Crystal Oscillator

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.
Now most of oscillator using PLL system because it can work with variable frequency, but still using Crystal oscillator as referency that making PLL stable.

This is a typical example of the type of crystal oscillators which may be used for say converters. Some points of interest on crystal oscillators in relation to figure 1.



The transistor could be a general purpose type with an Ft of at least 150 Mhz for HF use. A typical example would be a 2N2222A.

The turns ratio on the tuned circuit depicts an anticipated nominal load of 50 ohms. This allows a theoretical 2K5 ohms on the collector. If it is followed by a buffer amplifier (highly recommended) I would simply maintain the typical 7:1 turns ratio. I have included a formula for determining L and C in the tuned circuits of crystal oscillators in case you have forgotten earlier tutorials. Personally I would make L a reactance of around 250 ohms. In this case I'd make C a smaller trimmer in parallel with a standard fixed value.

edited from : epanorama.net & electronics-tutorial.com