A frequency detector (200) and method therein for measuring and tuning a frequency of a controlled oscillator are disclosed. The frequency detector (200) comprises a pulse generator (210) for generating sampling pulses; a sample circuitry (220) for sampling output states of the controlled oscillator (180); and a digital processing unit (230). The sample circuitry (220) is configured to sub-sample the output states of the controlled oscillator (180) at two or more sampling frequencies, and all sampling frequencies are lower than the frequency of the controlled oscillator. The digital processing unit (230) is configured to calculate a frequency offset of the oscillator based on the sampled states and generate a control signal based on the frequency offset to tune the frequency of the oscillator.