Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1999-11-02
2004-07-13
Barnie, Rexford (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
C379S093050
Reexamination Certificate
active
06763107
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a low noise telephone line interface for data access arrangements (DAA). Specifically, it relates to a low distortion line powered DAA having improved linearity and accuracy.
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 (10-1-97 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.
2
. Since the voice band modem signal is limited to the 100 to 3600 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 offices and various modulation techniques to couple the DAA through small capacitors to the PC.
FIG. 3
shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit of
FIG. 3
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 circuit is designed in low voltage CMOS technology and can handle only a small amount of voltage. 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 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 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.
The circuit depicted in
FIG. 3
works by monitoring the current through sense resistor R
S1
with a feedback loop around the amplifier A. Resistors R
T1
and R
B1
sense the differential voltage across R
S1
. By setting R
T1
=R
B1
, the current through R
T1
and R
B1
will accurately model the current through R
S1
. The desired signal to be modulated is introduced by a differential data source signal V
D
. The differential signal is created by adding signal V
D
/2 to 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 signal input current. The generation of the differential signal current is well known in the art and will not be further discussed herein. The control amplifier operates to force the current through resistor R
S1
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
and thereby through resistor R
S1
. In this circuit, the collector current of transistor Q
1
is well controlled by the control amplifier A. However, this arrangement incurs a degree of error which is problematic for new communication devices such as high speed data modems. The source of the error is due to current that is outside of the path containing the sense resistor R
S1
. This stray current will be discussed after a brief discussion of FIG.
4
.
FIG. 4
depicts an alternative circuit arrangement similar to the circuit depicted in FIG.
3
. However, the circuit in
FIG. 4
uses the output of amplifier A to control the emitter of transistor Q
2
, rather than the base of transistor Q
2
, and thereby the collector current of transistor Q
1
. As in the circuit depicted in
FIG. 3
, the collector current of transistor Q
1
is well controlled by the control amplifier A. This arrangement also incurs a degree of error which is problematic for new communication devices such as high speed data modems.
The error associated with the previously mentioned circuit designs of FIG.
3
and
FIG. 4
will now be discussed. Ideally, the current through R
S1
would equal the current, I
LINE
, introduced to the system by the telephone company. This would allow amplifier A to take all of I
Line
into account when modulating the differential signal source onto I
Line
. An error exists in the line modulation devices of FIG.
3
and
FIG. 4
due to the inclusion of only part of the total current I
LINE
through sense resistor R
S1
. In both circuits, the current from the telephone company is introduced to the system through the emitter of transistor Q
1
(hereinafter “I
E1
”). In the circuits depicted in FIG.
3
and
FIG. 4
, I
E1
is equal to I
LINE
, the resistances of R
T1
and R
B1
are a couple hundred thousand ohms, and the resistance of R
S1
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 circuits, 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
S1
is used by amplifier A in a feedback loop to modulate the desired signal onto I
LINE
. Since the current I
B1
is outside the feedback loop, an error term in the amount of I
B1
is introduced to the circuit, that is, I
C1
through resistor R
S1
is not equal to I
LINE
, but is equal to I
E1
−I
B1
or I
LINE
−I
B1
.
An additional problem arises from I
B1
being outside the amplifier feedback path. Since I
C1
and I
B1
are related by the &bgr; of Q
1
, and the &bgr; of a transistor is a function of the actual signal level, the error term introduced by not accounting for current I
B1
in the feedback loop is signal dependent. Signal dependent error terms are a source of harmonic distortion which is problematic for communication devices. In order for current 56 k modems (V.90 standard) to function, a signal to distortion ratio greater than 80 dB is needed. Unfortunately, due to the error term introduced by neglecting I
B1
, the circuits of FIG.
3
and
FIG. 4
can provide a signal to distortion ratio of only about 75dB, even when high quality components are utilized.
One method which has been used to reduce distortion is depicted in FIG.
5
. 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 incorporated herein by reference.
The circuit in
FIG. 5
reduces distortion by incorporating a larger portion of I
LINE
into the feedback path of the control amplifier A. A larger portion of I
LINE
is incorporated by including a second sense resistor R
S2
in a second feedback path to amplifier A in order to sense current introduced to the system by I
LINE
which does not flow through the first sense resistor R
S1
. The operation of the differential signal source and the shunt regulator are similar to the differential signal source and shunt regulator discussed above. In addition, as with R
T1
and R
B1
, R
T2
and R
B2
have a relatively high level of resistance and the current that flows through R
T2
and R
B2
can be neglected in the circuit analysis.
In
FIG. 5
, the output of amplifier A is electrically connected to the emitter of transistor Q
2
through the additional sense resistor R
S2
, the collector of transistor Q
2
is electrically connected to the base of transistor Q
1
, and the base of transistor Q
2
is electrically connected to the collec
Fischer Jonathan Herman
Hollenbach Keith Eugene
Laturell Donald Raymond
Smith Lane A.
Zhu Weilin
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