An oscillator in which the frequency is controlled by a piezoelectric crystal. Note 1: A crystal oscillator may require controlled temperature because its operating frequency is a function of temperature. Note 2: Types of crystal oscillators include voltage-controlled crystal oscillators (VCXO,) temperature-compensated crystal oscillators (TCXO,) oven-controlled crystal oscillators (OCXO,) temperature-compensated-voltage controlled crystal oscillators (TCVCXO,) oven-controlled voltage-controlled crystal oscillators (OCVCXO,) microcomputer-compensated crystal oscillators (MCXO,) and rubidium crystal oscillators (RbXO. )