Patent ID: 6985551

Claim:
A fractional-N phase-locked loop, comprising: a phase detector, comprising: a first input that receives a first signal; a second input that receives a second signal; and a comparison circuit that generates a phase detector output signal that is a function of a phase difference between the first signal and the second signal; a loop filter that generates a frequency control signal from the phase detector output signal, a circuit that generates a phase-locked loop output signal having a frequency that is controlled by the frequency control signal, a frequency divider that generates the second signal from the phase-locked loop output signal, a sigma-delta modulator that generates division values for the frequency divider, and an operating point circuit that maintains an operating point of the phase detector at a position with a nonzero output signal and a corresponding nonzero phase difference between the first and second signals, such that for a predetermined range of both positive and negative phase differences between the first and second signals, the output signal is generated as a substantially linear function of the phase difference between the first signal and the second signal, wherein the substantial linearity of the output signal is due at least in part to a shifting of the operating point so that a nonzero output signal corresponds to a nonzero phase difference between the first and second signals, and the nonzero phase difference is close to or larger than an amount of time equal to at least a number of cycles of the phase-locked loop output signal.