PURPOSE: To limit a beat noise generated when a phase synchronizing oscillator is unlocked and to prevent an abnormal operation such as false lock by connecting a circuit in which a nonlinear element is connected in series with a capacitor to a negative feedback circuit in parallel.

CONSTITUTION: When an alternating voltage with high amplitude is applied to a terminal Vi, the impedance of diodes 7 and 8 are reduced for voltages applied to the diodes 7 and 8 in point of index, consequently, output amplitude is limited to the on-voltages of the diodes 7 and 8 when a frequency is the one at which the impedance of the capacitor 5 is sufficiently lowered. Therefore, it is possible to make the circuit function as a limiter circuit against a signal with a high frequency to some degree and high amplitude and to limit an unrequired signal and to prevent the saturation of an operational amplifier 1 from occurring.