Frequency control

The subject of frequency control is closely bound up with that of timekeeping where the primary requirement is for a device or system to play the part of a pendulum. At the lower end of the frequency spectrum in the audio range up to several hundred megacycles, the “pendulum” consists most often of a mechanical resonance structure such as a tuning fork or quartz crystal vibrator whose frequency is determined from the classical theory of elasticity. The other major division of the frequency‐control field is occupied by devices that utilize quantum‐mechanical principles for their operation, where the “pendulum” is identified with an atomic or molecular resonance, and where the frequency, consequently, is in the gigacycle range.