The Realization of the Second
A primary cesium beam frequency standard serves to realize the unit of time, the Second, in accordance with the international definition as formulated at the XIII General Conference of Weights and Measures in 1967. The basic design of a cesium standard is shown in Fig. 1. The cesium beam emerges from an oven into a vacuum, passes a first state selecting magnet, traverses a Ramsey type cavity where it interacts with a microwave signal derived from a slave oscillator. The microwave signal changes the distribution of states in the atomic beam which is then analyzed and detected by means of the second state selector magnet and the atom detector. The detector signal is used in a feedback loop to automatically keep the slave oscillator tuned. The line-Q is determined by the interaction time between the atoms and the microwave cavity. Thus a beam of slow atoms and a long cavity leads to a high line-Q. Commercial devices which for obvious reasons are restricted in total size have line-Q’s of a few 107, whereas high performance laboratory standards with an overall device length of up to 6-m feature line-Q’s of up to 3 × 108.
KeywordsMicrowave Power Primary Standard Beam Optic Microwave Signal Frequency Bias
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