Bio-signal sensing system has deeply rooted at bio and information technology. In the hospital, bio-sensing system is actually used to detect a patient``s basic bio condition such as heart rate, respiration, blood pressure and so on. The study about sensing of respiration and heart rate places especially the focus on remote sensing. With this tendency, the part of remote sensing technology has been studied by many groups, .
Doppler radar system is very useful as sensor which can detect moving objects. The principle of operation for this sensing system is very simple. If the target has velocity toward the direction of the radar, the reflected signal on the surface of moving target is shifted in frequency in proportion to the velocity of target toward the direction of the radar. If the surface of moving target is the skin and muscle near the heart, heartbeats and respiration will be sensed by their periodic back and forth movement.
This highly sensitive, real time wireless sensor has used direct conversion method. The reflected signal from the object is mixed with the reference signal of transmitter to produce an output signal with its magnitude proportional to the phase difference between them. The dielectric constant of skin and muscle are 31.29 and 42.764 at 10GHz. The conductivities of two mediums are 8.0138 and 10.626 (S/m) at 10GHz. Since the skin and muscle have a high dielectric constant  at 10GHz frequency, the skin depth of incident signal is very short on the surface of them. Because the high dielectric constant of body influence on the polarization of the reflected wave, CP antennas that have the opposite circular polarization play an important role to improve sensitivity.
The efficiency of the remote sensor is verified by the test of speaker which has a regular vibration and by the direct experimentation for human bio-signal.
Doppler radar is consisted of several devices such as DRO, LNA, mixer, Wilkinson power divider, CP antenna, a...