Fractional-N PLL using multiple phase comparison frequencies to improve spurious signal performance

摘要

A fractional spur compensation technique is implemented in a fractional-N PLL using multiple phase comparison frequencies F_pd, one of which is selected for any channel frequency F_ch in a target frequency band to obtain a selected offset frequency F_os between the channel frequency F_ch and its primary fractional spur throughout the target frequency band. Other features of an exemplary implementation of the fractional spur compensation technique include (a) maintaining the phase comparison frequency at less than a predetermined maximum value, (b) using a programmable reference frequency multiplier with selectable multiplication factors and/or a programmable reference frequency divider with selectable divide ratios to generate multiple phase comparison frequencies derived from a predetermined reference frequency F_ref, and (c) using a programmable charge pump to select different charge pump currents for respective phase comparison frequencies to reduce loop gain variation.