Crystal oscillators are units where the primary frequency determining element is a quartz crystal. When a crystal is placed into an amplifier circuit, a small amount of energy or voltage is fed back into the crystal, which causes it to vibrate. The inherent high Q characteristic of quartz crystals enables the crystal oscillators to maintain high frequency accuracy and stability over temperature. Since the frequency-temperature characteristic of the quartz crystal is predictable, temperature compensation may be applied to crystal oscillators to further improve their thermal stability. Specific output logics, such as TTL, HCMOS, ECL, PECL, LVDS, Sine, or Clipped-Sine are available on oscillators from ILSI America.