| 摘要 |
PROBLEM TO BE SOLVED: To hold the input level of an FM demodulator fixed and to hold a fixed noise factor by setting the conversion gain of a frequency conversion circuit so as to perform an operation in a linear area changed in linear relation with the level of local oscillation signals. SOLUTION: When an applied voltage, that is an AGC voltage AG, is made larger, a current becomes larger and resistance becomes smaller. Inversely, when the voltage AG is made smaller, the resistance becomes larger. Thus, the level of the local oscillation signals LOT to be injected to a mixer 4 is raised by enlarging the voltage AG and the level of the local isolator LOT is lowered by making the voltage AG smaller inversely. Thus, by using the positive proportional area of the conversion gain GC and the level of the local oscillation signals LOT and varying the level of the local oscillation signals LOT by a local oscillation level varying circuit 3, the conversion gain of the mixer 4 is controlled. Also since the noise factor of the mixer 4 is decided by the noise factor of the respective bias states of constituting elements, it is turned to a stable value regardless of the conversion gain.
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