| 摘要 |
PURPOSE:To realize an automatic compensation for a shift of receiving frequency, by controlling the receiving frequency by a PLL loop in a channel selecting mode and then releasing the PLL loop to switch it to AFC control. CONSTITUTION:When a channel number is supplied through a channel selecting keyboard 1, a dividing ratio is set in accordance with the channel selected by a frequency divider 8. At the same time, the AFC output SW of a tuning control circuit 10 is turned off by an SW (internal switch) switch signal from a controller 3, and the output SW of a phase comparator is turned on. Thus the local oscillating frequency of an electronic tuner 5 is set at a frequency corresponding to a desired channel by the feedback of a PLL loop comprising a fixed frequency divider 7, a variable frequency divider 8, a phase comparator 9 and a low band active filter 45. Then the PLL loop is released by the signal given from the controller 3 and then switched to the control given by an AFC circuit. |
