A sub-0.75°RMS-phase-error differentially-tuned fractional-N synthesizer with on-chip LDO regulator and analog-enhanced AFC technique

This paper presents a low-phase-error wideband fractional-N frequency synthesizer. Differential tuning is described and a level shift circuit is proposed to obtain symmetrical tuning range. On-chip LDO regulator is designed to improve the power supply rejection for VCO. A voltage monitor is used to enhance the digital AFC technique to overcome the temperature variation. The synthesizer was implemented in a 0.18-mum CMOS process with a 16-mA supply current and a 1.47-mm² die area. The measured in-band phase noise is less than -97 dBc/Hz at a 10-kHz frequency offset and the integrated phase error is less than 0.75deg_RMS. The measured reference spur is less than -71 dBc and the locking time is smaller than 20 mus.