Single-channel receiver limitations in Doppler radar measurements of periodic motion

Periodic motion, such as that resulting from cardiopulmonary activity can be measured by direct-conversion microwave Doppler radar. In a direct-conversion receiver, motion is measured as phase modulation, and the baseline phase relationship between the received signal and local oscillator signal for a given position has a significant effect on the demodulation sensitivity, resulting in optimum and null case extreme target positions. Presented here is a mathematical analysis of this target-position sensitivity verified with measurements using a custom compact Doppler radar transceiver with separate quadrature outputs. The results indicate that increased error in respiration and heart rate measurements can be expected for null case positions due to significant changes in mathematical conditions as well as reduced sensitivity, and that these limitations can be accommodated using two receiver channels in quadrature.