  Many modern large-scale facilities, like European X-ray Free Electron Laser
(E-XFEL), require precise synchronisation, often down to femtosecond level.
Even a very short interruption or an excessive glitch in the reference signal
might break the precise time relations between subsystems. In such event, a
time-consuming resynchronization process is required that renders the facility
not available for the users until it is completed. Therefore, such events are
highly undesirable.
  In this paper, we present an autonomous redundancy solution for the
European-XFEL's master oscillator that will guarantee a continuous delivery of
the high-quality reference signal even in case of most of the potential
failures. The concept and implementation are presented, as well as results from
testing in the laboratory environment.
