A detector includes an oscillation source, a frequency multiplier, a transceiver and a demodulator. The oscillation source generates a first injection signal with a first frequency. The frequency multiplier receives the first injection signal, outputs an output signal and receives a second injection signal with a second frequency. The frequency multiplier uses injection locking to lock a frequency of the output signal at a multiple of the first frequency, and uses injection pulling to pull the frequency of the output signal to the second frequency. The transceiver transmits the output signal and receives a received signal with a third frequency for updating the second injection signal. The demodulator performs a demodulation operation according to the output signal so as to generate a displacement signal.