| 摘要 |
<p>PURPOSE:To eliminate the need for provision of a phase difference of 180 deg. to an optical signal by deciding a threshold value at an amplifier having a feedback diode and an automatic threshold value set circuit so as to compare the optical signal with the value in detecting the optical signal. CONSTITUTION:A photodetector 1 converts the optical signal into an electric signal, which is inputted to an amplifier 2. Moreover, the input range of the amplifier 2 is expanded by the feedback diode 3. Then the automatic threshold value set circuit 4 holds a peak voltage of an output of the amplifier 2 and outputs a value being a half the held peak voltage so as to use it as a threshold value of a comparator 5. The comparator 5 has a hysteresis characteristic and converts stably the input into a pulse voltage. Thus, it is not required to provide a 180 deg. phase difference to an encoder circuit and it is not necessary to provide two fixed slits and two photodetectors are not required and only one photodetector is enough for the purpose.</p> |
