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Subscriber Line Interface Circuit

IP.com Disclosure Number: IPCOM000042917D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Ferry, M: AUTHOR [+2]

Abstract

A simple subscriber line interface circuit (SLIC) is proposed that withstands specific equipment in different line switching environments, for instance, European requirements, at the cost of a few more electrical components. The basic circuit, schematically represented in Fig. 1, comprises two quasi-identical blocks each one representing a differential operational amplifier, OA1 or OA2, with a gain of 2 and referenced to a voltage level VS1 or VS2, respectively, other than ground. By splitting resistor R1 into two parts, i.e., R11 and R12, side 1 of the basic circuit is double-ended between nodes M and N, while side 2 with resistor R2 is single ended.

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Subscriber Line Interface Circuit

A simple subscriber line interface circuit (SLIC) is proposed that withstands specific equipment in different line switching environments, for instance, European requirements, at the cost of a few more electrical components. The basic circuit, schematically represented in Fig. 1, comprises two quasi-identical blocks each one representing a differential operational amplifier, OA1 or OA2, with a gain of 2 and referenced to a voltage level VS1 or VS2, respectively, other than ground. By splitting resistor R1 into two parts, i.e., R11 and R12, side 1 of the basic circuit is double-ended between nodes M and N, while side 2 with resistor R2 is single ended. Simple equations show that VS1=VS2-2V2 (1) and VS2=VS1-2V1 (2) which leads to: V2 = - 1 and i2 = R1 __ __ __ V1 i1
R2 with VS1 representing the voltage at OA1 output P and VS2 at OA2 output. Node Q represents the connecting node of resistor R12 to the battery (-48 volts). These equations explain that the basic circuit feeds a DC current in the telephone set from the battery through R1, performs double- to single-ended conversion, performs an impedance conversion with a ratio k = R2/R1 like a transformer. Unlike a transformer, this basic circuit keeps voltage values and performs a current conversion with a ratio 1/k = R1/R2 instead of 1/k. This last property is interesting as it does not change the voltage dynamics although it allows 2 wire/4 wire-conversion and matching with higher impedances. The drawbacks are twofold: a) battery noise is fed directly to the attached station, and
b) since DC and AC input impedances are different and VS1 is applied non- symmetrically with respect to ground, part of the voice signal is converted into longitudinal current and leads to audible crosstalk on long telephone lines. Battery noise is overcome by means of coupled inductors or differential transformers or by referencing AC signals to grou...