Null point elimination using biphase states in a direct conversion vital signal detection radar

A novel noncontact heartbeat and respiration monitoring system that can eliminate the null point in the detection of vital signal is reported. The new type of vital signal radar is composed of a variable-gain low-noise amplifier, direct conversion mixer, biphase phase shifter, and local oscillator, which are all homemade circuits constructed with Si bipolar transistors. Moreover, the radar consumes less than 20 mA under a 3 V biasing voltage. A 2.4 GHz experimental prototype was fabricated to verify the proposed null point elimination method. Measurements show good agreement with the theoretical prediction that the radar can clearly detect heartbeat and respiration signals at any point within the detectable range of the radar. The proposed radar system was shown to be a low-power and cost-effective solution for vital signal detection without the problem of null point.