Continuous-wave (CW) radar sensor has been used for monitoring physiological signals of respiration and heartbeat. Complete respiration pattern monitoring is of vital importance in motion-adaptive cancer radiotherapy which strictly relies on the respiration pattern to generate gating signals and track the tumor motion. However, the conventional AC-coupled radar sensor is subject to signal distortion due to the harmonics generated by nonlinear phase modulation. The distortion problem was often overlooked in the past but it may lead to false demodulation. In this paper, a DC-coupled architecture has been analyzed for complete respiration pattern monitoring. The DC-coupled approach allows the radar sensor to precisely measure movement with stationary moment, while the AC-coupled radar sensor is incapable of doing so. This paper further analyzes the distortion problem in AC-coupled radar sensor and demonstrates that the distortion comes from the loss of harmonic characteristics as the signal goes through the AC-coupled receiver chain. Both analysis and experiments show that the distortion decreases as the target frequency increases. It is shown that distortion-free measurement using AC-coupled radar sensor is also possible based on careful choice of the component parameters, but it results in tradeoffs with settling time, hardware cost, etc.