| 摘要 |
PURPOSE:To prevent a local oscillation voltage from being leaked to an antenna terminal via an inter-stage of a variable capacitor mainly by leading an antenna input to a gate input of a common source field effect transistor (TR) and connecting the 1st tuning circuit to the drain output of the said field effect TR. CONSTITUTION:As an antenna input circuit 2, an antenna matching circuit or a band pass filter is used in general. Further, the 1st and 2nd turning circuits 4, 6 and a local oscillation circuit 8 are controlled by using a four-blade variable capacitor. Then the field effect TR 3 is used as a common source, the input gate is connected to the antenna input circuit 2 and the drain output is matched and connected to the 1st turning circuit 4. A local oscillation voltage of the local oscillation circuit 8 is fed to a mixer 7, while being leaked to the tuning circuits 4, 6 by the internal inter-stage coupling of the variable capacitor. The local oscillation voltage leaked to the 1st tuning circuit 4 is not generated at the antenna input terminal because the passage from the drain output to the gate input is blocked by the field effect TR 3.
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