Abstract: An automatic gain control and its method are disclosed. An average power computing unit computes the power of a received signal. A power range monitoring unit monitors its change. If the signal power fluctuates around a threshold, the system enters an acquisition mode and the gain is tuned back to a default value. If the signal power is above an upper limit of a tracking range, a gain decreasing unit reduces the gain. If the signal power goes below a lower limit of the tracking range, a gain increasing unit raises the gain. Once the power falls in the tracking range, the system enters a tracking mode to make immediate responses toward the gain of the received signal.