Atomic frequency standards and clocks

The principles of operation of atomic frequency standards are described. Details are given of their physical construction. Their characteristics as well as their availability, principal uses and impact are specified. The basic concepts involved in atomic frequency standards and the terminology commonly used in this field are summarized. The role of quartz crystal oscillators which are slaved to active and passive atomic frequency standards is emphasized. The traditional atomic frequency standards are reviewed, i.e. the hydrogen maser, the caesium beam tube and the passive rubidium cell frequency standards.