In this paper, a phase locked oscillator under injection is designed with proximity coupled vital sign sensor at 2.4 GHz. The proposed system is composed of a sensor oscillator with a planar resonator, a voltage controlled oscillator (VCO), and a phase locked loop (PLL) synthesizer. The planar resonator functions not only as a series feedback component to an oscillator but also as a detector which measures respiration and heartbeat signal. The input impedance of the planar resonator varies with the distance between the human body and the planar resonator. This impedance variation changes the oscillation frequency of the sensor oscillator. The sensor oscillator injects its output into the VCO so that the VCO is locked to the sensor oscillator's frequency. Then the PLL applies the control voltage to the VCO to prevent frequency shifting by vital signs. Therefore, the VCO control voltage reflects vital signs. Measurement results prove that this sensor can detect vital sign up to 40 mm away from a subject.