| 摘要 |
PROBLEM TO BE SOLVED: To prevent a voltage controlled oscillator from being controlled to oscillate an erroneous frequency due to reduction in a reception electric field strength by storing frequency difference information between an intermediate frequency signal and a frequency of a comparison signal from a reference oscillator as just preceding frequency in formation when an amplitude of the intermediate frequency signal is reduced more than a specified amplitude. SOLUTION: A level measurement device 4 always measures an amplitude of an intermediate frequency signal S2 denoting a reception electric field level and converts the measured amplitude into a DC voltage S5 and inputs the voltage to a comparator 5. The comparator 5 compares the DC voltage S5 with a reference voltage S4 from a reference power supply 14 to provide an output of a low gate control level S6 to gates 6, 8 so as to shut an amplified intermediate frequency signal S3 to be given to a frequency comparator section 10 and a comparison signal S8 from a reference oscillator 7 when the DC voltage S5 is lower than the reference voltage S4. Thus, while the reception electric field level is low, the oscillated frequency from a voltage controlled oscillator 13 is not fluctuated and the oscillated frequency is prevented from converging to an erroneous frequency. |
