| 摘要 |
A motor rotational pulse generating circuit for a motor is provided which generates a correct pulse signal even at an initial turning-on stage of the motor, by adjusting a filter cutoff frequency in response to motor rotational condition. The motor rotational pulse generating circuit includes a rotational pulse generation circuit 20 which generates ripple pulses based on a signal being inputted from the DC motor 1, in which a ripple is superposed whose frequency is in proportion to a rotation number of the DC motor 1. A filter 3 makes a cutoff frequency variable on the basis of a clock signal issued from a PLL circuit 6. An oscillation frequency at an oscillator VCO10 is determined by the ripple pulses and a motor rotation condition signal inputted by way of circuits 12 to 16 inclusive. A microcomputer 20 operates, when the motor is turned on, to cause the oscillator VCO10 to issue a preliminary clock signal. The microcomputer 20 measures the oscillation frequency on the basis of the resultant preliminary clock signal to judge whether or not an initial cutoff frequency of the filter 3 is correct. If the result is negative, a switching circuit 17 is controlled in switching mode to adjust an attenuance of an attenuator 16, thereby correcting the initial cutoff frequency.
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