| 摘要 |
45MR00592 ELECTRICALLY-TUNABLE BANDPASS FILTER A bandpass filter is disclosed which may be used in a superheterodyne receiver in front of the first mixer. The bandpass filter provides a narrow front end filter which is tuned automatically as the local oscillator frequency is changed. In the present exemplary embodiment, a series tuned reflective notch filter is coupled to a node of a balanced resistive bridge in such a manner that the tuned circuit presents a short circuit to the node at the filter's resonant frequency to thereby assure a maximum transfer of signal from the input to the output of the circuit at a desired frequency. At all other frequencies, the series tuned reflective notch filter will present essentially an open circuit across the node of the resistive bridge so that the node is balanced and no transmission occurs. The inclusion of the series reflective notch filter thereby produces a bandpass characteristic from the circuit input to output. By using back to back varactor diodes in the reflective notch filter, the center frequency of the passband can be changed by changing the tuning voltage applied to the varactors. In the configuration of the present invention, the varactor diodes are protected from high signal levels which are not within the narrow passband of the filter. For this reason, the filter is inherently less susceptible to the generation of intermod products than filters of a conventional design.
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