| 摘要 |
PURPOSE:To reduce distortion generated by a filter without degrading selectivity by permitting a control signal to cancel distortion in a signal of the 2nd intermediate frequency when converting an FM received signal from the 1st intermediate frequency to the 2nd intermediate frequency. CONSTITUTION:The FM received signal is converted by a front end 21 into a signal of frequenct f1, i.e. the 1st intermediate frequency IF, which is applied to a BPF22 and a delay device 25, so that transmission distortion appears at the output of the BPF22. A mixer 23 performs conversion to frequencies f1+ or -f2 by a signal of frequency f2 from a VCO31 and an LPF24 extracts the 2nd IF signal of frequency f1-f2. The 2nd IF signal and the 1st IF signal from the delay device 25 are mixed by a mixer 26 and a BPF27 extracts a signal of frequency f2. The 1st and the 2nd IF signals have the same frequency shift direction, so a modulated signal S is canceled to obtain only the distortion D. The distortion D is made into fine distortion (d) through the negative feedback loop of the system with an input to a detector 8. The distortion (d) detected by the detector 28 is passed through an LPF29 and phase-inverted and amplified by an inverting amplifier 30 to control the output of the VCO31 so that the distortion D in the 2nd IF signal is canceled. |
