Pulse or digital communications – Receivers – Automatic baseline or threshold adjustment
Reexamination Certificate
2000-12-26
2004-06-22
Booure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Automatic baseline or threshold adjustment
C327S558000
Reexamination Certificate
active
06754288
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to line receivers and particularly to line receivers having desirable dynamic range and filtering capabilities in a single amplifier stage.
BACKGROUND OF THE INVENTION
DSL (digital subscriber line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephones lines. xDSL refers to different variations of DSL such as ADSL (asymmetric DSL), G.Lite DSL (ITU-T standard G-992.2), HDASL (high bit-rate DSL) and RADSL (rate-adaptive DSL).
DSL modems are typically installed in pairs, with one of the modems installed in a home (customer's premises) and the other in the telephone company's central office servicing that home. The pair of xDSL modems are connected to the opposite ends of the same twisted-pair transmission line.
Referring to
FIG. 1
a conventional xDSL communication system
100
comprises a CO (central office)
101
. The CO
101
has a plurality of xDSL modems
102
(only one shown). The xDSL modem
102
has a D/A (digital to analog) converter
104
. An output of the D/A converter
104
is connected
105
to an input of an xDSL driver
106
. An output of the xDSL driver
106
is connected
107
to a 4-wire input of a hybrid
108
. A 4-wire output of the hybrid
108
is connected
109
to an input of an xDSL receiver
110
. An output of the xDSL receiver
110
is connected
111
to the input of an A/D (analog to digital) converter
112
. An AGC (automatic gain control) output of the A/D converter
112
is connected
113
to an AGC input of the xDSL receiver
110
. A 2-wire port of the hybrid
108
is connected to a transmission line
114
, such as copper twisted pair.
The xDSL communication system
100
also comprises CPE (customer premises equipment)
126
. The CPE
126
has an xDSL modem
122
having a D/A converter
124
. An output of the D/A converter
124
is connected
125
to an input of an xDSL driver
126
. An output of the xDSL driver
126
is connected
127
to a 4-wire input of a hybrid
128
. A 4-wire output of the hybrid
128
is connected
129
to an input of an xDSL receiver
130
. An output of the xDSL receiver
130
is connected
131
to an input of an A/D converter
132
. An AGC output of the A/D converter
132
is connected
133
to an AGC input of the xDSL receiver
130
. The 2-wire port of the hybrid
128
is connected to the transmission line
114
.
Since an xDSL modem operates at frequencies higher than the voice-band frequencies, an xDSL modem may operate simultaneously with a voice-band modem or a telephone conversation. Referring to
FIG. 2
, there is shown an example of a frequency spectrum plan
200
for a G.Lite DSL system on the transmission line
114
of FIG.
1
. The frequency range from 0.3 to 4 kHz
202
is occupied by conventional voice communications. The frequency range from 30 to 120 kHz
204
is occupied by upstream (CPE
126
to CO
101
) data transmission. The frequency range from 150 kHz to approximately 500 kHz
206
is occupied by downstream (CO
101
to CPE
126
) data transmission. The upper frequency limit of the downstream data transmission is determined by the length and quality of the transmission line
114
.
A problem in xDSL communications systems is that the hybrid
108
in the CO
101
does not provide adequate isolation between the xDSL driver
106
in the CO
101
and the xDSL receiver
110
in the CO
101
. Similarly, the hybrid
128
in the CPE
126
does not provide adequate isolation between the xDSL driver
126
in the CPE
126
and the xDSL receiver
130
in the CPE
126
. This lack of isolation causes unwanted noise, distortion and saturation.
One possible solution is to use a Salen and Key circuit for the xDSL receiver
110
and the xDSL receiver
130
of FIG.
1
. Referring to
FIG. 3
, there is shown a conventional implementation of a Salen and Key circuit
300
that comprises a first impedance
302
(Z
1
) having a first terminal connected to an input node
320
and a second terminal connected to a first junction node
322
; a second impedance
304
(Z
2
) having a first terminal connected to the first junction node
322
and a second terminal connected to a second junction node
324
; a third impedance
306
(Z
3
) having a first terminal connected to the second junction node
324
and a second terminal connected to a ground reference
314
; a fourth impedance
308
(Z
4
) having a first terminal connected to the first junction node
322
and a second terminal connected to an output node
326
; and an amplifier
310
that is preferably an operational amplifier (opamp) having a non-inverting input
310
A connected to the second junction node
324
, an output
310
C connected to the output node
326
and an inverting input
310
B connected to the output
310
C. The input node
320
is connectable to a voltage source (shown in dotted outline at
312
) such as a 4-wire output of a hybrid. The output node
326
is connectable to a load (not shown) such as an input of an A/D converter.
In the xDSL receiver
110
in the CO
101
the downstream data transmission
206
must be rejected and the upstream data transmission
204
be passed. It is therefore advantageous that the xDSL receiver
110
in the CO
101
be a low-pass filter. To this end, referring to
FIG. 3
, the first impedance
302
and second impedance
304
are implemented as resistors while the third impedance
306
and fourth impedance
308
are implemented as capacitors. The values of the four impedances
302
,
304
,
306
,
308
are chosen such that the cut-off frequency of the low-pass filter is between the frequency of the upstream data transmission and the frequency of the downstream data transmission. For example, in the case of G.Lite DSL, the cut-off frequency would be between 120 kHz and 150 kHz.
Conversely, in the xDSL receiver
130
in the CPE
126
the upstream data transmission
204
must be rejected and the downstream data transmission
206
be passed. It is therefore advantageous that the xDSL receiver
130
in the CPE
126
be a high-pass filter. To this end, referring to
FIG. 3
, the first impedance
302
and second impedance
304
are implemented as capacitors while the third impedance
306
and fourth impedance
308
are implemented as resistors. The values of the four impedances
302
,
304
,
306
,
308
are chosen such that the cut-off frequency of the high-pass filter is between the frequency of the upstream data transmission and the frequency of the downstream data transmission. For example, in the case of G.Lite DSL, the cut-off frequency would be between 120 kHz and 150 kHz.
A problem with the conventional Salen and Key circuit of
FIG. 3
is that the pass-band gain is fixed. As well, the dynamic range is limited by the presence of leakage from the xDSL drivers
106
,
126
through the hybrids
108
,
128
.
Two possible solutions are shown in
FIGS. 4A and 4B
. In
FIG. 4A
the topology is the same as
FIG. 3
except that an adjustable gain amplifier
404
having an input
404
A and an output
404
B is inserted such that the output
404
B is connected to the input node
320
. However, there is no protection against large interfering signals in the adjustable gain amplifier
404
. In
FIG. 4B
the topology is the same as
FIG. 3
except that an adjustable gain amplifier
406
having an input
406
A and an output
406
B is inserted such that the input
406
A is connected to the output node
326
. However, the circuits of
FIGS. 4A and 4B
have the disadvantages of high noise, high power, distortion and complexity.
Thus there is a need in the industry to provide an xDSL receiver with improved dynamic range. Furthermore, it would be advantageous to provide an xDSL receiver that would also have filtering capabilities, low power consumption, less complexity and good noise figure.
SUMMARY OF THE INVENTION
The invention may be summarised according to a first broad aspect as a line receiver having an input and an output, equipped with an amplifier that is preferably an operational amplifier, a network of four impe
Booure Tesfaldet
Tran Khanh Cong
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