Crystal Oscillator

http://www.GNoorPCtronics.com/updates/OSCheck.pdf

An oscillator is an electronic device used for the purpose of generating a signal. Oscillators are found for example in computers, wireless receivers and transmitters, and audio-frequency equipment, and music synthesizers. Every oscillator has at least one active device.This active device acts as an amplifier.There are many types of oscillator devices, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback. Crystal oscillators are oscillators where the primary frequency determining element is a quartz crystal. Because of the inherent characteristics of the quartz crystal the crystal oscillator may be held to extreme accuracy of frequency stability. Crystal oscillators are usually, fixed frequency oscillators where stability and accuracy are the primary considerations.Temperature compensation may be applied to crystal oscillators to improve thermal stability of the crystal oscillator. The value of feedback resistor and capacitor determines the stability of oscillation with respect to crystal. Use formula R:= 1/(2Pi*F*C) Where F is frequency, C is capacitance in Farad.
The selection of feedback resistor and capacitor is very critical, it is often found that oscillator does not work.