| 摘要 |
PURPOSE:To surely decide the presence of a signal by comparing a signal level from a band pass filter with a signal level from a band stop filter. CONSTITUTION:In receiving a signal of frequency fs from a sender side, an output level L2 from an out-band signal level measuring circuit 11 is subjected to addition of a prescribed level alpha by an adder circuit 12 and the result is given to one input terminal of a level comparator circuit 13. An output level LOUT=L4 from an in-band signal level measuring circuit 10 is given as it is to the other input terminal of the circuit 13. because the relation of LOUT=L4> L2+alpha exists, the level comaprison circuit 13 sends signal detection information via a terminal 3. At the time of no signal is obtained from the sender, the level comparator circuit 13 gives the result of LOUT=L2<L2+alpha, the signal undetection information is sent via the terminal 3.
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