PURPOSE:To make it unnecessary to do a complicated consideration for completing two characteristics of the existing system and the reserve system, by changing for use one set of a diode detector and a logarithmic amplifier to the existing unit and the reserve unit by the chopper method. CONSTITUTION:An input signal of the carrier wave input terminal 1 is equally divided by the branch circuit 2 and is provided to both the existing unit and the reserve unit. The directional couplers 4 and 14 divide a part of the input carrier wave, and these two carrier waves are supplied to a change-over device 31 by which a chopper signal is provided to the change-over control input, and are changed mutually. An output of the change-over device 31 is provided to the phase synchronous amplitude detector 35 by which a chopper signal is provided to the control input terminal through the diode detector 33 and the logarithmic amplifier 34. An output of the detector 35 is provided to the threshold detector 37 through the low band filter 36, and the change-over device 38 of the existing unit and the reseve unit is driven by the above output.
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