| 摘要 |
PURPOSE:To receive signals of two bands with one mixer only by switching power sources by connecting high frequency amplifiers and local oscillators for the 1st and the 2nd bands to the common mixer common to the 1st and the 2nd bands through a coupling capacitor with small capacity respectively. CONSTITUTION:When a band changeover switch 8 is placed at the side of the 1st band of the 4th - the 12th channels of TV, a high-frequency amplifier 1 and a local oscillator 3 are powered on to receive a TVch. At this time, the impedance of the tuning circuit of the 1st band is high and that of the 2nd band is low, so coupling capacitors 11 and 13 are interposed between the input of the mixer 9 and the ground and there is no influence of the circuit of the 2nd band exerted. Then, when the switch 8 is placed at the side of the 2nd band of the FM wide band (76-90MHz and the 1st - the 3rd channels of TV), the FM wide band is received by a high-frequency amplifier 2 and a local oscillator 4 without any influence of the 1st band. In such a case, the coupling capacitors 10-13 have 0.5-3PF capacity. |
