Design of Low Phase Noise and Low Spurs Fractional-N PLL Based Frequency Generator for Millimeter Radar

The front end of the radar is a key part of the entire millimeter wave system, and millimeter radars with high performance require the generation of low-spurs, low-phase-noise and highly linear chirp signal. In this paper, source of the phase noise and spurs of a fractional-N PLL based frequency generator are analyzed, and a series of methods are proposed to suppress the spurs and reduce the phase noise. To verify the theoretical analysis, a 24GHz millimeter wave signal source is designed and fabricated, and the measurement results show that the signal source performs well at 24GHz, while the loop bandwidth of the PLL is 35KHz, the phase noise is −74.6dBc/Hz at 50KHz and −110.3dBc/Hz at 1MHz. Moreover, there is no observable spurs near the central frequency.