Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-05-12
2001-09-11
Chin, Stephen (Department: 2734)
Pulse or digital communications
Transceivers
Modems
C375S345000, C455S234100, C455S241100
Reexamination Certificate
active
06289044
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of automatic gain control (AGC) circuits, and more particularly, for AGC circuits of modem receivers.
BACKGROUND OF THE INVENTION
In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. The transmission speed of digital subscriber loop (DSL) modems has exhibited a remarkable increase in recent years, and as the increase of the transmission speed progresses, it becomes increasingly necessary to reduce the occurrence of errors in data communication arising from disturbances of the circuit to as few as possible.
In addition, due to the large range of different twisted pair loops over which high speed modems operate and the varying amount of interference, the received signal at the analog-to-digital converter input (in the demodulation section of a modem) can present a dynamic range of over 70 dB.
An automatic gain control circuit is provided in the demodulation section of a modem to monitor the input signal level and to provide the appropriate gain to bring the signal to a desired level.
Traditional AGC circuits in modem receivers working over the necessary full dynamic range present significant differences in performance, convergence times, and gain variability at different regions of operation.
Consequently, there is a need for a AGC circuit in an modem receiver that provides gain tracking for a large range of twisted pair loops with over 70 dB range (for example), various signal bandwidths, and in the presence of radio frequency (RF) and other asynchronous digital subscriber line (ADSL) interference. Further, the AGC circuit must improve gain ripple after convergence to minimize its noise contribution to the system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an automatic gain control circuit for a modem receiver that improves gain tracking for a large range of twisted pair loops.
Another object of the present invention is to provide an automatic gain control circuit for a modem receiver that improves gain ripple after modem convergence to minimize noise contribution.
In accordance with an aspect of the present invention there is provided 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 the signal Y to a first prescribed level during a first mode of operation and to a second prescribed level during a second mode of operation; and means for converting the scaled signal Y into an analog gain control signal for input to the analog AGC.
In accordance with another aspect of the present invention there is provided 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) means for obtaining an AGC signal from the signal Y; (b) means for subtracting the AGC signal from a prescribed reference signal to form a delta signal; (c) scaling means for reducing the delta signal by a prescribed value; (d) an integrator for integrating the scaled delta signal to form an AGC level control signal; and (e) conversion means for converting the AGC level control signal to an analog AGC control signal for the analog AGC amplifier to attenuate the input signal X.
In accordance with another aspect of the present invention there is provided an 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 during a first mode of operation and to a second prescribed level during a second mode of operation; and (b) converting the scaled signal Y into an analog gain control signal for input to the analog AGC.
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Dublin Ian
Noel Peter
Velez Edgar
Chin Stephen
Ha Dac V.
Nortel Networks Limited
Swabey Ogilvy Renault
Wood Max R.
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