Telephonic communications – Substation or terminal circuitry – Sidetone control or hybrid circuit
Reexamination Certificate
1999-03-30
2003-12-09
Harvey, Minsun Oh (Department: 2644)
Telephonic communications
Substation or terminal circuitry
Sidetone control or hybrid circuit
C379S412000
Reexamination Certificate
active
06661892
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 line powered DAA having significantly improved linearity and accuracy over the prior art.
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.
3
. Id. 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 office and various modulation techniques to couple the DAA through small capacitors to the PC.
FIG. 4
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 is 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 signal source, V
D
, with a line modulator so as to place the data signal on the telephone line. The prior art 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 prior art circuit depicted in
FIG. 4
works by monitoring the current through sense resistor R
S1
with a feedback loop around the amplifier A. Resistors R
TI
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 signal source 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 prior 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.
Ideally, the current through R
S1
would equal the current, I
LINE
, introduced to the system by the telephone company. An error exists in the prior art line modulation device of
FIG. 4
due to the inclusion of only part of the total current I
LINE
in R
S1
. The current from the telephone company is introduced to the system through the emitter of transistor Q
1
(hereinafter “I
E1
”). In the prior art circuit depicted in
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 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
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 Beta of Q
1
, and the Beta 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 circuit of
FIG. 4
can provide a signal to distortion ratio of only about 75 dB even when high quality components are utilized.
SUMMARY
The present invention proposes a novel method and apparatus for increasing the signal to distortion ratio in a line powered telephone line interface or data access arrangement (DAA). The present invention accomplishes this task by reducing the amount of error which is inherent to the prior art circuit design by adapting the circuit of
FIG. 4
to incorporate more of the total line current supplied by the telephone company into the feedback circuit. The invention modifies the prior art circuit by inserting an additional sense resistor to sense the current that is flowing through the line modulator but not through the shunt regulator.
According to one aspect of the present invention, the prior art is improved upon by inserting an additional sense resistor in series with the emitter output of transistor Q
2
, found within the line modulator. In the prior art circuit, the telephone line current I
LINE
enters the DAA through the emitter of transistor Q
1
. The current that is allowed to flow through Q
1
results in a base current of Q
1
, I
B1
, and a collector current of Q
1
, I
C1
. The prior art only incorporates I
C1
into the feedback path of the amplifier, leaving I
B1
outside of the feedback path. Since transistor Q
2
is electrically connected to the base of Q
1
, the current I
B1
flows through transistor Q
2
. Therefore, the emitter of transistor Q
2
is the path for most of the total circuit current I
LINE
which is not flowing through the prior art sense resistor or equivalently the shunt regulator.
In accordance with this aspect of the present invention, by serially connecting an additional sense resistor to the emitter output of transistor Q
2
and including the current across the additional sense resistor, in addition to the current across the original sense resistor, in the feedback path, the amplifier can sense substantially all of I
LINE
within the circuit and process accordingly to remove noise and distortion. This arrangement results in an increase of the overall signal to distortion ratio in the circuit.
REFERENCES:
patent: 4358645 (1982-11-01), Brown
patent: 5636273 (1997-06-01), Schopfer et al.
patent: 5640451 (1997-06-01), Schopfer
patent: 5809109 (1998-09-01), Moyal et al.
patent: 6298037 (2001-10-01), Sharifi
U.S. patent application Ser. No. 09/028,061, Hollenbach et al., filed Feb. 23, 1998.
U.S. patent application Ser. No. 09/055,991, Hollenbach et al.,
Agere Systems Inc.
Harold Jeffrey
Harvey Minsun Oh
Synnestvedt & Lechner LLP
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