PHASE LOCKED LOOP CIRCUIT INCLUDING FREQUENCY CONVERTER

PURPOSE:To restore the output voltage of a phase comparator to a normal voltage region by detecting a fault of the operating state of a phase locked loop and interrupting tentatively the comparison of the phase comparator. CONSTITUTION:Since the increasing speed of an oscillating frequency of a VCO 16 is slower than the frequency change speed of a local oscillating signal source 14, the frequency of a signal FPG passes ahead an oscillating frequency f8 of the VCO 16 and the relation of the quantity of the frequency is reversed. Then a comparison output voltage Ec of a phase comparator 17 having a slope increasing positively changes to a slope decreasing negatively and finally the comparison output voltage Ec goes to a negative voltage. When the comparison output voltage Ec enters a negative voltage region, a voltage comparator 31 goes to an L logic and a flip-flop 33 inverts the state and outputs L logic to an output terminal Q to control a stop controller 40. Thus, the comparison output voltage Ec of the phase comparator 17 is restored to the positive polarity.