In this study, we propose a novel RF sensor system to enhance
both sensitivity and stability using a free-running voltagecontrolled
oscillator (VCO). The oscillation circuit of the proposed
sensor system is realized by a planar resonator, which
functions as a positive-feedback element, as well as a near-field
radiator to sense vital signals. A surface acoustic wave filter
and a power detector are used to increase the sensitivity of the
system, as well as to transform the frequency variation into a
voltage waveform. In addition, the relation of the frequency deviation
of the oscillator and the Q value of the resonator was
analyzed to enhance the system sensitivity [10].