| 摘要 |
PURPOSE:To obtain horizontally symmetric frequency characteristics even in case of slight detuning, by using a digital filter as an IF filter and by setting its center frequency to a reference frequency at any time. CONSTITUTION:Between IF amplifying circuits 2A and 2B, narrow-band BPF30 is provided and IF signal S1, after band-limited by filter 30, is supplied to divider 3, and thus obtained voice signal component SA is supplied to synchronous detectors 4 and 5. Filter 30 is composed of a digital filter and as closk signal CK for driving this filter 30, a pulse obtained by frequency-dividing the oscillation output of variable oscillator 13 of PLL circuit 12 by M at counter 31 is utilized. Therefore, while circuit 12 is being locked, the center frequency of filter 30 is equal to the intermediate frequency. Then, when the intermediate frequency shifts slightly from the normal frequency in tuning, the center frequency of filter 30 shifts by the extent and filter characteristics symmetric to left and right on the center frequency can be obtained. |
