A device for controlling frequency synchronization includes a processor for controlling a phase-controlled clock signal to achieve phase-locking with a reference clock signal, and for controlling a frequency-controlled clock signal so as to achieve frequency-locking with the reference clock signal. The processor is also configured to monitor a deviation between the frequency and phase-controlled clock signals, detect a change of circumstances such as temperature changes causing frequency drifting of the frequency-controlled clock signal, and replace or correct the frequency-controlled clock signal with, or on the basis of, the phase-controlled clock signal when both the monitored deviation and the detected change of circumstances show correlation confirming frequency drift of the frequency-controlled clock signal.