| 摘要 |
An RF input filter circuit (19) inserted between the antenna connection (8) and the input (3) of the RF input amplifier (2) of a tuner (1) comprises a tunable and switchable resonant circuit (6) and an antenna adapter circuit (7) which is connected downstream of the resonant circuit and which can also be switched to be set to a plurality of receive frequency bands. Seen from the antenna connection, the antenna adapter circuit has a base inductor (10) and a series inductor (12) which connects the antenna connection to the resonant circuit. The two inductors are formed from the series connection of two further inductors (L3, L4; L5, L6), the latter connection points (21, 22) of which are interconnected via a switching diode (D3). When the switching diode is in the conducting state, it connects in parallel the inductors (L4, L5) lying between it and, with a capacitor (C2) which lies in parallel with one of these inductors, forms a relatively broadband blocking circuit which is tuned to an interfering receive frequency band and with which the attenuating antenna resistor (Ra) lies in parallel via the small remaining base inductor (L3). <IMAGE>
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