Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2000-01-06
2002-04-16
Ghayour, Mohammad H. (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S220000
Reexamination Certificate
active
06373886
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates broadly to telecommunications. More particularly, this invention relates to modems.
2. State of the Art
A modem is a computer hardware peripheral designed to convert digital signals generated by the computer into analog signals that can be modulated onto an analog transmission medium, such as a twisted pair copper wire telephone line, and demodulate the signals back into digital data at the receiving end. Modems used on a public switched telephone network (PSTN) telephone line (i.e., a “public line”) have an interface to the telephone line called a data access arrangement, and commonly referred to as a DAA. The DAA provides functions such as dielectric isolation, dc termination, ac termination, ring detection, and protection from transients. This functionality can be implemented with transformers, inductors, and electromechanical relays, which are relatively bulky and expensive.
Various techniques have been used to reduce the size and cost of the DAA. As shown in prior art
FIG. 1
, a modem
10
includes modem circuitry
12
coupled to a “silicon” DAA
14
. The silicon DAA
14
includes two integrated circuits
16
,
18
(e.g., the Si3021 and Si3014, available from Silicon Laboratories) separated by a dielectric barrier (in this case a capacitor)
20
, and minimal external components
22
. This silicon DAA
14
implements the DAA functions and also integrates “analog front end” functions of analog-digital conversion, digital-analog conversion, and filtering. As a PSTN line to which the modem is coupled has 48V inherently present at all times, integrated circuit
18
is powered from the voltage on the public telephone line
24
.
A “private line” or “leased line” is a telephone line directly connecting two businesses, a business and a home, or different departments in a business or institution, without going through the public switch telephone network. Modems intended to be used over a private line do not use many of the DAA functions required for modem use over the PSTN. Nevertheless, the silicon DAA would be a cost effective way in which to implement the analog front end functions of a modem over a private line even if the other DAA functions are not utilized. Therefore, it is desirable to allow modems designed for operation over public lines to also operate over private lines. However, unlike public lines, private lines are not powered, and therefore cannot power the integrated circuits of the silicon DAA. Therefore, prior art silicon DAAs cannot be used over private lines.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a power supply for a DAA to enable a DAA intended for use over a public line to operate over a private line.
It is another object of the invention to provide a power supply which does not adversely affect dielectric isolation, dc termination, and protection from transients.
It is also an object of the invention to provide a power supply which is relatively simple in design and relatively inexpensive to manufacture.
In accord with these objects, which will be discussed in detail below, a power supply is provided which includes a dc voltage source and circuitry which powers the DAA while preventing dc voltages from being applied to the private line; i.e., the private line is effectively isolated from dc voltages generated by the voltage sources. The circuitry further permits ac signals from the modem to travel substantially unhindered over the private line.
According to one embodiment of the invention, the circuitry coupled to the dc voltage source includes a split winding transformer, and a capacitor coupled across the voltage source. A first primary winding of the transformer is coupled in series between a positive pole of the voltage source and a positive input to the DAA and a second primary winding of the transformer is coupled in series between a negative pole of the voltage source and a negative input to the DAA.
According to a second embodiment, the circuitry coupled to the dc voltage source includes a standard transformer, an inductor and a capacitor. The inductor, the capacitor, and the primary winding of the transformer are arranged in series with the voltage source, with the capacitor coupled between the positive input to the DAA and the primary winding, and the inductor coupled between the positive pole of the voltage source and the positive input to the DAA.
According to a third embodiment of the invention, the circuitry coupled to the dc voltage source includes two capacitors and two inductors; no transformer is utilized. The capacitors are coupled to the inputs of the DAA, while the inductors are coupled between the poles of the voltage source and respective inputs of the DAA.
The voltage source in each embodiment externally supplies power to the DAA, while the circuitry couples the ac modem signals to and from the private line. The transformer (in the first and second embodiments) and capacitors (in the third embodiment) provide a dc dielectric barrier between the private line and the dc voltage source; i.e., isolate the private line from the dc voltage, yet permit ac signals to and from the DAA circuit to pass therethrough.
While each circuit has different advantages and disadvantages, each meets regulatory and performance requirements including dielectric isolation, longitudinal balance, frequency response, return loss/impedance, and tolerance to lightning surges and other transient voltages. In addition, since the power applied to the DAA is a known voltage and polarity, the DAA does not have to tolerate a wide range of dc voltages and current of either polarity. As such, components otherwise required for a silicon DAA adapted for use over the PSTN may be eliminated, thereby reducing the cost of the DAA.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
REFERENCES:
patent: 5140630 (1992-08-01), Fry et al.
patent: 5148144 (1992-09-01), Sutterlin et al.
patent: 6147963 (2000-11-01), Walker et al.
patent: 6212226 (2001-04-01), Newton
patent: 6226331 (2001-05-01), Gambuzza
Specifications from Silicon Laboratories on 3.3 V International Digital Access Arrangement, 1999 revised, 43 pages.
Gallagher Thomas A.
Ghayour Mohammad H.
Gordon David P.
Jacobson David S.
PC Tel Inc.
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