A novel ΣΔ fractional-N PLL architecture for fast locking and fractional spur suppressing is proposed based on the capacitance scaling scheme. It changes the effective capacitance of loop filter (LF) by increasing and decreasing current to the capac...
A novel ΣΔ fractional-N PLL architecture for fast locking and fractional spur suppressing is proposed based on the capacitance scaling scheme. It changes the effective capacitance of loop filter (LF) by increasing and decreasing current to the capacitor via different paths with multiple charge pumps. The effective capacitance of loop filter (LF) can be scaled up/down depending on operating status while keeping LF capacitors small enough to be integrated into a single PLL chip. Fractional spurs suppressing have been achieved by reducing the magnitude of charge pump current when the PLL is in-lock without degrading fast locking characteristic. It has been simulated by HSPICE in a CMOS 0.35㎛ process, and shows that locking time is less than 8㎲ with the small size of LF capacitors, 200㎊ and 17㎊, and 2.8㏀ resistor.