  The requirements on the stability of the frequency reference in the Square
Kilometre Array (SKA), as a radio astronomy interferometer, are given in terms
of maximum accepted degree of coherence loss caused by the instability of the
frequency reference. In this paper we analyse the relationship between the
characterisation of the instability of the frequency reference in the radio
astronomy array and the coherence loss. The calculation of the coherence loss
from the instability characterisation given by the Allan deviation is reviewed.
Some practical aspects and limitations are analysed.
