A 32kHz-Reference 2.4GHz Fractional-N Oversampling PLL with 200kHz Loop Bandwidth

In this article, a mixed-signal, 32-kHz reference-based 2.4-GHz fractional-N over-sampling phase-locked loop (OSPLL) is proposed. Different from the conventional 1x sampling PLL, which only uses zero-crossing timing information of the reference signal, the proposed OSPLL fully utilizes both the voltage and timing domain information of the reference signal and realizes oversampling ratio (OSR) times phase detection (PD) in one reference cycle. The proposed OSPLL employs the digital-to-analog converter (DAC) to construct the reference-like feedback signal in the voltage domain and utilizes the digital-to-time converter (DTC) to improve PD resolution in the time domain. The adaptive lookup table (LuT)-based calibration is proposed to generate the correct information for DAC and DTC control. A clocked passive comparator works as a bang-bang phase detector (BBPD) for the PLL control and LuTs' construction. The co-design of low-noise analog circuits and digital calibrations enables good jitter and spur performance. The proposed OSPLL is fabricated in 65-nm CMOS technology, with the core area of 0.58 mm², and the power consumption is 4.97 mW with a 1-V power supply. It achieves 5.79-ps root-mean-square (rms) jitter in fractional-N modes with the loop-bandwidth (BW $_{loop}$ ) of 200 kHz, corresponding to the figures of merit (FoMs) of -217.8 dB. The measured fractional spur is less than -36 dBc, and the reference spur is -78 dBc, respectively.