A system adapted to analyse free play in a rotating shaft, comprising: a continuous wave, CW, signal source for producing an CW signal directed at a surface of the rotating shaft, the CW signal being produced at a first frequency, a receiving element for receiving a signal reflected from the rotating shaft's surface when the CW signal hits this surface, and a down-converting frequency mixer for converting the received signal into a signal of a second frequency lower than the first frequency, the frequency mixer being configured to produce an output signal representing an AM component of the received signal that represents movement of the rotating shaft's surface, wherein the frequency mixer comprises a local oscillator, LO, and a port for receiving a coherent LO injection from the CW signal source to produce the output signal representing the AM component, wherein an amplitude of the output signal, corresponding to an amplitude of the amplitude-modulated, AM, component of the received signal represents movement of the rotating shaft's surface, wherein the output signal of the mixer represents an envelope of the received signal, and further comprising: a frequency filter coupled to the output of the frequency mixer for filtering the output signal of the mixer to produce a filtered signal within a predetermined bandwidth; and a signal analyzer that analyzes the free play in the rotating shaft based on the amplitude modulated component of the received signal in the filtered signal. a continuous wave, CW, signal source for producing an CW signal directed at a surface of the rotating shaft, the CW signal being produced at a first frequency, a receiving element for receiving a signal reflected from the rotating shaft's surface when the CW signal hits this surface, and a down-converting frequency mixer for converting the received signal into a signal of a second frequency lower than the first frequency, the frequency mixer being configured to produce an output signal representing an AM component of the received signal that represents movement of the rotating shaft's surface, wherein the frequency mixer comprises a local oscillator, LO, and a port for receiving a coherent LO injection from the CW signal source to produce the output signal representing the AM component, wherein an amplitude of the output signal, corresponding to an amplitude of the amplitude-modulated, AM, component of the received signal represents movement of the rotating shaft's surface, wherein the output signal of the mixer represents an envelope of the received signal, and further comprising: a frequency filter coupled to the output of the frequency mixer for filtering the output signal of the mixer to produce a filtered signal within a predetermined bandwidth; and a signal analyzer that analyzes the free play in the rotating shaft based on the amplitude modulated component of the received signal in the filtered signal. a frequency filter coupled to the output of the frequency mixer for filtering the output signal of the mixer to produce a filtered signal within a predetermined bandwidth; and a signal analyzer that analyzes the free play in the rotating shaft based on the amplitude modulated component of the received signal in the filtered signal.