| 摘要 |
A channel selector for a television receiver having a low gain, high-frequency radio frequency-intermediate frequency section as combined with a relatively high-gain, lower intermediate frequency in the intermediate frequency section to reduce noise, such as intermodulation distortion and cross-modulation distortion. The channel selector comprises a frequency spectrum filter in the RF section, a mixer which preferably includes a metal semiconductor field effect transistor, i.e. MESFET, and a channel selecting filter. The frequency spectrum filter is coupled to the antenna of the television receiver and filters a frequency spectrum of radio frequency signals as received from the antenna, such as a plurality of television channels. The mixer has an input coupled to the frequency spectrum filter and frequency shifts selected channels of the frequency spectrum to a predetermined relatively high intermediate frequency. The RF section has a low gain RF amplifier connected between the frequency spectrum filter and the mixer or may have no RF amplifier at all. The channel selecting filter has an input coupled to the mixer and filters the selected channel at the predetermined intermediate frequency. The total gain from the RF section through the channel selecting filter is no larger than necessary in order to obtain a desired system noise figure. Another mixer is provided in the IF section, this IF mixer being coupled to the output of the channel selecting filter. The IF mixer receives the filtered selected channel at the relatively high intermediate frequency from the channel selecting filter and frequency shifts the filtered selected channel to a substantially lower second intermediate frequency. An IF amplifier having a relatively high gain is connected to the output of the IF mixer and adds gain to the filtered selected channel at the lower second intermediate frequency, the added gain being substantially in excess of the total gain provided before the channel selecting filter.
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