| 摘要 |
<p>A low power consuming signal detection device which is attached to a telephone line is provided. The device has means for supplying power (16) within the device with an impedance of at least 5 megohms across the phone line (10, 12), means for amplifying signals (20) on the line, phase-locked-loop signal sensing means (22, 24, 26) for determining when the frequency of the amplified signal is within a predefined bandwidth, and lock detection means (28) 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.</p> |
