Index signal generating circuit

An index signal generating circuit comprising a double-pole magnet attached onto the circumference of the rotor of a spindle motor, a magnetic flux detecting coil for detecting magnetic flux of the double-pole magnet when the double-pole magnet moves past the magnetic flux detecting coil and for generating a detected output having two peaks of the same polarity, a feedback comparator circuit for comparing a direct current voltage fed to a first input of the comparator circuit with the detected output of the magnetic flux detecting coil fed to a second input of the comparator circuit to produce a comparison output, and a mask voltage generating circuit for generating a detected signal that lasts during a fixed duration of time from the moment of a transition of the comparison output, and for supplying the detected signal as a mask voltage to the first input of the comparator circuit, wherein the direct current voltage fed to the first input is set to be higher than the level of the detected output of the magnetic flux detecting coil with no magnetic flux being detected but lower than the two peaks of the same polarity of the output magnetic flux detecting coil with magnetic flux being detected;

• the signal level at the first input as the mask voltage is set to be higher than the two peaks of the same polarity of the detected output;
• the timing of the supplying of the mask voltage is set such that the mask voltage masks one of the two peaks of the same polarity;
• and an index signal is generated based on the timing of a transition of the comparison output.