| 摘要 |
PROBLEM TO BE SOLVED: To provide a PLL frequency synthesizer which can shorten a pull-in time, when a frequency is changed. SOLUTION: A frequency synthesizer 10 is constituted by including a phase comparator 20, a charge pump 22, LPFs 30 and 32, analog switches 34 and 36, a loop filter 40, an adder 42, a D/A converter 44, a VCO 50, a mixer 52 and a frequency divider 54. When the frequency is changed, since the analog switches 34 and 36 are turned off to stop the operation of the charge pump 22 and the output voltage Vpd of the loop filter 40 is converged on 0 V, the oscillation frequency of the VCO 50 is set stably in the vicinity of a target frequency through the precharging operation by the D/A converter 44. Thereafter, the analog switches 34 and 36 are turned on, the charge pump 22 begins to operate, and the oscillation frequency of the VCO 50 is synchronized speedily with the target frequency.
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