Highly Sensitive Frequency- and Self-Injection-Locked Radar for Precise Vital Sign Detection

Noncontact vital sign sensing usually utilizes highly sensitive Doppler radars. Self-injection-locked (SIL) radars have been studied to have better sensing capabilities than traditional continuous wave (CW) radars. However, certain injection phases may cause degradation of SIL radar's detection capabilities. Besides, most SIL radars require a bulky delay line for frequency demodulation. In this work, a frequency-locked loop (FLL) was incorporated into an SIL radar to form a frequency- and self-injection-locked (FSIL) radar. The FLL can track the phase of the subject under test (SUT), keeping the injection at optimal angles. In addition, using FLL can further enhance radar's sensing capabilities and eliminate the need for delay lines. Experimental results show that the developed 2.3 GHz FSIL radar exhibited great phase tracking and sensing capabilities. The minimum detectable vibration amplitude of the FSIL radar with a transmit power of 10 dBm was as small as 0.1 mm at a detection distance of 1.5 m.