Optimizing FMCW Radar Architecture for LO Chirp Detected Noise Reduction
FMCW radar is widely used today for precise range-velocity detection. The measurement is performed by generating and radiating a frequency chirp toward the target and down-converting the received reflection using the same chirp for the LO. The target range is encoded in a beat frequency, which is obtained at baseband, and can be retrieved by FFT. It is usually neglected that the down-converting mixer of the receiver also performs an undesired AM detection of the LO, resulting in a DC signal. This parasitic signal has strong dependence upon LO frequency. In fact, when a wideband FMCW chirp is applied to the mixer, the AM detection level changes rapidly over time. The result is a wideband 1/f² parasitic signal at the baseband, which moves to higher frequency as the chirp rate increases. This “noise” conceals short range targets. In order to allow the detection of short range objects it is required that the mixer’s design considerations include frequency flatness of the AM detection for the LO input. Moreover, to mitigate the problem at system level, we propose to separate the signal from the LO detection “noise” by applying IF modulation of the transmitted signal. Demodulation is performed with a subsampling ADC, which moves the clean signal back to baseband. The method was demonstrated in a wideband W-band FMCW radar system, resulting in more than 30 dB improvement in SNR over the conventional method.