Automatic gain control circuit for a modem receiver

An automatic gain control circuit for a modem receiver, said receiver having an analog automatic gain control (AGC) amplifier to attenuate an input signal X prior to being digitized to a signal Y by an analog-to-digital converter, said automatic gain control circuit comprising:

• a programmable loop gain for scaling down the signal Y to a first prescribed level using a first gain factor during a first mode of operation and to a second prescribed level using a second gain factor during a second mode of operation, the programmable loop gain being characterised by 2−k1, wherein k1 represents the gain factors;
• an up/down counter with variable step size for integrating the scaled down signal Y to form an AGC level control signal;
• and
• an analog reconstruction filter for converting the scaled down signal Y into an analog gain control signal for input to the analog AGC.

A method of attenuating an input signal X in an automatic gain control circuit of a modem receiver, said receiver having an analog automatic gain control (AGC) amplifier to attenuate the input signal X prior to being digitized to a signal Y by an analog-to-digital converter, the method comprising the steps of:

• (a) scaling the signal Y to a first prescribed level using a first gain factor during a first mode of operation and to a second prescribed level using a second gain factor during a second mode of operation, using a programmable loop gain characterized by 2−k1, wherein k1 represents the first and second gain factors;
• (b) integrating the scaled signal Y using an up/down counter with variable step size to produce an AGC level control signal;
• and
• (c) converting the AGC level control signal into an analog gain control signal for input to the analog AGC using an analog reconstruction filter.