| 摘要 |
PURPOSE:To realize the optimum AGC characteristic at both fading time and normal time of the input receiving state, by providing a timeconstant changing means which detects the fading time and normal time of the input receiving state and automatically changes the time constant of the smoothing circuit of a feedback controlling circuit. CONSTITUTION:Part of a DC voltage Edet produced when the output Pout of a gain amplifier 1 is detected by means of a detector 21 is compared with a reference voltage Er at the level comparator 32a of a timeconstant changing circuit 3. When the Edet is lower than the Er (at the time of fading), the comparator 32a outputs a control signal EC and a relay RL is actuated by driving a transistor 31. Since the capacitor C1 of a smoothing circuit 22 is cut off when the relay RL is actuated, the time constant becomes smaller and the DC output current Ic of a feedback controlling circuit 2 changes following the change in time constant. Accordingly, the gain of the amplifier 1 is controlled. When the Edet is higher than the Er (standard state), the EC is not outputted and the time constant of the circuit 22 remains as a large value. Therefore, an AGC circuit does not follow the instantaneous amplitude change of the PAM signal of a QAM and the code error ratio characteristic does not become worse. |
