Universal impedance matching network for the subscriber line integrated circuits

Abstract

A codifier/decodifier (CODEC) filter circuit () connected in a subscriber line interface circuit () includes a transmit section () for converting differential voltage audio transmit signals representing voice transmissions from the subscriber instrument () into encoded digital data for transmission to the digital switching network. A receive section () coupled between the digital switching network and subscriber instrument () within CODEC () for converting encoded digital data representing voice signals switched through the digital switching network to differential voltage audio receive signals for transmission to the subscriber instrument (). The subscriber loop and subscriber instrument () reflect the digital voltage audio signals to the transmit section (). An impedance section () within CODEC () connects between the transmit section () and the receive section () and is disposed to provide an audio band feedback signal between the transmit section () and the receive section () for synthesizing a source impedance for the subscriber line that matches the subscriber loop impedance. Impedance section () includes an analog impedance scaling network () coupled between the transmit section () and receive section (). The impedance section () also includes a programmable digital filter () coupled to the transmit section () having a transfer function equal to:()(1)/((2)(1+(−2)/(2))where Ris the second subscriber loop impedance, Cis the subscriber loop capacitance, Ris the first subscriber loop impedance, T is the sampling rate of the analog-to-digital converter and z is the frequency of the signal. Furthermore, a summer circuit () provides feedback between the programmable digital filter () and the receive section () by summing the single-ended audio receive signals from the digital switching network with the audio band feedback signal output by the programmable digital filter ().