  A compact, continuous-wave, mmWave radar sensor is developed for non-contact
detection of micron-scale motions. This board-integrated radar system consists
of a pair of mmWave transmitter and receiver, two series-fed microstrip patch
arrays, an IF subsystem, and a microcontroller. Working at 60-GHz frequency,
this super-heterodyne, digital-IF radar sensor exhibits an agile sensitivity
and a robust anti-noise performance. Assisted by a gradient-descent DC offset
estimation and a phase demodulation algorithm based on extended differential
and cross-multiply, a 45-{\mu}m pendulum swing can be experimentally detected
with a 20.6-dB SNR, verifying its strong ability in Doppler micro-motion
detections.
