| 摘要 |
PURPOSE:To shorten pull in time without impairing jitter characteristic by operating a circuit with a large current when a PLL circuit performs pull in, and operating it with a small current when the pull in is completed. CONSTITUTION:In a charge pump circuit, a FETQ20 is used for a source type current circuit, and a FETQ17 for a pull-in type current circuit, and FETs 19, 18 for switching, respectively. FETs Q14, Q15, Q20, and Q16, Q17 comprise current mirror circuits, respectively, and FETs 11, 13 are used for a current source, and a FETQ12 for the control of a current iI, and they are controlled by the output of a synchronism decision circuit 9 which inputs the output of a digital phase comparator 3. The decision circuit 9 decides whether the pull in is being performed by the output from the comparator 3 or it is completed. When the pull in is being performed, quick pull in can be performed by charging/discharging a capacitor C1 by the large current i3 or i4 in proportion to a current (iI+i2). Also, when the pull in is completed, charge/discharge is performed by the small current i3 or i4 in proportion to only the current i2. |
