| 摘要 |
PROBLEM TO BE SOLVED: To provide a cutoff frequency variable filter that automatically controls cutoff frequency of a filter with high precision through a comparatively simple constitution. SOLUTION: A reference signal generation circuit 14 comprises an oscillating circuit 12 that generates a vibration signal repeating level transition with predetermined recurrence frequency between a ground level and power supply level; and a clip circuit 13 that, by clipping the vibration signal into a first level higher by a predetermined levels than the ground level and a second level lower by a predetermined level than the power supply level, generates a reference signal repeating level transition between the first level and the second level. A reference signal generated in the reference signal generation circuit 14 is input into an LPF 11 and its outputs are monitored to control the cutoff frequency of the LPF 11. COPYRIGHT: (C)2003,JPO
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