Telephonic communications – Subscriber line or transmission line interface – Power supply
Utility Patent
1998-03-16
2001-01-02
{haeck over (S)}mits, T{overscore (a)}livaldis I. (Department: 2741)
Telephonic communications
Subscriber line or transmission line interface
Power supply
C379S412000, C379S399010, C370S279000, C370S286000, C700S094000
Utility Patent
active
06169801
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of telephonic devices, and more particularly to a digital isolation apparatus and method and a power supply and method for the telephonic devices.
2. Description of Related Art
Generally, before a device can be legally connected to a public telephone network
40
as shown in
FIG. 1
, it must be granted approval by one or more safety agencies. Approval status (pass or fail) is determined by testing the device according to the agency's safety requirements. The testing typically includes the verification that an effective electrical isolation barrier
48
as shown in
FIG. 1
exists between a user-connected device (equipment-side) and the telephone network (loop-side). Verification that the isolation barrier meets the safety requirements set forth by the relevant agency is a key to proving that a product can safely be connected to the public telephone network
40
. It also means that an electronic device, e.g. a Digital Signal Processing (DSP) device
52
, will be sufficiently isolated from the telephone network
40
in the event lightning strikes the telephone line or an electric power line falls across or otherwise contacts the telephone line.
In addition, to maintain isolation, the telephone network-side (loop-side) electronic circuits (as shown in
FIGS. 1
,
2
, and
5
, left of isolation lines
46
,
56
, and
72
) are generally powered by the telephone network
40
. Typically, the electronic circuits on the loop-side that are powered by the telephone network
40
require additional complex power supply circuits. Alternatively, some of the electronic circuits may be powered by means of a conventional battery power supply. Accordingly, these conventional power supplies add significant cost and complexity to telephone network interface designs. Therefore, a need exists for a low cost, efficient technique of providing an isolated source of power from a safe-side, for instance, equipment-side to the loop-side.
FIG. 1
illustrates a conventional arrangement where an isolation barrier
48
resides in a section of circuitry commonly referred to in the telephone industry as a Data Access Arrangement (DAA)
42
(FIG.
1
). Originally, the function of the DAA was solely to protect the telephone network
40
from a malfunctioning user-connected device. The DAA was designed to be a stand alone, separate piece of equipment. Today, the term DAA can be more broadly applied to encompass the circuitry in a piece of telephonic equipment that provides a similar isolation function for safety purposes as the original stand alone DAA
42
.
FIG. 1
illustrates the conventional isolation safety barrier
48
coupled directly to a telephone line interface circuit
44
on one end and coupled to a Coder/Decoder (CODEC)
50
as shown in FIGS.
1
-
5
on the opposite end. The CODEC
50
usually contains both an analog-to-digital (A/D) converter and a digital-to-analog (D/A) converter.
The conventional isolation device
48
is an analog isolation device, such as an analog transformer whose function is to isolate loop-side from the equipment-side. The analog transformer passes the entire frequency band of analog signals from the telephone network via the telephone line interface circuit
44
to the CODEC
50
which is connected to the DSP device
52
. The interface between the telephone network
40
and the DSP device
52
can be arranged and configured to function as a modem. In the example of a modem, the band of frequencies ranges from 10 Hz to 4,000 Hz.
Several disadvantages exist with a conventional analog transformer as an isolation device. First, the size of an analog transformer is generally too big. Excursions along the analog transformer's electromagnetic B-H loop causes distortions in the analog signal passed by the transformer. Smaller transformers have greater excursions along the B-H loop as compared with larger transformers. Therefore, larger analog transformers perform better than smaller ones. As more and more demand for a smaller size of the transformer, a point is reached where it becomes physically too small to function properly due to the distortions. Additionally, further size reduction of a transformer reduces the internal creepage and clearance distances required for isolation. If the creepage and clearance distances become too small, the transformer will not function as an effective isolation barrier, and the user-connected device may become unsafe for use on the public telephone network. Larger, higher performance, transformers are too large for applications such as a PCMCIA modem requiring a relatively small transformer. Currently, the PCMCIA modem is constrained to fit in a space equivalent to a stack of three credit cards. In this application, the transformer's small size can result in analog signal distortion that severely hampers the modem's performance.
A proposed approach of reducing the size of the transformer is to move the isolation barrier
48
from the analog interface to the digital interface
56
as shown in FIG.
2
. However, this method requires a power source for the loop-side coding and decoding (CODEC) circuitry. In the existing proposal, the power source is provided by the telephone network
40
or by a separate battery supply.
One disadvantage associated with powering the loop-side circuitry from the telephone network
40
relates to sensing the state of the telephone line. It requires detecting a voltage between a telephone network connection's TIP and RING lines. TIP and RING are typically two lines that carry data, ringing signal, and dial tone from the telephone network's central office to a user connection. The TIP and RING lines have two states, ON-HOOK and OFF-HOOK. The state of the telephone line, when not in use, is referred to as ON-HOOK. Whereas OFF-HOOK refers to the state of the telephone line when it is in use. Typically, the voltage between TIP and RING while the telephone line is ON-HOOK is approximately 48 VDC. Alternatively, while in the OFF-HOOK state, the voltage drops to a level between 6 and 12 VDC. Sensing the voltage across TIP and RING while the telephone line is ON-HOOK, for the purpose of obtaining power for the proposed approach, requires that the circuitry draw an extremely small amount of current. Otherwise, the telephone network central office switching circuits will sense the current flowing through the telephone line causing it to go OFF-HOOK. Generally, telephone networks require the ON-HOOK current thresholds to be much lower than 3 milli-amps and are typically set at 10 micro-amps. Accordingly, sensing the state of the telephone line for obtaining power for the loop-side CODEC in the above proposed approach cannot be readily accomplished.
Alternatively, power may be obtained from the telephone line while it is OFF-HOOK via the ringing signal. Generally, this technique is difficult to implement because the ringing signal has a much higher voltage than that of standard silicon circuits. In addition, the ringing signal is typically shared with other telephonic devices connected to the same line. Drawing too much current from the ringing signal may cause the telephone network central office equipment to assume the device(s) went OFF-HOOK, thus failing a registration test.
It can be seen that there is a need for an improved apparatus and method of isolating signals between the telephone network and digital processing devices.
It can also be seen that there is a need for an improved apparatus and method of supplying power to the loop-side circuitry.
SUMMARY OF THE INVENTION
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an improved apparatus and method for providing digital isolation between the telephone network and telephone devices and an improved power supply.
The present invention solves the above-described problems by providing an isol
Levasseur David James
Rigdon Donald Burnell
Wetzel Richard Miles
Chawan Vijay B.
Merchant & Gould P.C.
Midcom, Inc.
{haeck over (S)}mits T{overscore (a)}livaldis I.
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