A method for analyzing the overall noise performance of vital sign detectors is introduced. For demonstration purpose, the noise analysis on a quadrature direct-conversion vital sign detector is performed. The detector is divided into three sub-systems including RF Front-end amplifiers, mixer with LO, and baseband amplifiers. The noise characteristics of the sub-systems are studied individually and combined to form the overall noise evaluation of the detector. The study on mixer sub-system results in a key design consideration of mixer. The signal-to-noise ratio at the sampler input is defined as the output SNR and is used to measure the overall noise performance of the vital sign detectors. The effect of output SNR on vital sign detection is presented. Tradeoff between output SNR and detection distance is verified by the experimental results.