| 摘要 |
PURPOSE:To prevent the generation of a beat tone by controlling the switching frequency so that a harmonic being a noise source is not included in the tuning frequency band or coincident with the tuning frequency. CONSTITUTION:A switching pulse outputted from an oscillator 1 is inputted to a duty control circuit 2, controlled in response to a prescribed duty ratio and outputted to a transistor (TR) 3. The TR 3 is subjected to ON/OFF control and a DC motor 4 is subjected to duty control. The oscillator 1 is formed as a CR oscillator and oscillated at a frequency in response to a resistor 5 selected by a switching circuit 6. The movable iron piece 8 of the circuit 6 is subjected to switching control by using a switching signal from a microcomputer 7, a fluorescent display tube driving circuit 10 and the computer 7 detect the tunining frequency and the switching frequency is controlled so that the harmonics being a noise source are not included in the tuning frequency band and coincident with the tuning frequency. Thus, the generation of beat tone is prevented.
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