Abstract: Interfacing between radio units in a base station in a mobile communication system may use synchronized clocks. A controller device has a tracking clock circuit for generating a transmit clock, the tracking clock circuit comprising a clock input for receiving a reference clock and a sync input for receiving an external synchronization signal. A multiplying phase locked loop generates the transmit clock in dependence on the reference clock and a divider output of a controllable divider coupled to the transmit clock. A tracking loop has a phase detector coupled to the sync input and the divider output for detecting a phase error between the external synchronization signal and transmit clock, and a phase control circuit for generating a phase control signal based on the phase error, the phase control signal being coupled to a control input of the controllable divider for adapting the division function.