| 摘要 |
PURPOSE:To eliminate the defect that a PLL lock fault detecting circuit responds too much to the operation of a switch by providing a voltage discriminating device detecting a prescribed level of an output voltage of a phase comparator in a phase locked circuit, an integration circuit integrating an output voltage of the voltage discriminating device and a memory circuit storing an output of the integration circuit. CONSTITUTION:The voltage discriminating device 12 and the memory circuit 14 correspond to the voltage discriminating device 10 and the memory circuit 11 of a conventional circuit. Moreover, the integration circuit 13 consists of a simple CR circuit having a time constant tau or the like. The integration circuit 13 suppresses a very fast change in the output voltage of the voltage discriminating device 12 and selects a comparatively slow change and gives the result to the memory circuit 14. The time required for the switching operation of the switch 8 is very short normally and the relocking of the PLL as the result is finished in a very short time. An output voltage change of PD based on a fault of the PLL is normally slow against the result. Thus, the discrimination of fault at the operation of the switch is made ineffective by utilizing this point. |
