| 摘要 |
PURPOSE:To improve the transmission quality and reliability by detecting a control signal controlling the oscillation frequency of a voltage controlled oscillator of a phase locked loop (PLL), comparing the signal with a reference voltage and outputting an alarm signal so as to detect the increase in a phase error even with the PLL in the locking state. CONSTITUTION:A control signal detection circuit 2 detects a control signal VC controlling the oscillated frequency of the voltage controlled oscillator 14 and a comparator circuit 3 outputs an alarm signal VA when the output voltage of the control signal detection circuit 2 exceeds a preset reference voltage VR within the frequency holding range of the PLL 1. The circuit 2 acts like an absolute value circuit with the relation of VD=V>=C at VC>0, VD=-VC at VC<0, that is VD=VC in the relation of resistors R1=R2=R3=2R4=R5. The comparator circuit 3 having an amplifier 31 compares the voltage VD with the reference voltage VR and when the voltage VD exceeds the reference voltage VR, the alarm signal VA is outputted. |
