| 摘要 |
PURPOSE:To improve the noise resistance of a receiver by detecting an AC signal, charging a capacitor, and closing a switch circuit inserted into an AC signal passage when the terminal voltage of the capacitor becomes a predetermined voltage or higher. CONSTITUTION:An AC signal Sa of specific frequency is supplied through a band filter 1, a preamplifier 2, a switch 31 for forming a noise preventing circuit 3 and a Schmitt trigger 4 to a driver circuit 5 such as a power amplifier. A detector 32 detects an AC signal applied from the output of the amplifier 2 to charge a capacitor, and a switch circuit SW31 is closed only when the terminal voltage of the capacitor becomes a predetermined voltage or higher. A discharger 33 abruptly discharge the charge stored in the capacitor of the detector 32 in response to the erasure of the AC signal. Thus, SW31 is closed by the signal Sa continuing for a predetermined time or longer. |
