Line interface for telephone
A line interface includes a transmit path for an electrical signal, a receive path for an electrical signal, and a reference path coupled from the output of the transmit path to the receive path. The reference path including a filter for matching the impedance of the telephone line. The transmit path has an output coupled to the primary winding of a transformer. The secondary winding of the transformer is adapted to be coupled to a telephone line. The receive path has an input coupled to the primary winding and the reference path includes means for inverting an electrical signal, thereby providing a hybrid coupling function.
1 . A hybrid circuit including a transformer having a primary winding and a secondary winding, wherein the secondary winding can be coupled to a telephone line, said hybrid circuit further comprising:
a transmit path for an electrical signal, said transmit path having an output coupled to the primary winding;
a receive path for an electrical signal, said receive path having an input coupled to the primary winding;
a reference path having an input coupled to the output of the transmit path and an input coupled to the input of the receive path, said reference path including a filter for matching the impedance of the telephone line.
2 . The hybrid circuit as set forth in claim 1 wherein said reference path includes means for inverting an electrical signal.
3 . The hybrid circuit as set forth in claim 2 wherein the transmit path and the receive path each include differential signal paths.
4 . The hybrid circuit as set forth in claim 3 wherein said means includes a cross-over between the differential signal paths.
5 . A line interface including a hybrid network for coupling a telephone having l INE IN and LINE OUT leads to a telephone line including at least a pair of wires, wherein said hybrid network comprises:
a transformer having a primary winding and a secondary winding, wherein the secondary winding can be coupled to said telephone line;
a first amplifier having a first input coupled to a LINE OUT lead and an output coupled to said primary winding;
a second amplifier having a first input coupled to a LINE IN lead and an output coupled to said primary winding;
a filter circuit;
means for inverting a signal coupling the filter circuit to the first input of the second amplifier;
wherein a signal from said first amplifier is subtracted from a signal on the first input of said second amplifier; and
wherein said filter circuit substantially matches the impedance of the telephone line to the telephone.
6 . The line interface as set forth in claim 5 wherein the filter circuit includes a high pass filter and a low pass filter coupled in series.
7 . The line interface as set forth in claim 5 wherein the filter circuit includes a pre-emphasis circuit.
8 . The line interface as set forth in claim 5 wherein said filter circuit includes means for inverting an electrical signal.
9 . The line interface as set forth in claim 5 wherein said interface is implemented with differential signal paths.
10 . The line interface as set forth in claim 9 wherein said filter circuit includes a cross-over between the differential signal paths.