| 摘要 |
The object of this invention is to design the radio-frequency stages of a superhet (direct mixer) so selectively that all unwanted frequencies are less strongly amplified or the required signal component is more strongly amplified. Immunity to interference signals, selectivity, sensitivity and far-off selectivity are thereby intended to be improved. For this purpose, the preliminary stage must therefore already operate selectively. However, this is difficult to achieve at radio frequencies using conventional selection means. The circuit is used in FSH in such a way that the signal is selected at a lower intermediate frequency, as in normal superheterodyne receivers, but is then mixed upwards again. This upward-mixed signal is then added in the preliminary stage to the input signal. The phase shift is selected in such a way that the upward-mixed signal and the input signal amplify one another. Therefore, the preliminary stage then amplifies the required frequency spectrum only. <IMAGE>
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