Crystal Oscillators, usually abbreviated as XO, are piezoelectric devices that vibrates at a specific frequency in the presence of an electric field. This vibration is then turned into an electrical signal due to the piezoelectric effect. The frequency and other characteristics of the signal are determined by the crystal material and also the cut angle of the crystal itself. Quartz is a common crystal material. The basic XO does not have any method to control the frequency of the signal, and the frequency of the signal will change with temperature based on the intrinsic properties of the crystal material itself.