| 摘要 |
PROBLEM TO BE SOLVED: To reduce the cost of a filter circuit by giving a common impedance to a rejection (blocking) circuit and a sires turning circuit to realize a selected characteristic equivalent to a general circuit design. SOLUTION: The filter circuit incorporates an amplifier stage 1, a two-circuit band-pass filter 3 connected to the downstream side of the stage 1, and a filter 2 which is connected to the upstream side of the stage 1 and constituted of the combination of a rejection circuit and a series tuning circuit. The rejection circuit of the filter 2 is tuned to a desirable frequency fn and formed of aλ/4 coaxial stub 4 short-circuited to a reference grounding potential through its grounding terminal. In additon, a capacitor 5 is arranged between the live terminal of the stub 4 and the input of the amplifier stage 1. The capacitor 5 is sized so that the capacitor 5 may form the series tuning circuit which is tuned to an image frequency fs together with the stub 4 which works as an inductance with respect to the frequency fs . In additon, a common impedance is given to the rejection circuit and series tuning circuit.
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