| 摘要 |
PROBLEM TO BE SOLVED: To prevent a signal from a local oscillation circuit from appearing at the RF signal input edge of a mixer due to the inside isolation shortage of a mixer, and from leaking through an input terminal to the outside. SOLUTION: A microwave band RF signal from an input terminal 1 is amplified by a low noise amplifier 2, and an image signal outside the band is removed by a BPF3, and inputted through an impedance matching circuit 5, a second circuit 7, and a first circuit 6 to a mixer 8, and mixed with a local signal from a local oscillation circuit 9, and converted into an IF frequency, and amplified and outputted by an IF amplifier 10. The local frequency impedance of a circuit constituted of the first and second circuits 6 and 7 is made infinite when viewed from the mixer side, and turned to be almost zero when viewed from the input terminal side so that the local signal can be interrupted by the first and second circuits, and prevented from appearing at the input terminal. On the other hand, the impedance viewed from the input terminal is turned into almost zero. Then, an impedance matching circuit 5 is interposed so that the impedance can be matched with the output impedance of an external circuit connected to the input terminal.
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