| 摘要 |
A constant output signal can be achieved by controlling a gain variable amplifier 2 based on a level of an output signal which is derived by variably amplifying a receiving signal S1 (input signal). An AGC output signal S11 is detected by a detector circuit 3, then a detected voltage signal S7 is converted into a digital signal by an A/D converter 4 to output it to a CPU 5. Then, the CPU 5 generates a control signal S8 based on the detected voltage signal S7, then an A/D converter 6 converts the control signal S8 into the analogue signal, and then the signal is sent out to the gain variable amplifier 2 to execute gain control. At this time, an RSSI detector portion 9 detects an RSSI signal S10 corresponding to a transmitting signal level from the receiving signal S1, and then the CPU 5 sends out the control signal S8 stored at the time of preceding execution of the gain control when the RSSI signal S10 is reduced below a predetermined threshold value S12. In the weak received electric field strength, the control voltage for the gain variable amplifier is prevented reduction, and the proper automatic gain control can be made possible to thus achieve the good receiving characteristics.
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