| 摘要 |
An automatic gain control circuit which is digitally embodied to prevent deterioration of bit error rate generated by saturation of a low noise amplifier located at the front stage of a receiver through an input of high power to an antenna, thereby simplifying construction thereof and improving commercialization. The automatic gain control circuit of a radio data communication system using a spread spectrum method utilizes a low noise amplifier for low noise amplifying a bandpass filtered signal received through an antenna; a limiter for generating a received signal strength indicator voltage in order to constantly maintain the signal received as a local oscillation frequency through the antenna before downward frequency conversion; a unit for separating the a signal transmitted through the limiter to I and Q channels; a comparator for comparing the received signal strength indicator voltage with a given reference voltage; a D flip-flop for inputting an output of the comparator applied to a data input terminal in synchronization to a given first control signal; an AND gate for ANDing the output of the D flip-flop and the first control signal; a field effect transistor having a gate for receiving the output of the AND gate therein, a drain provided with a given voltage, and source connected to power supply terminal of the low noise amplifier through a resistor; and, a modem for sensing carrier, generating the sensed signal as the first control signal, and outputting digital demodulation data from a signal separated to the I and Q channels.
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