IQ demodulator for DC coupled Doppler radar

Abstract

The work in this thesis demonstrates the possibility of using DC coupled signal in Doppler radar systems. The microwave radar system can detect the periodic motion which can extend to the cardiopulmonary activity of human beings. One of the challenges in Doppler radar systems for physiological monitoring is a large DC offset in the baseband outputs. This DC offset is largely resulting from the parasitic signal leakage between radar ports. Since the physiological signals of interest include frequency content near DC, it is not desirable to simply AC couple radar outputs. While AC coupling effectively removes DC offset, it also introduces a large time delay and distortion. This thesis present the first DC coupled Doppler radar design and measurements. The DC coupling is achieved by using a mixer with high LO to RF port isolation, resulting in very low radar DC offset, on the order of mW. The printed circuit board (PCB) quadrature (IQ) demodulator was designed and fabricated using the high isolation mixers. The IQ demodulator was tested in the radar system to detect the motion of the moving target. DC offset in radar system was analyzed and measured. Two quadrature radar systems were tested for comparison (PCB system and coaxial system). Due to the lower LO leakage in the PCB system, significantly reduced DC offset was measured for both I and Q channels. The DC coupled signals from the PCB radar system were successfully detected before saturation of LNA. The DC coupled and AC coupled data were compared. The DC coupled results show great advantages of less signal distortion and more accurate rate estimation. This is the first reported DC coupled Doppler radar measurement result.