| 摘要 |
A phase locked loop has a digitally controlled oscillator (DCO) for generating a DCO output signal (fOSC), a clock divider coupled to the DCO and receiving the DCO output signal and outputting a feedback clock signal (fN), and a phase frequency detector (PFD) coupled to the DCO and controlling the DCO by a DCO control signal (dCNTL). The PFD has a first input for receiving the feedback clock signal (fN), a second input for receiving a reference clock signal (fREF), and comprises a frequency detection stage (FD) adapted to calculate a frequency difference between the feedback clock signal (fN) and the reference clock signal (fREF) in a frequency detection mode and to adjust the DCO control signal based on said frequency difference, a phase detection (PD) stage for calculating a phase error between the feedback clock signal and the reference clock signal in a phase detection mode, and a switch for switching between the frequency detection mode and the phase detection mode upon the frequency of the feedback clock signal reaching a predetermined value.
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