Method and apparatus for providing for automatic gain control of incoming signals in a modem

摘要

An improved circuit for providing automatic gain control (AGC) for incoming phase shift keyed (PSK) and quadrature amplitude modulated (QAM) signals. An absolute value circuit (193) and a comparator (195) provide a first error signal (197). An integrater (200,202) smoothes the first error signal (197) to provide a second error signal (201) to a variable-threshold threshold detector (204). The threshold (209) is initially set at a low value to allow the AGC circuit to quickly respond. A larger value is then used to reduce susceptibility to noise and provide for proper QAM operation. An error circuit (208,211,213) provides a non-linear response so that the gain variations will be small when the input signal (190) is large. This non-linear response further reduces the effects of noise on the AGC circuit.