Method and apparatus for stabilization of a line powered...

Telephonic communications – Subscriber line or transmission line interface

Reexamination Certificate

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Details

C379S093060

Reexamination Certificate

active

06728371

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a telephone line interface for data access arrangements (DAA). Specifically, it relates to a line powered DAA with enhanced stability.
BACKGROUND OF THE INVENTION
The telephone lines to a residence in the United States and elsewhere can have common mode voltages of over 100V, and the FCC requires the telephone lines to be isolated from any electric main powered device (such as a PC) connected to the telephone lines (through a modem for example) to prevent damage to the telephone network. 47 CFR 68.302,4 (Oct. 1, 1997 Edition). A data access arrangement (DAA) is specified by the FCC to isolate the telephone lines from electric main powered devices, such as illustrated in FIG.
4
. Since a voice band modem signal is limited to the 50 to 4000 Hz band, a DAA can be constructed using a transformer which operates as a bandpass filter to isolate the electric main powered device from the telephone lines.
A smaller size and potentially lower cost solution uses active circuits to communicate with the central telephone office and various modulation techniques to couple the DAA through small capacitors to a device, such as a PC.
FIG. 5
shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/028,061 filed on Feb. 26, 1998, entitled “Low Noise Line Powered DAA With Feedback,” assigned to the same assignee as the present application, and incorporated herein by reference.
The main function of the circuit is to take the incoming current I
LINE
supplied by the telephone company and modulate it with a data signal developed by processing a differential data signal source V
D
with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q
1
as a line modulator, and contains a shunt regulator in series with the line modulator Q
1
. A sense resistor R
S
is placed in series between the line modulator Q
1
and the shunt regulator to monitor the current through the shunt regulator.
The circuit depicted in
FIG. 5
works by monitoring the current I
S
through sense resistor R
S
with a feedback loop around the amplifier A. Resistors R
T
and R
B
sense the differential voltage across R
S
. By setting R
T
=R
B
, the current through R
T
and R
B
will accurately model the current through R
S
. The desired signal to be modulated is introduced by a differential signal source V
D
. The differential signal is created by adding signal V
D
/2 to common mode voltage, V
CM
, to create V
P
, and subtracting V
D
/2 from V
CM
to create V
N
. This differential signal then drives the input resistors R
IP
and R
IN
to provide a differential input current signal. The generation of the differential current signal is well known in the art. The control amplifier operates to force the current through resistor R
S
to equal the desired signal current by regulating transistor Q
2
to control the base of transistor Q
1
, which in turn regulates the current through the collector-emitter path of transistor Q
1
and thereby through resistor R
S
. In this circuit, the collector current of transistor Q
1
is controlled by the control amplifier A.
Ideally, the current through R
S
would equal the current, I
LINE
, introduced to the system by the telephone company. However, this is not the case in actuality. The current from the telephone company is introduced to the system through the emitter of transistor Q
1
(hereinafter “I
E1
”). In the circuit depicted in
FIG. 5
, I
E1
is equal to I
LINE
, the resistances of R
T
and R
B
are a couple hundred thousand ohms, and the resistance of R
S
is 10-20 ohms. Because of the relatively high level of resistance of R
T1
and R
B1
, the current that flows through R
T1
and R
B1
can be neglected in the circuit analysis. As current flows through the circuit, I
E1
is divided into the transistor Q
1
base current (hereinafter “I
B1
”) and the transistor Q
1
collector current (hereinafter “I
C1
”) The collector current I
C1
through the resistor R
S
is used by amplifier A in a feedback loop to modulate the desired signal onto I
LINE
.
FIG. 6
shows another known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/280,473 filed on Mar. 30, 1999, entitled “Method and Apparatus for Decreasing Distortion in a Line Powered Modulator Circuit,” assigned to the same assignee as the present application, and is incorporated herein by reference.
As in the circuit described above in reference to
FIG. 5
, the main function of the circuit in
FIG. 6
is to take the incoming current I
LINE
supplied by the telephone company and modulate it with a data signal developed by processing a differential data source signal V
D
with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q
1
as a line modulator, and contains a shunt regulator in series with the line modulator Q
1
. A first sense resistor R
S1
is placed in series between the line modulator Q
1
and the shunt regulator to monitor the current through the shunt regulator. In addition, a second sense resistor R
S2
is added within the modulator to pick up “stray components” of line current I
LINE
which are outside of the feedback path containing the first sense resistor R
S1
, and incorporate the “stray components” into an additional feedback path around the amplifier A.
The circuit depicted in
FIG. 6
works by monitoring the current through sense resistor R
S1
and R
S2
with feedback loops around the amplifier A. The method of sensing the current through R
S1
and R
S2
, and for generating the differential signal current is similar to the circuit setup described in reference to FIG.
5
. The control amplifier operates to force the sum of the current through resistors R
S1
and R
S2
, and thereby I
LINE
, to equal the desired signal current by regulating transistor Q
2
to control the base of transistor Q
1
, which in turn regulates the current through the source-emitter path of transistor Q
1
.
FIG. 7
shows another known line powered telephone line interface circuit designed in low voltage CMOS technology for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/407,444 filed on Sep. 29, 1999, entitled “Pre-Charging Line Modem Capacitors to Reduce DC Setup Time,” assigned to the same assignee as the present application, and is incorporated herein by reference.
As in the circuits described above in reference to FIG.
5
and
FIG. 6
, the main function of the circuit in
FIG. 7
is to take the incoming current, I
LINE
, supplied by the telephone company and modulate it with a data signal developed by processing a differential data signal source, V
D
, with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q
1
as a line modulator, and contains a shunt regulator in series with the line modulator Q
1
. A sense resistor R
S
is placed in series between the line modulator Q
1
and the shunt regulator to monitor the current through the shunt regulator. In addition, resistors R
1
and R
2
and capacitors C
1
and C
2
are included to set the AC gain of the modulator. Because the circuit is used to control AC and DC characteristics, additional components are required to obtain desired AC and DC values.
To have a low enough frequency response for the full voice band (e.g., down to about 50 Hz and up to about 4 kHz), capacitors C
1
and C
2
need to be large enough so there is not undue frequency response distortion of the signal. However, larger capacitors take a longer time to charge up, i.e., to finish settling down. The settling time may be as large as 400 ms, for example. During the time that the capacitors are charging, the transient charging curren

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