A crystal oscillator, Crystal Oscillator (XO), uses a quartz crystal as the frequency generating component whose high Q factor is used to generate the frequency stable oscillator frequency. Such an oscillator circuit has about a thousand times higher Q and frequency stability compared to an oscillator with LC resonant circuit.
In crystal oscillators, the crystal is the frequency-determining component of the oscillating circuit. Depending on the circuit concept, it can be located in the oscillating circuit, but also as a series resonance in the positive feedback. In this circuit, the crystal forms a very low resistance at series resonance, via which the oscillator frequency is fed back. The resonant circuit thus oscillates at the crystal frequency.
As with any other oscillator, the feedback can start at the collector, base or emitter of the transistor.
To further increase the frequency stability, which is about 5 ppm to 100 ppm (`10^-6`) for the normal crystal oscillator, crystal oscillators are temperature compensated, these crystal oscillators are called Temperature Compensated Crystal Oscillator( TCXO), or they are placed in a heatable package, a small temperature stable oven. These crystal oscillators are called Oven Controlled Crystal Oscillator( OCXO).