| 摘要 |
PURPOSE:To detect a rotation signal with a high S/N by using a variable frequency type band-pass filter, controlling its center frequency to an output side frequency, and narrowing down the passing band width sufficiently. CONSTITUTION:The rotation signal is amplified 1 and then inputted to the band- pass filter 2. The center frequency of the filter 2 is determined by the frequency of a control signal C. Then, the rotation signal having a noise removed is shaped by a waveform shaping circuit 3, and inputted to a phase-locked loop 4 while appearing at an output terminal. The loop 4 synchronizes the output frequency of a VCO43 with a frequency Nf which is N times as high as the rotation signal to obtain the control signal C to the filter 2, so that the center frequency of the filter 2 is also controlled to the frequency (f) of the rotation signal. Thus, the center frequency of the filter 2 follows up the frequency of the input rotation frequency continuously and automatically, so the passing band width of the filter 2 is set sufficiently narrow and the rotation signal with the high S/N is detected.
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