Frequency-Offset Self-Injection-Locked Radar With Digital Frequency Demodulation for SNR Improvement, Elimination of EMI Issue, and DC Offset Calibration

The proposed frequency-offset self-injection-locked (FOSIL) radar combines two independent SIL radar front-ends with up/down-mixers. The transmit frequency can be held constant by properly adjusting the injection phases and amplitudes of two oscillators. Therefore, electromagnetic interference (EMI) in the monitoring of a moving subject using conventional SIL radars is eliminated, and multiple FOSIL radars can be operated simultaneously within a limited bandwidth. In this work, one RF mixer with digital frequency demodulation is used to extract the motion of the target relative to the radar, reducing the system complexity and eliminating the I/ Q imbalance and dc offset calibration. The proposed system substantially improves the SNR over that of the conventional SIL radar by 28 dB, by mitigating the flicker noise and the large Doppler shift. In noncontact vital sign sensing experiments, the 5.8-GHz industrial scientific medical (ISM) band FOSIL radar prototype had a sensing range of 8 m, and its frequency swing range was reduced by over 96% without or with clutter signals. Two FOSIL radars with a frequency spacing of 2 MHz can yield exact information on the cardiopulmonary activities of two subjects at the same time in the experimental environment.