| 摘要 |
PURPOSE:To save a narrow band filter or the like being an externally mounted component in case of circuit integration by obtaining a sound signal and a so- called stop signal for conducting digital tuning from a synchronism detecting circuit. CONSTITUTION:In receiving a certain broadcast radio wave corresponding to a frequency dividing ratio M of a programmable divider 10, when the level of an intermediate frequency signal obtained from a frequency mixing circuit 3 of a PLL circuit comprising 7, 8, 9, 3, 16 and 17 is large and the signal is locked accurately to the output of a reference frequency oscillating circuit 5, its lock detecting signal indicates the automatic search of a broadcast station. On the other hand, accurate locking is impossible, a central processing unit 20 changes the frequency dividing ratio of the divider 10. Since the PLL circuits have a prescribed lock-in range or capture range and the synchronism detecting circuits 18, 18' detect the said relation of synchronism, no narrow band filter is required. |
