| 摘要 |
PURPOSE:To prevent a comparator from malfunctioning and to secure the detection of a specific signal by setting the time constant of a 2nd rectifying and smoothing circuit which rectifies an input signal and generates a reference voltage longer larger than the time constant of the 1st rectifying and smoothing circuit of a detecting circuit. CONSTITUTION:When there is no signal, the difference between the outputs of a detecting circuit 5 and a reference voltage generating circuit 11 becomes invariably constant because of the difference in amplification factor between amplifying circuits 6 and 12 to preclude the malfunction due to a noise (c). Further, when a specific signal (d) is inputted, it is inputted to both circuits 5 and 11 and amplified 6 and 12. The output of the circuit 6 has the specific signal extracted by an LC tuning circuit 7 and this signal and the output of the circuit 12 are rectified and smoothed by the 1st rectifying and smoothing circuit 8 and the 2nd rectifying and smoothing circuit 13 respectively. Here, the capacity of the capacitor C3 of the circuit 13 is set larger than the capacity of the capacitor C1 of the circuit 8, so the output (e) of the circuit 13 is led slower than the output (f) of the circuit 8 as shown in a figure (b). Therefore, when the specific signal is inputted, the input voltage rises above the reference voltage of the comparator CP1 and whether or not there is the specific signal is detected.
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