Abstract: A digital phase locked loop keeps an output signal exactly in phase and frequency with a reference signal. An oscillator has a plurality of delay elements which are connected to one another in a loop to produce the output signal. Each of the delay elements has a delay which is controlled by one of digital control signal sets supplied from a controller. A total delay of the delay elements decides a frequency of the output signal. A phase comparator is connected to the oscillator to compare the output signal with the reference signal in phase and frequency to produce an error signal. A controller is connected between the phase comparator and the oscillator to produce the digital control signal sets in response to the error signal.