A 2.6–3.4 GHz Fractional- $N$ Sub-Sampling Phase-Locked Loop Using a Calibration-Free Phase-Switching-Sub-Sampling Technique

Sub-sampling phase-locked loop (SSPLL) achieves lower in-band phase noise compared to a conventional charge-pump phase-locked loop with frequency dividers. Recently, several works have been reported to enable fractional-N operation of SSPLL to broaden its applications. However, they require careful calibrations with extra silicon area and adjusting time (~20 ms measured) to achieve low phase noise. For scenarios requiring short settling time such as frequency modulation, such a time consuming calibration is undesirable. This letter presents a phase-switching technique for fractional-N mode SSPLL to eliminate this calibration. The principle of the technique is analyzed and a prototype is fabricated in 65-nm CMOS technology. Even without calibration, the frequency synthesizer achieves a figure of merit of -234.3 dB under fractional-N operation, with 13.3-mW power consumption at 1.2-V supply.