Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1998-07-01
2001-02-20
Chan, Wing F. (Department: 2743)
Telephonic communications
Subscriber line or transmission line interface
C379S379000, C379S093050
Reexamination Certificate
active
06192125
ABSTRACT:
REFERENCE TO RELATED APPLICATIONS
The present application is being concurrently filed with commonly assigned, U.S. patent application entitled ELECTRONICALLY SWITCHED OPTICALLY COUPLED LINE INTERFACE, the disclosure of which is incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
This invention relates to a circuit for providing communication between one computer and another via the public switched telephone network while simultaneously providing a high degree of electrical isolation therein between.
BACKGROUND OF THE INVENTION
In telephone systems there is a requirement that devices, such as modems, which are coupled to telephone lines must provide isolation between the device and the central office. Typically, prior art systems provide isolation by using a transformer between the telephone line and the device. The telephone line is usually connected to the primary side of the transformer and the device is on the secondary side. Signals from the transmission line and from the device are coupled across the transformer so that current does not flow directly from the device to the central office.
Instead of using a transformer to couple a device to a transmission line, some prior art systems use optically coupled circuits to provide the required isolation. This type of system is disclosed in U.S. Pat. Nos. 4,190,747 and 4,228,323, both to Feiner et al and in co-pending Patent application Ser. No. 08/957,916, assigned to a common assignee, the disclosures of which are hereby incorporated by reference. Optically coupled systems use photo emitters and photo detectors to couple the two sides of the circuit using light waves.
However transformers have disadvantages due to their physical size as well as their cost. Optocouplers are used for PC/MCIA applications where size constraints are severe. However, these circuits are very complicated and costly since they require feedback circuits to cancel errors due to component variation, especially the current transfer ratio (CTR) of the optocoupler. CTR, is the ratio of induced photo current in a photo-transistor to the forward current of an LED.
The circuit must provide a load to the telephone network consistent with certain requirements. These requirements are usually in the form of a permissible DC voltage and current. Additionally, the AC input impedance is specified as well, which typically is 600 Ohms in the U.S.
The electrical isolation provided by the circuit accomplishes two objectives: 1) isolates the common voltage or ground of the telephone line from that of the computer's allowing them to be substantially different and helps isolate; 2) voltage surges which may be present on the telephone line which could prove to be destructive to sensitive integrated circuits within the computer.
A final requirement for the circuit is that it performs a 4-wire to 2-wire conversion. From the figure, the “4 wires” are the receive signal (Rx) and its reference (typically the computer system's ground) and the transmit signal (Tx) and its reference. The “2 wires” are the tip and ring, which carry the standard signals which telephones and modems use.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a circuit which uses optocouplers that are manufactured in such a way as to guarantee the CTR of the device over a controlled range, while still costing little more than a standard optocoupler. Thus, by employing these devices, a minimal circuit involving only two optocouplers can be used which yields minimum cost.
The major blocks of the circuit are the electronic inductor, the receive optocoupler and its associated biasing circuitry, the transmit optocoupler and its associated biasing circuitry, and the transmit canceller, which is often referred to in the industry as a near-end echo canceller. A central concept of the overall circuit is to take advantage of the availability of optocouplers with controlled current transfer ratios, thus eliminating the need for expensive feedback circuits. By using such controlled transfer ratio optocouplers, a DAA circuit is designed to produce a linear response over the entire range of operation. Since feedback is not required, the minimum number of optocouplers can be employed to meet the requirements of the DAA circuit.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
REFERENCES:
patent: 5245654 (1993-09-01), Wilkison et al.
patent: 5946394 (1999-08-01), Gambuzza
Kincaid Darin
Norsworthy John
Brady W. James
Chan Wing F.
Courtney Mark E.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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