| 摘要 |
PURPOSE:To attain the locking even when a polarity of a voltage controlled oscillator is changed by providing a voltage comparator in a circuit applying a control voltage to the voltage controlled oscillator in the phase locked loop oscillating circuit using a frequency mixer and providing a one-shot multivibrator and a switch at the output of the voltage comparator. CONSTITUTION:A voltage comparator 7 is provided in a conventional circuit and its output is fed back to a voltage controlled oscillator 1 through a voltage application means comprising a one-shot multivibrator 8 and a switch 9. When a variable frequency width of the voltage controlled oscillator 1 is 880-920MHz and an output frequency of the voltage controlled oscillator 1 reaches 897MHz or below, an intermediate frequency outputted from a frequency mixer reaches an image band, the output frequency of the voltage controlled oscillator 1 is locked to 880MHz and fixed thereat. The switch 9 is closed by the voltage comparator 7 via the one-shot multivibrator 8 to increase the control voltage to the voltage controlled oscillator 1 at that time for a predetermined time to increase its output frequency higher than 897MHz. |
