| 摘要 |
A circuit comprises a frequency synthesizing circuit with a voltage-controlled oscillator whose frequency is preset to a preset value. The voltage-controlled oscillator generates an oscillating signal in response to an input voltage. The frequency synthesizing circuit is configured to operate in a locked loop mode under control of an error signal representative of a phase frequency differential between the divided oscillating signal and a reference signal. A digital processing unit can disable the frequency synthesizing circuit to operate in phase locked loop mode. Once the synthesizing circuit is disabled, the digital processing unit determines a first and a second frequency of the oscillating signal in response to respective first and second loop filter input voltage values. The unit further generates a control value from the two frequencies, the frequency divider dividing ratio and the reference signal. The circuit further comprises a digital to analog converter configured to preset the loop filter input voltage to a preset value in response to the control value. Once the voltage controlled oscillator output oscillates at the corresponding input preset value, the digital processing unit disables the digital to analog converter and enables the frequency synthesizing to operate in phase locked loop mode.
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