Abstract: In digital systems clock signals are generated from an external clock reference signal for the purpose of synchronization with the aid of a phase-control loop 1 incorporating a frequency-controllable oscillator 2. In practice synchronization problems occur due to the phase inaccuracy of these clock signals relative to the clock reference.By a two-level control of the oscillator frequency in a plurality of cycles, located within a period of the clock reference signal, using a pulse-width modulator 12, the phase accuracy of a clock signal to be derived from the oscillator output signal is improved.In a preferred embodiment the loop 1 comprises: storage means 16, control means 17 and selection means 18 for providing in accordance with a given phase control strategy a phase-accurate oscillator output signal, this accuracy already being obtained on a small time scale.