MONITORING CONTROL CIRCUIT OF FM MODULATOR

PURPOSE:To prevent a malfunction of a power supply at the rise time, by adding a power supply delay circuit having a rise time larger than that of the power supply of an AFC circuit to a circuit that monitors a shift of the intermediate frequency of a heterodyne type AFC circuit of an FM modulator. CONSTITUTION:The output of an FM modulating circuit 1 receives a frequency conversion through a frequency converting circuit 6 of an AFC circuit 2 and with the output of an oscillator 5. The intermediate frequency is fed to a frequency monitor circuit 3 as well as to the circuit 1 via a frequency discriminating circuit 8. The power given from a power supply input terminal 11 is supplied to the circuit 1 via a switch 14 to actuate the circuit 1 and at the same time supplied to the circuit 2 for actuation. During the power supply rise time of the circuit 2, the power supply rise time is delayed by the time constant of a power supply delay circuit 12. Thus the circuit 3 has no actuation, and a frequency shift detecting circuit 13 is actuated after a delay time. This can aviod a malfunction for detection of a frequency shift of the power supply rise time and the breaking of power supply due to the control of the switch 14.