| 摘要 |
PURPOSE:To improve the reliability of mis-detection to white noise, by providing a band pass filter taking the entire frequency band of reception signals as the pass band and transmitting a detection signal when the rectified signal level of an output signal is lower than the reference level. CONSTITUTION:An output signal of a band pass filter 23 taking a low group frequency component 21 and a high group frequency component 22 of a reception signal as the pass band, is rectified at the 3rd rectifier circuit 33, and this output signal is applied to the 3rd comparison circuit 43 which produces an output signal when the level is the 2nd reference level VS2 or below set corresponding to the reception level higher than the 1st reference level VS1. When the output signal of the 3rd comparison circuit 43 and that of the 1st and 2nd comparison circuits 41, 42 are simultaneously outputted consecutively for a prescribed time or over, a multifrequency signal can be detected. |
