| 摘要 |
PURPOSE:To attain sure switching of a transmission line when the line is subjected to radio wave interference from other system in a relay section by detecting the level of a frequency component at both end of a spectrum band of a desired wave. CONSTITUTION:The component at frequencies f, f at both ends of the band of the spectrum of a received input signal converted into an intermediate frequency signal at an intermediate frequency amplifier 3 and subject to digital modulation is extracted respectively by band pass filters 13, 14, and an output obtained through the detection at detectors 15, 16 is fed to an AND circuit 17. The circuit 17 gives an output when the detected output level of the component at the frequencies f, f reaches a certain level or below. The output of the circuit 17 and a signal representing interruption of the intermediate frequency output level from a DC amplifier 4 are applied to an OR circuit 18. When either one signal is applied, the circuit 18 is operated, a squelch circuit 5 is activated so as to disconnect the connection between a reception section 20 and a transmission section 21, a switch circuit 7 is activated and an output of an oscillator 8 corresponding to the intermediate signal is fed to a transmission mixer 9 via a hybrid circuit 6.
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