A method of detecting a vital sign comprising at least one of a heart rate and a respiratory rate of a subject is provided. In one aspect, the method includes transmitting a radio frequency signal towards the subject； and receiving a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate to form the reflected signal. The method also includes mixing the reflected signal with a first reference signal； and providing a vital sign carrying signal based on the mixing to a first input of a phase or frequency comparator. The method further includes generating an adjustable second reference signal and providing the reference signal to a second input of the phase or frequency comparator； and generating an output signal, by the phase or frequency comparator. The method includes varying at least one of a phase and a frequency of the adjustable second reference signal based on the output signal to track a phase or frequency of the vital sign carrying signal.
