LOCAL OSCILLATOR CIRCUIT

PURPOSE:To obtain a highly stable circuit by providing a switch means keeping a reception input level to a level before it is decreased even when it is decreased, inputting the reception input level to a low pass filter and interrupting a path from a voltage controlled oscillator till a phase comparator for the period of the level decrease. CONSTITUTION:When an input frequency reaches a prescribed level or below, a switch circuit 15 is closed, a capacitor 20 is sufficiently charged up till just a preceding state and the voltage is the same as that of an output of a low pass filter 8, then the voltage is fed to the input of the low pass filter 8 via a buffer amplifier circuit 17. Since the output impedance of the buffer amplifier circuit 17 is sufficiently lower than the line impedance at the input of the low pass filter 8, the input voltage depends on the output of the buffer amplifier circuit 17. Thus, the output voltage of the low pass filter 8 is fixed to a voltage just before the input frequency level reaches a prescribed level or below. Thus, the power supply is closed and after the input frequency reaches a prescribed level or over, the oscillator circuit is very stable.