6-2
When an atomic system changes energy from an exited state to a lower energy state, a photon is emitted.  The photon frequency n is given by Planck’s law
where E2 and E1 are the energies of the upper and lower states, respectively, and h is Planck’s constant.  An atomic frequency standard produces an output signal the frequency of which is determined by this intrinsic frequency rather than by the properties of a solid object and how it is fabricated (as it is in quartz oscillators). The properties of isolated atoms at rest, and in free space, would not change with space and time.  Therefore, the frequency of an ideal atomic standard would not change with time or with changes in the environment.  Unfortunately, in real atomic frequency standards: 1) the atoms are moving at thermal velocities, 2) the atoms are not isolated but experience collisions and electric and magnetic fields, and 3) some of the components needed for producing and observing the atomic transitions contribute to instabilities.
Atomic Frequency Standard Basic Concepts
   Atomic frequency standards must be understood in terms of the concepts of quantum mechanics.  The properties of simple atomic systems cannot assume arbitrary values.   For example, the energies of the bound states of an atomic system are constrained to discrete values called energy levels.  When an atomic system changes energy from an excited state to a state with lower energy, it emits a quantity of electromagnetic energy called a photon, the frequency of which is determined by the energy difference between the two states, in accordance with Planck’s law, shown above.
   Atomic systems can be isolated from unwanted perturbations, which result in small sensitivities to temperature, pressure, and other environmental conditions.  The low level of interaction also results in extremely sharp resonance features, and reduces errors due to imperfections in the electronics.  All atoms of an element are identical, and atomic properties are time invariant, which makes it possible to build very stable devices.
   Atomic frequency standards are categorized in several ways; most often, they are referred to by the type of atom: hydrogen, rubidium, or cesium.  Actually, these three devices are based on the same type of atomic interaction, but there are great practical differences in their implementation.  Some atomic frequency standards, called oscillators, are active, in which case the output signal is derived from the radiation emitted by the atom.  Others are passive; the atoms are then employed as a discriminator to measure and control the frequency of an electronic oscillator, such as a quartz oscillator.  The third classification follows the method of interaction.  In atomic beams, the atoms are observed "on the fly"; they pass through the interaction region and are not used again.  In contrast, storage devices contain some type of cell that holds the atoms to be observed indefinitely (ideally).


S. R. Stein and J. R. Vig, "Frequency Standards for Communications," U. S. Army Laboratory Command Research and Development Technical Report SLCET-TR-91-2 (Rev. 1), October 1991, AD-A243211.  This report is a reprint of a chapter, "Communications Frequency Standards," in The Froehlich/Kent Encyclopedia of Telecommunications, Vol. 3, pp. 445-500, Marcel Dekker, Inc., 1992.