Improving Frequency Sweep Linearity in FMCW Vital Sign Radar Using Frequency Locking

Conventional voltage-controlled oscillator (VCO) based frequency-modulated continuous-wave (FMCW) vital sign radars often suffer from nonlinear frequency sweeps. The nonlinearity distorts the beat frequency of the radar's baseband output signal and lead to inaccurate distance measurements. To address this issue, this work developed a novel FMCW vital sign radar that incorporates a frequency-locked loop (FLL). The frequency locking mechanism of the FLL effectively mitigates the frequency sweep nonlinearity of a VCO, thereby improving the accuracy of distance detection. Experimental results demonstrated that incorporating FLL into an FMCW vital sign radar effectively reduced the distance detection error by up to 9.5% for a 2-meter range.