Combined active impedance and filter in line drivers

Amplifiers – Signal feedback

Reexamination Certificate

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Details

C330S069000, C330S079000, C330S085000, C330S103000, C330S195000

Reexamination Certificate

active

06566947

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to line drivers and particularly to line drivers having desirable output impedance 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
. 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
. The xDSL modem
122
has 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
. 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
.
Referring to
FIG. 3
, there is shown a conventional implementation of the xDSL driver
106
of FIG.
1
. The input of the xDSL driver
106
is connected
105
to an input of a bandpass filter
302
. The output of the bandpass filter
302
is connected to a first non-inverting input
304
of a summation circuit
306
. The output of the summation circuit
306
is connected to an input
312
of an amplifier
314
. An output of the amplifier
314
is connected
316
to a first terminal of a reference resistor R
e
318
. A second terminal of the reference resistor R
e
318
is connected to the output of the xDSL driver
107
. A resistor R
1
320
is connected from a second non-inverting input
308
of the summation circuit to ground
328
. A resistor R
2
322
is connected from output
107
of the xDSL driver
106
to the second non-inverting input
308
of the summation circuit
306
. A resistor R
3
324
is connected from an inverting input
310
of the summation circuit
306
to ground
328
. A resistor R
4
326
is connected
316
from the output of the power amplifier
314
to the inverting input
310
of the summation circuit
306
.
Referring to
FIG. 4
, there is shown a conventional implementation of the xDSL driver
126
of FIG.
1
. The topology of the xDSL driver
126
is the same as the topology of the xDSL driver
106
of FIG.
3
. The differences are in the upper and lower cut-off frequencies of the filters, a bandpass filter
302
in xDSL driver
106
and bandpass filter
402
in xDSL driver
126
. For example, in the case of G.Lite DSL, the lower cut-off frequency of filter
302
in xDSL driver
106
is 150 kHz, the upper cut-off frequency of filter
302
in xDSL driver
106
is 500 kHz, the lower cut-off frequency of filter
402
in xDSL driver
126
is 30 kHz and the upper cut-off frequency of filter
402
in xDSL driver
126
is 120 kHz. The gain and output impedance of xDSL driver
106
and xDSL driver
126
are substantially the same.
Unfortunately, the performance characteristics such as gain and output impedance of the conventional xDSL drivers
106
,
126
are severely affected by the tolerances of the components in the positive (R
1
320
,
420
, R
2
322
,
422
) and negative (R
3
324
,
424
, R
4
326
,
426
) feedback loops and in the reference resistor (R
e
318
,
418
). Another disadvantage of this circuit is that the active impedance generation and filtering are realized in different stages.
Thus there is a need in the industry to provide an xDSL driver that combines active impedance generation and filtering capabilities in a single amplifier stage. Furthermore, it would be advantageous to provide a line driver that would also have an independently specified gain and output impedance as well as gain that is relatively insensitive to component tolerances.
SUMMARY OF THE INVENTION
The invention may be summarized according to a first broad aspect as a line driver having an amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance and a second feedback impedance. Preferably, the amplifier is an operational amplifier connected in an inverting configuration with an input and an output. The transformer has a primary winding and a secondary winding with a ratio of 1:n. The primary winding having a first terminal connected to the output of the amplifier and having a second terminal. The secondary winding is connectable to a transmission line having a characteristic impedance. The reference impedance is connected from the second terminal of the primary winding at a junction node to a ground reference. The input impedance having one terminal connected to the input of the amplifier and another terminal connectable to a voltage source. The first feedback impedance is connected from the junction node to the input of the amplifier and the second feedback impedance is connected from the output of the amplifier to the input of the amplifier. The second feedback impedance preferably has a value equal to (K−1) times the value of the first feedback impedance.
In accordance with this first broad aspect of the invention, the reference impedance has a value equal to n
2
/K
times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to (K+1) times the voltage at the junction node, for a predetermined value of K. The resulting output impedance will be equal to K times the reference impedance and the gain will be equal to half of the negative of the ratio of the value of the second feedback impedance to the value of the input impedance.
According to a second broad aspect, the invention may be summarized as a line driver having a first amplifier, a transformer, a reference impedance, an input impedance, a first feedback impedance, a second feedback impedance and a second amplifier. Preferably, the first amplifier is an operational amplifier connected in an inverting configuration with a non-inverting input, an inverting input and an output and the seco

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