An ultra-fast and high-precision VCO frequency calibration technique for fractional-N frequency synthesizers

Abstract

A novel VCO auto frequency calibration(AFC) technique is presented in this paper. It provides ultra-fast and high-precision search for an optimum VCO discrete tuning curve among a group of frequency sub-bands in the PLL frequency synthesizer. The calibration circuit consists of a table of frequency (TOF) and a finite state machine (FSM). Asynchronous work of TOF and FSM makes it possible to realize ultra-fast and high-precision calibration simultaneously. TOF affords the high-precision of the calibration and FSM with a high-speed clock offers the ultra-fast search. A 2.8G-3.5GHz Fractional-N PLL incorporating the proposed calibration technique is implemented in 55nm CMOS process. The measured calibration time for a 5-bit capacitor bank is 90ns for a frequency resolution of 203KHz. Such small calibration time significantly reduces the effects of calibration time on the total lock time of the PLL.