| 摘要 |
PURPOSE:To reduce the synchronism restoring time at re-input of reference frequency signal by sampling and holding an output of a LPF controlling a VCO and controlling the VCO with a sample-and-hold output when the reference frequency signal is interrupted. CONSTITUTION:When the reference frequency signal is inputted to a signal line 102, a phase comparator 1 compares the phase of an output signal of the VCO3 with that of the reference frequency signal and a signal through an LPF2 from an output signal of a comparator 1 controls the VCO3 via a switch 6. Similarly, a sample-and-hold circuit 4 samples always an output of te LPF2. When the reference frequency signal is interrupted, a reference frequency signal interruption detector 5 detects it, the detected signal controls the circuit 4 to stop sampling and the signal just before is held and the switch 6 is changed over to the position of the circuit 4, and the holding signal of the circuit 4 controls the VCO3. Thus, since a large frequency fluctuation is not caused even if the reference frequency is interrupted, the synthronism restoring time when the reference frequency is inputted again is shortened. |
