| 摘要 |
PURPOSE:To attain a good SN ratio to the noise level and to perform satisfactory demodulation by controlling the gain of an amplifier so that this gain is an optimum value in accordance with the change of the noise level inputted to a demodulating circuit. CONSTITUTION:A demodulating circuit 5 demodulates an inputted signal of the frequency shift keying system to a digital signal of '1' and '0'. The signal demodulated to the digital signal before modulation by a demodulating device 6 is transmitted to a CPU 7. A command is issued from the CPU 7 to a noise measuring circuit 11 in an idle time when a carrier signal is not transmitted, and the noise measuring circuit 11 measures the noise level inputted to the demodulating circuit 5 and sends the result to the CPU 7. The CPU 7 sends a command to a gain control circuit 12 to set the gain of an amplifier 3 so that the reception level of the demodulating circuit 5 is set to an optimum level to the measured noise level. The gain control circuit 12 which receives the command controls the gain of the amplifier 3 to an optimum value, thereby performing satisfactory demodulation in the demodulating device 6.
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