Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1998-06-17
2001-03-06
Dorvil, Richemond (Department: 2741)
Telephonic communications
Subscriber line or transmission line interface
C379S387020
Reexamination Certificate
active
06198818
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an analog front end circuit and, more particularly, to an analog front end circuit of a data communications equipment device (DCE) or other equipment for use in a multipoint environment wherein the analog front end circuit incorporates a feedback circuit that forces the input current of the analog front end circuit to zero, thereby generating a high input impedance and eliminating the impedance null that would otherwise occur in the frequency bands being used by the DCE or other equipment.
BACKGROUND OF THE INVENTION
Conventional communications systems typically include two DCEs connected across a communication connection, such as a subscriber loop associated with a public-switched-telephone-network (PSTN). The communication connection is comprised of a pair of copper wires that are terminated at the DCE so that the DCEs may communicate with each other via either half-duplex or full-duplex communications. These DCEs normally include a voltage driver that has a low source impedance for driving the data signal across the connection. Consequently, to effect multipoint communication, bridging additional voltage-driven devices to an end of the communications connection significantly loads the connection making it difficult to successfully communicate across the connection,
As a result of the difficulty of bridging multiple voltage-driven DCE tributaries to the connection, in order to increase the number of DCEs at a particular location, prior art systems typically increase the number of communications connections such that each tributary at a customer premises is serviced by a separate communications connection. The insertion of additional connections into the system increases the overall cost of the system, particularly when the connection is a subscriber loop associated with a PSTN.
Recently, a multipoint system has been developed by the assignee of the present application that allows a plurality of tributaries to be bridged to a standard PSTN telephone twisted pair. The DCEs coupled to each of the tributaries communicate with a DCE located at the central office via a particular polling protocol. Each of the DCEs coupled to the tributaries comprises a current source-driven (i.e., driven by a high-impedance source) analog front end circuit that couples each DCE to its respective tributary via a dry transformer. In order to prevent the DCEs connected to the communications connection from pulling DC from the PSTN central office equipment, a DC blocking capacitor is inserted into the analog front end circuit of each DCE.
This DC blocking capacitor and the transformer of the analog front end circuit form an equivalent series RLC circuit. Multi-band communications over the multipoint system may include frequencies ranging from 15 Hz for the ring signal on the low end, from 200 Hz to 4000 Hz for voice-band modem communications and voice communications, and from approximately 20 KHz to 80 KHz at the upper end for data transmission. Practical values for the inductors of the transformer and for the DC blocking capacitor cause an impedance null to be created in or near the voice band with a low quality factor Q, usually resulting in a large band of attenuation in the voice band.
Accordingly, a need exists for a DCE that can be employed in a multipoint environment and which has an analog front end circuit designed to eliminate the impedance null that would otherwise occur in or near the voice band.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for use with an analog front end circuit of a DCE or other equipment utilized in a multipoint system for preventing an impedance null from being generated in the frequency bands being used by the DCE or other equipment bridged to the multipoint system, such as, for example, a telephone. A feedback circuit is employed with the analog front end circuit to force the input current of the analog front end circuit to zero, thus providing the analog front end circuit with a high input impedance, which prevents an impedance null from forming in the frequency bands being used by the DCE or other equipment.
In accordance with the preferred embodiment of the present invention, the analog front end circuit of the present invention is utilized in equipment that is bridged to a twisted wire pair of a PSTN. The meaning of the term “DCE”, as that term is used herein, is intended to encompass any and all types of equipment that may be bridged to a multipoint system, which preferably is a twisted wire pair of a PSTN. Therefore, in the interest of brevity, the analog front end circuit of the present invention will only be described in relation to its use in conjunction with a “DCE”, since this term is intended to cover all types of equipment that can bu used in conjunction with the analog front end circuit of the present invention. It will be understood by those skilled in the art that the present invention is not limited to use with any particular type of equipment connected to the multipoint system. It will also be apparent to those skilled in te art that the present invention is not limited to use with a PSTN, but that the present invention can be used with any type of network.
In accordance with the preferred embodiment of the present invention, a sensing resistor is incorporated into the analog front end circuit of the DCE for sensing the input current of the analog front end circuit. A feedback circuit coupled to the sensing resistor detects the input current sensed by the sensing resistor and causes a power driver of the analog front end circuit to force the input current to zero, thus providing the analog front end circuit with a high input impedance that prevents an impedance null from being created in the frequency bands being used by the DCE.
These and other features and advantages of the present invention will become apparent from the following discussion, drawings and claims.
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patent: 5850436 (1998-12-01), Rosen et al.
patent: 5864607 (1999-01-01), Rosen et al.
patent: 5982852 (1999-11-01), Schwartz
Dorvil Richemond
Paradyne Corporation
Thomas Kayden Horstemeyer & Risley LLP
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