| 摘要 |
PURPOSE:To eliminate the fluctuation of a response speed and to reduce the cost by adopting a circuit comprising passive components only being resistors and a capacitor for a low pass filter applying a control voltage to a varactor diode of a tuner section to make the charge/ discharge characteristic of the capacitor linear. CONSTITUTION:The LPF is formed of externally mounted components comprising passive elements fo resistors Rc, Rd and a capacitor C only. Then a PLL IC is provided with constant current source switches Is1, Is2 and a power terminal (10V) for driving a tuner section VD higher than the IC operating power supply (5V), the switches is1, Is2 are connected in series between the 10V terminal and ground, a series connecting point is used as the output terminal, to which the input terminal of the LPF is connected. Since the LPF employs passive elements only, the cost reduction is attained and the output voltage TV is linear because of the charge/discharge of the capacitor C by the constant current sources Is1, Is2. Further, a sufficient voltage is applied to the tuner VD through the provision of the 10V terminal to the PLL IC. Thus, the capacitor charge/discharge characteristic is made linear to eliminate the change in the response speed. |
