Where extra high stability is required for, say, laboratory standards or in quasi-synchronous wide area coverage systems, oscillators utilizing the atomic resonances of substances like caesium and rubidium, although expensive, may be employed.
Caesium oscillators are used to provide standard frequencies such as 1, 5 and 10 MHz with accuracies of ±7× 10−12 over a temperature range of 0 to 50æC with a long-term stability of 2× 10−12.
Rubidium oscillators are used to provide secondary standards and in some quasi-synchronous radio systems. Their accuracy is less than that of caesium, the long-term drift being of the order of 1× 10−11 per month.