The invention is concerned with a hybrid-line circuit for connecting a two-wire line to a four-wire switching system.
In telephone networks a hybrid-line circuit is required for connecting each two-wire line to the four wires of a switching center, that circuit providing transfer of voice signals in both directions, providing line termination impedance, providing high voltage separation from the switching center, and fulfilling a number of other functions. The main component of the hybrid line circuit in use today is a transformer which provides many of the necessary functions (signal transfer, electrical separation, symmetric feeding). Because of its simplicity and flexibility the transformer is particularly useful for this application and it has proven to be a reliable component.
But the use of transformers also has disadvantages. These include particularly its space requirements which prevent the high packing density of circuit cards now usual in modern equipment, as well as its cost. It is, therefore, desirable to eliminate the use of transformers in hybrid line circuits. It has previously been suggested that high-voltage capacitors be used for signal transfer. However, rather large capacitors were required, and, furthermore, these solutions did not provide for a very good rejection of common-mode signals.
It has heretofore been proposed that isolation between the lines and the central equipment can be provided at a substantial reduction in both size and cost through the use of optical couplers. U.S. Pat. No. 3,987,257, issued Oct. 19, 1976, utilizes a light-emitting diode and a phototransistor connected in series in each wire of a two-wire line, a pair of photo-transistors connected in series in one wire of the four-wire line, and a pair of light-emitting diodes connected in series in another wire of the four-wire line. Optical coupling between the two-wire and four-wire lines is provided by light generated within the light-emitting diodes and received by the photo-transistors of the respective two-wire and four-wire lines.
One of the difficulties believed to reside in such an arrangement is matching the transfer characteristics of the optical couplers used so that signals to or from the two-wire line are symmetrical. The transfer characteristics of such opto-couplers also may vary with temperature and age, causing further non-symmetry.
It is an object of the present invention to provide a circuit arrangement using opto-couplers which has the advantages of high voltage protection of the switching system (up to .+-. 1500V) and medium voltage protection of the hybrid-line circuit itelf (up to 300V), stable insertion loss in both transfer directions, high return loss (20 dB at 600.OMEGA. source impedance, high earth symmetry independent of frequency, high longitudinal balance of outgoing signals, and low idling noise. Furthermore, cross-talk over the battery is prevented because almost no voice signal current flows through the battery. The disclosed hybrid circuit further provides line termination at 600.OMEGA. and the necessary 2-to-4 wire conversion.
In addition, selection of the opto-couplers in the incoming and outgoing paths is less critical in the present invention compared to the above identified patented circuit. Also, the arrangement is less sensitive to temperature and aging effects with influence gain and linearity.