This paper reviews the recent developments of a non-contact vital sign (heartbeat and respiration) detection system using low power microwave radar sensor. Two key techniques are employed in this system. The technique of double-sideband transmission and detection replaces the quadrature detection and simplifies the architecture. The other technique of using short wavelength (high frequency carrier) improves the sensitivity while reducing the transmitted power. Theories and experimental results of these techniques will be described. Theory of nonlinear Doppler phase modulation discovered through the use of short wavelength will be presented and its potential applications will be discussed.