| 摘要 |
PURPOSE:To shorten loop locking time regardless of the limit of the power supply of a charge pump and of the supply capability by providing a switching circuit to a position where a capacitor and a resistor being components of a lag lead filter is short-circuited or opened. CONSTITUTION:The circuit consists of a digital phase comparator 2 detecting a phase difference of outputs between a reference oscillator 1 and a voltage controlled oscillator 4, a charge pump 3 converting the output into an analog voltage, resistors 7, 8 and a capacitor 9. Then an abnormal detection circuit 5 detecting the unlocked state of a loop is provided on a PLL circuit having the lag lead loop filter receiving an output of the charge pump 3 and outputting an APC voltage controlling the oscillated frequency of the oscillator 4. A switch 10 is arranged between the resistor 8 and the capacitor 9, disconnects the resistor 8 and the capacitor 9 in the case of the loop unlock by an output signal from the circuit 5 and short-circuits the resistor 8 and the capacitor 9 in the case of loop lock to switch the loop gain. |
