A 12-GHz all-digital calibration-free frequency-modulated continuous-wave (FMCW) signal generator is presented in this paper based on a retiming fractional frequency divider (FFD). Instead of modulating a multi-modulus divider (MMD) by a $\Delta\Sigma$ modulator, a fractional divider is utilized to release the narrow loop bandwidth limitation and achieve better phase noise without active noise cancellation techniques. Thus, the FMCW signal generator can achieve low root-mean-square (RMS) frequency error under a fast chirp slop. A calibration-free retiming FFD architecture which is not sensitive to process, voltage and temperature (PVT) variations is proposed to avoid complex and slow calibration process. Implemented in 40-nm CMOS, the generator consumes 33.8 mW power and 0.32 mm 2 chip area. Measurement results show that the generator achieves 78 $\mathrm{MHz}/\mu\mathrm{s}$ maximum chirp slope and 6.0 kHz RMS frequency error when 300 MHz frequency is swept in 2 ms. The phase noise from 12 GHz carrier is -113.6 dBc/Hz at 1 MHz offset.