| 摘要 |
PURPOSE:To obtain a good receiving characteristic with a simple circuit constitution by setting the first intermediate frequency on the outside of upper band of a receiving band and connecting a tuning circuit having a series resonance and a parallel resonance closely to the output, and setting the frequency of an image signal at a minimum output point of the series resonance point. CONSTITUTION:The transmission characteristic appearing across both ends of a capacitor 19 is an output characteristic shown in a figure, and a maximum output point due to the parallel resonance characteristic and a minimum output point due to the series resonance characteristic are obtained. In this case, values of a coil 16 and capacitors 17, 18, and 19 are so selected properly that the maximum output point has the first intermediate frequency and the minimum output point has the image frequency generated by the second local oscillating circuit. By this constitution, an ANT input circuit secures sufficient image characteristic, degree of amplification, and degree of selection even with an LPF, and further, the local oscillation can be covered with a single variable capacitor to make a band changeover switch unnecessary. |
