Telephonic communications – Subscriber line or transmission line interface – Hybrid circuit
Reexamination Certificate
1998-02-12
2001-03-27
Chan, Wing F. (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
Hybrid circuit
C379S398000
Reexamination Certificate
active
06208732
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hybrid circuit for performing two-wire-to-four-wire conversion, and vice versa. More particularly, the present invention relates to a hybrid circuit that employs an impedance matching network comprising circuit modules that can be switched on or off to produce a balancing impedance that matches the impedance of the subscriber loop. The hybrid circuit of the present invention can be used for all wire-bound communications systems for which a two-wire-to-four-wire conversion, or vice versa, must be carried out.
BACKGROUND OF THE INVENTION
Normally, hybrid circuits are comprised within data communications equipment (DCE) located at a central office. A four-wire circuit contained in the DCE connects the hybrid circuit to the receiver and transmitter of the DCE. A two-wire circuit, which corresponds to the subscriber loop, couples customer premises to the hybrid circuit at the central office. The hybrid circuit performs a conversion from the two-wire circuit to the four-wire circuit, and vice versa. The hybrid circuit provides the capability of full-duplex communication between the central office and the customer premises.
A typical hybrid circuit comprises a bridge circuit that is coupled to the two-wire circuit via a transformer. The bridge circuit comprises an impedance matching network for matching the impedance of the hybrid circuit with the impedance of the subscriber loop. Transhybrid loss, which corresponds to the portion of the signal transmitted by the transmitter of the DCE that is sent over the subscriber loop without being received by the receiver of the DCE, is controlled by matching the impedance of the impedance matching network of the bridge circuit with the impedance of the subscriber loop. Therefore, it is desirable to maximize the transhybrid loss, thus minimizing the amount of the transmitted signal which is reflected at the interface of the bridge circuit and the subscriber loop and received by the receiver of the DCE, which is commonly referred to as near-end echo.
In the past, attempts have been made to maximize the transhybrid loss by designing the impedance of the impedance matching network of the hybrid circuit so that it matches, as closely as possible, the impedance of the subscriber loop. However, once the impedance matching network was designed and implemented in the hybrid circuit, the impedance of the impedance matching network was fixed and could not be altered.
Accordingly, a need exists for a hybrid circuit having a variable impedance matching network that can be implemented in a practical manner and that is cost-effective.
SUMMARY OF THE INVENTION
The present invention provides a hybrid circuit having a switched impedance matching network. The hybrid circuit of the present invention performs two-wire-to-four-wire conversion, and vice versa, and can be used in any wire-bound communications system for which two-wire-to-four-wire conversion, and vice versa, must be performed. The hybrid circuit of the present invention can be implemented in a digital subscriber line (DSL) system and can be used with high-speed XDSL systems. In accordance with the preferred embodiment of the present invention, the matching network comprises switchable circuit modules of discrete-valued resistor and/or capacitor components. Preferably, the circuit modules are assembled into an array of buffer-isolated first-order RC circuits, although higher-order circuit modules may also be used. The buffers comprise amplifiers, which serve two purposes. A first purpose is to isolate the individual circuit modules from each other when they are connected in a cascade of circuit modules. A second purpose is to avoid having to implement actual transmission switches between the circuit modules.
The switching of the buffer amplifiers clears or blocks the transmission paths between the individual circuit modules, thereby providing the optimum path through the module array. By controlling the transmission paths through the module array, the balancing impedance of the matching network is optimized, thus maximizing transbybrid loss and minimizing near-end echo.
At least one circuit module of the array is switched on at any given time. Preferably, two or three of the circuit modules of the array are switched on at any given time to form a cascade of modules. The number of modules switched on at any given time in the array will depend on many factors, such as the impedance of the subscriber loop, as will be understood by those skilled in the art. Also, it is possible to obtain the desired balancing impedance by merely varying the gain of the switched-on circuit module(s). This feature of the present invention allows the power requirements of the array, and consequently, the power requirements of the overall hybrid circuit, to be minimized.
The optimum component settings for the circuit modules are determined through testing and will depend on the impedances of the subscriber loops with which the hybrid circuit is implemented. Once the component values for the circuit modules of the matching network have been selected, a learning phase begins during which different combinations of the circuit modules are connected for the different subscriber loops. During the learning phase, different combinations of circuit modules are switched on for each subscriber loop and the transhybrid power loss for each combination is measured. The combination of circuit modules resulting in the greatest maximization of transhybrid power loss is selected as the best combination for the particular subscriber loop. During operation, the appropriate combination of circuit modules for a particular subscriber loop are switched on to provide the hybrid circuit with a balancing impedance that substantially matches the impedance of the subscriber loop. Once a combination of circuit modules has been selected, the proper balancing impedance may be obtained by merely varying the gain of the array of circuit modules.
REFERENCES:
patent: 4757530 (1988-07-01), Arnon
patent: 5175763 (1992-12-01), Gazsi
Moschytz George S.
Muralt Arnold
Chan Wing F.
Globespan Semiconductor Inc.
Thomas Kayden Horstemeyer & Risley
LandOfFree
Switched hybrid circuit for use with digital subscriber lines does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Switched hybrid circuit for use with digital subscriber lines, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switched hybrid circuit for use with digital subscriber lines will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2458276