An oscillator produces signal at a set frequency. They are useful for creating the base signals upon which modulation can be performed to encode information. An Oscillator is also known as a Clock.
Often osciallators are created using an amplifier and a feeback circuit to route some of the amplifiers output signal back into its input. To make this oscialator produce a single frequency output, the feedback circuit must include a filter so feedback is present only in the intended frequency.
Any oscillator's frequency can be variable or fixed.
An RC oscialltor's feedback circuit is made up of Resistors (R) and Capactiors (C). Because of the combination of reacrance and resistance, the RC circuit shifts the phase of the feedback circuit so that the resulting signal, when amplified, reinforces the original signal
An LC oscialltor is made up of an inductor(L) and Capacitor(C) connected in parallel or series to create a resonant circuit. This is sometimes called a tank circuit, as it stores electrical engergy, similar to a flywheel with mechanical energy. LC oscialltors are more stable than RC and can be used with a wider range of frequencies.
A quartz crystal with a voltage applied creates a steady frequency that can be used as an osciallator, this is fare more accurate than an LC circuit.
A Variable Frequency Osciallator (VFO) whose frequency can be adjusted, is used in adjustable tuners for radios. The frequency is alterted by changing the value of one or more components in a feedback circuit. A VFO is often made by having a variable capactior in an LC circuit.
Other Variable Oscillators include a Phase Locked Loop (PLL) oscillator and a Direct Digital Syntehsizer (DDS). The accuracy of a DDS is comparable to a crystal