| 摘要 |
PURPOSE:To attain small size, light weight, low cost and stable characteristic by converting a supervisory signal into a digital level signal, inputting the result to a digital phase locked loop circuit, integrating an output of a phase detector and converting the result into a DC, thereby detecting the frequency of the supervising signal. CONSTITUTION:A 6kHz supervisory signal S1 modulated by a receiver is converted into a digital level signal S2 through a band pass filter 1 at a slicer 2. The signal S2 is inputted to a digital phase locked loop circuit 3. Then an output S4 corresponding to the digital level signal S2 is obtained and set to a modulator 9 and a phase signal S3 is obtained from a phase detector built in the said circuit 3. The phase signal S3 is inputted to an integration circuit 4, integrated and converted into a DC. The DC is inputted to a frequency detection circuit 5, from which the frequency of the supervisory signal S1 is detected, thereby detecting the supervisory signal S1. |
