| 摘要 |
PURPOSE:To suppress the spurious radiation due to a channel other than a desired channel, by selecting the 1st intermediate frequency to a specific multiple of the upper limit of a receiving frequency, in a TV receiver of the double superheterodyne system. CONSTITUTION:An input signal from a terminal 60 is inputted to a variable attenuator 62 via an input filter circuit 61 comprising an FM signal trap and an HPF of variable frequency, in a tuner of the double superheterodyne system for CATV. A signal attenuated by a prescribed amount by the attenuator 62 is amplified 63 and inputted to a variable attenuator 64, where the signal is mixed with a signal amplifying 60 an output of a voltage controlled oscillator 65 of variabl frequency the result is frequency-converted, and its output is inputted to the 1st intermediate frequency BPF67. The BPF67 selects the difference between the frequency of the oscillator 65 and the receiving frequency, and its frequency is selected to 1,350-2,250MHz equivalent to 3-5 times the receiving highest frequency 450MHz. The output of the BPF67 is mixed with the output of a fixed frequency oscillator 69 by a mixer 68, and its output is outputted via the 2nd intermediate frequency BPF70 and an amplifier 71. |
