| 摘要 |
PURPOSE:To design the titled control circuit to allow the AGC amplifier to follow the fast level fluctuation of an input signal by using a signal excluding the amplitude modulation component included in the output signal of the amplifier of a multivalue QAM signal as control information. CONSTITUTION:A logic circuit 2 multiplies an output data of an A/D converter 1 with a reciprocal of a ratio with a substantial amplitude obtained from output data I, Q of a demodulator to arrange the level into a substantial amplitude. The output of the logic circuit 2 represents only the phase modulation component being eliminated the amplitude modulation component of a carrier signal (multi-value QAM signal) subject to multi-value orthogonal amplitude modulation from the output of an IF amplifier 5B. A binary error signal with a setting value outputted from a comparator 3 is converted into an analog signal A by a D/A converter 4 and a voltage obtained by integrating the signal A by an integration device 5 is used as a control voltage Vc of a variable attenuator 6 to vary the attenuation. Thus, the time constant of the integration device 5 is selected to a small value and even when the input signal level of the AGC (automatic gain control) amplifier is fluctuated at fast speed, the circuit can follow sufficiently the input signal level. |
