Bandwidth of non-contact vital sign detection with a noise suppression phase locked loop

Abstract

In a dual-carrier vital sign detection system, we have designed a noise suppression scheme that uses phase locked loop (PLL) to automatically suppress the noise induced by range correlation and transmission paths. The system uses two microwave carriers at 5.6 and 5.68 GHz generated by two phase locked signal generators to extract the noise and vital sign respectively. The feedback microwave signals are mixed with local 5.68-GHz signal to transfer to the vial sign signal and low frequency intermediate frequency (IF) signal. When the IF signal corresponding to 5.68 GHz microwave signal is locked to a highly stable low noise reference, the noises of IF signal and vital sign signal are suppressed as their corresponding microwave signals are highly correlated. In this system, the noise suppression performance is related to the bandwidth of the PLL, which needs to be carefully designed. Through the theoretical analysis, initial bandwidth is chosen to be 200 Hz. Then the charge pump current is changed to adjust the bandwidth and the corresponding noise suppression performance is evaluated using experiments. The results show the system with a charge pump current 0.625 mA, which corresponds to about 50 Hz bandwidth, exhibits a better noise performance. In addition, at different bandwidth, the vital sign detection system is compared with a design scheme with unlocked PLL and demonstrates superior performance at all bandwidths.