Different schemes and structure of the accuracy budget in cesium frequency standards

Many systematic effects have been identified, which can shift the frequency of Cs standards from the ideal frequency of the unperturbed hyperfine separation. They limit the accuracy with which the definition of the second is realized to the uncertainty at which they can be evaluated. The list of known systematics is reviewed in this paper in a critical way, by looking at the nonlinear interaction which occurs among the various effects in the design stage. Consideration is given to several different configurations from the point of view of accuracy analysis and the kind of interplay that ties the various systematic effects. It is the author\´s opinion that all these schemes are valid and should be independently perfected. In fact, the quality of the realization of the definition of a fundamental unit is a matter of accuracy, and type B uncertainties-are therefore the limit. Building more standards does not seem to be the correct approach in an effort toward the improvement of the realization of the second. More convincing appears the idea to build and perfect more different types of standards, with a different set of systematics and different interplay among them. In this way the risk of overlooking technological "phase lock" and undesired human coherence should be minimized, and the independence of different realizations of the second best guaranteed.<>