| 摘要 |
A low power consuming signal detection device which is attached to a telephone line is provided. The device has means for supplying power within the device with an impedance of at least 5 megohms across the phone line, means for amplifying signals on the line, phase-locked-loop signal sensing means for determining when the frequency of the amplified signal is within a predefined banwidth, and lock detection means for outputting a control signal when the frequency of the signal is within the predetermined bandwidth for a predetermined period of time. The device may be powered by a telephone central office battery by using means for converting telephone line battery potential to the required supply voltage. |
