Fractional-n phase lock loop (pll) with noise cancelation

Abstract

A PLL circuit includes a fractional-N divider generating a feedback signal, a first phase-frequency detector that compares the feedback signal to a reference signal to generate first up/down control signals that control a charge pump to generate a charge pump output current. A noise cancelation circuit includes a synchronization circuit that generates first and second synchronized feedback signals from the PLL circuit output and the feedback signal, where the first and second synchronized feedback signals are offset by an integer number of cycles of the PLL circuit output. A second phase-frequency detector circuit compares the first and second synchronized feedback clock signals to generate second up/down control signals whose pulse widths differ by the integer number of PLL cycles. A current digital to analog converter circuit is controlled in response to the second up/down control signals to apply noise canceling sourcing and sinking currents to the charge pump output current.