensated, such as the Temperature Compensated Crystal Oscillator( TCXO), or use temperature controlled ovens, such as the Oven Controlled Crystal Oscillator( OCXO).
In addition to the oscillators mentioned above, there are also MEMS resonators, which have better aging characteristics compared to quartz oscillators. In addition, there is the Voltage Controlled Oscillator( VCO) with higher accuracy and stability as Voltage Controlled Crystal Oscillator( VCXO). In addition, there is Digital Controlled Crystal Oscillator( DCXO), a digitally controlled crystal oscillator( XO) with the highest frequency accuracy.