A Wall-Clutter Rejection Technique Using Two PLLs and a Phase Controller for Wall-Penetrating FMCW Radar

摘要

This letter proposes a novel frequency-modulated continuous-wave radar to achieve high wall-clutter rejection with a low-order high-pass filter (HPF). The proposed radar has two phase-locked loops (PLLs) and a phase controller. One transmitter PLL generates a chirp signal for transmitting (TX chirp) signal, the other a local-oscillator (LO) PLL generates a chirp signal for mixing (LO chirp), and the phase controller controls a phase of a reference clock entering the transmitter PLL. When the phase controller advances by a half-period of the reference clock, the PLL tracks and locks onto the advanced reference clock, and the TX chirp signal is advanced by a half-period of the reference clock. If longer advanced time is needed, the above-mentioned process (advance, track, and lock) is repeated after PLL locking. In this manner, long advanced time can be achieved with a fine time-resolution. The use of appropriate advanced time decreases the time gap between a wall-reflection signal and the LO chirp signal, and increases the ratio of target beat-frequency to wall beat-frequency. This enables a low-order HPF to fully attenuate wall-clutter. Moreover, this technique decouples the relationship between the wall’s distance and the HPF’s cut-off frequency. The radar was implemented and measured. The wall was located at 1.5 m and the target was located at 3 m. The measured results show that a second-order HPF attenuates by more than 20 dB for the wall beat-frequency signal, while it does not attenuate the target beat-frequency signal.