| 摘要 |
PURPOSE:To reduce the sensitivity change at the antenna circuit part to cover the entire tuning band as well as to prevent the disturbance by leading the pi-matching circuit between the antenna terminal and the 1st-step input amplifier via the LC circuit in the AM car radio set. CONSTITUTION:Variable capacity element C4 is provided at the antenna input circuit part in order to increase the input impedance at 1st-step amplification part A and to vary the frequency for selection and amplification. And fixed capacity element C3 featuring a larger capacity than the maximum value of element C4 is installed in parallel to the antenna terminal part, and as a result the variable range of the tuning frequency is increased to secure coverage over the entire band region. In addition, inductor L2 featuring the inductance suited to give a selective reception for the reception frequency range in cooperation with elements C3 and C4 is connected in parallel to the antenna terminal part, thus constitution the pi-matching circuit. Accordingly, the capacity of the parallel arrangement of C2 and C3 is shown by C23, and thus the sensitivity slope can be eliminated up to part A regardless of variation of C4 by increasing C23 more than the maximum value of C4. Furthermore, the cross modulation and the image disturbance can be avoided owing to the selective tuning circuit. |
