| 摘要 |
PURPOSE:To narrow down band characteristics in a demodulation loop and to improve a threshold level by varying the center frequency of the FM demodulation slope of a PLL type FM demodulation circuit with an FM-demodulation signal. CONSTITUTION:A differential amplifier 14 having a constant current source 13 is arranged behind the amplifier 3' of the PLL.FM demodulation circuit. One output 15 of the differential amplifier 14 is connected to a loop filter 4 and the other output 16 is applied to the constant current source 13 through a voltage convertig circuit 17 to vary the current of the constant current source 13 with the demodulated signal of the differential amplifier 14. Demodulation characteristics 11 of a center frequency f1 change into demodulation characteristics 12 of a center frequency f2 by the variation of the current along oscillation frequency characteristics 7 to the impressed voltage of a voltage-controlled oscillator. At this time, neither the loop phase nor the phase is unchanged, so the demodulation characteristics 11 and 12 have the same demodulation band. |
