|A 24-GHz CMOS injection-locked frequency divider (ILFD) based on the push-push oscillator topology is designed and fabricated. It is further treated as the kernel component of a self-injection-locked (SIL) radar sensor for noncontact vital-sign detection. The push-push topology is employed to double the oscillation frequency of the 12-GHz LC oscillator to provide the 24-GHz transmitting signal of the radar sensor, while the direct injection-locked configuration is used to divide the receiving 24-GHz injection signal by 2. Hence, the 12-GHz output of the ILFD can be connected with a differentiator-based envelope detector for vital-sign demodulation. By packaging the developed ILFD chip and connecting it with two antennas, a gain block, and a differentiator-based envelope detector, a 24-GHz SIL radar sensor can be built to detect the physiological chest movement of a human subject. The feasibility of developing a SIL radar sensor in the millimeter-wave range is also demonstrated.