In this paper, affect of unevenness of Frequency Modulated Continuous Wave radar is observed and analyzed in simulation. Through the observation, it is concluded that the waveform can be viewed as Amplitude Modulated signal, and that this “modulation” can affect the frequency spectrum of the radar system output. In order to calibrate this modulation, AM demodulation and envelope detection is studied and applied in the simulation. After confirming envelope detection helps resolve the problem, the same method is applied in real measured data. In the progress, limitations of the envelope detection for real data is examined and improved.