Electronic digital logic circuitry – Signal sensitivity or transmission integrity – Bus or line termination
Reexamination Certificate
2003-01-03
2004-11-30
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Signal sensitivity or transmission integrity
Bus or line termination
C326S082000, C326S086000, C327S108000, C327S109000, C327S110000, C375S222000, C379S399010
Reexamination Certificate
active
06825686
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to line interface circuits and line drivers and in particular to a line interface circuit or line driver which includes a novel impedance matching arrangement which is suitable for use in a digital subscriber line (xDSL) modem as well as methods of operating the same.
BACKGROUND TO THE INVENTION
Line drivers such as Asymmetric Digital Subscriber Line (ADSL) drivers, Very High Speed Digital Subscriber Line (VSDL) drivers and others generally called in the art xDSL line drivers (where “x” represents the type of technology) are known. Some such prior art arrangements are discussed in, for example, Bill Schweber “Analogue front ends bridge the xDSL to real world chasm”, EDN, Apr. 1, 1999; E. Nash, “Line driver design for broadband communications applications”, Electronic design, Dec. 1, 1997; and from M. Steffes, “Optimising performance in an xDSL line driver”, Electronic design, Apr. 19, 1999.
It is well known to provide transformer based line drivers and an example of a typical hybrid arrangement of this general type is shown with reference to
FIGS. 1
to
3
. In the example shown, a simple series termination is employed.
There is no unified terminology for the whole circuit, which is used to convert of an analogue signal to the appropriate line level. In one convention it is referred to as a discrete AFE (but the amplifier can be integrated). Others sources call it a line interface circuit. The term line driver is sometimes use for the complete circuit and sometimes for only part of this circuit. Although there are differences in terminology, the principles of operation remain the same. Generally, the word line driver will be used in the following when describing the invention and when used in the attached claims and it should be understood that a line driver means the circuitry that performs conversion of the analogue signal to the appropriate line level, foresees a termination impedance and eventually provides power splitting between transmitted and received signal.
FIG. 1
is a schematic block diagram of a conventional line driver comprising an amplifier
2
feeding a hybrid circuit
1
with a termination impedance
10
, the hybrid circuit being for transmitting signals to and receiving signals from a transmission line
9
. The hybrid circuit is assumed almost loss free. The hybrid circuit
1
has an input
11
and an output
12
to receiver circuitry. The impedance of the hybrid circuit
1
is matched to that (Z
ref
) of the line
9
. If the output voltage of the amplifier
2
is m.V volts and the power consumed by the circuit is P=2×V (volts)×I (amps), then the current entering the hybrid circuit
1
is 2.I/m amps. The power consumed in the line
9
is I×V. The impedance of the termination load is selected as k
2
Z
ref
and the current in the termination impedance is I/k. The power consumed in the termination load
10
is I/k×V×k=I×V, i.e. 50% of the power is lost in the termination impedance.
FIGS. 2 and 3
show a conventional circuit of this type, whereby
FIG. 3
shows the voltages and currents present in the circuit. An operational amplifier
2
has an input of V
NE
/G (V
NE
refers to a near end voltage) where G is the gain represented by the line signal/input signal. The output of the operational amplifier
2
is connected to the centre-tapping of a primary winding
4
,
5
of a splitting transformer
3
(1:1:N). A first primary winding
4
of the transformer 1:1:N (3) is terminated in known fashion to ground for load matching to the line
9
via a termination resistance R (7), giving rise to a termination impedance Z
ref
.
A second primary winding
5
of the transformer 1:1:N (3) is connected to the line
9
, represented here by a load
8
in the form of an equivalent resistance R
load
. For transmission, a near end signal having an amplitude V
NE
is placed on the load R
load
, while during reception the circuit receives a far end signal V
FE
from the line at the same point. The symmetry in the operation of the circuit can be seen clearly as the voltages and currents in the termination load and in the line are the same.
The input power to the hybrid circuit is represented by P=2.V.I and it will be apparent to the skilled person that the power dissipated in the termination impedance Z
ref
in this kind of arrangement is typically P/2, which means of course that half of the power generated is dissipated in the line driver itself.
One arrangement which improves the prior art approach of
FIGS. 1
to
3
is shown in EP0901221, the disclosure of which is incorporated herein by way of reference and to which the skilled person is referred for a better understanding of known systems of this type. The arrangement of EP0901221 is suitable for matching a line driver with a predetermined load impedance, but with reduced power consumption when compared with the arrangement of
FIGS. 1
to
3
.
Although an improvement over the basic line driver, the arrangement of EP0901221 still requires a conventionally highly powered operational amplifier (op-amp) for its successful implementation and line driver power consumption remains at what might still be considered to be a high level. There continues to exist, therefore, a need for line driver arrangements that consume less power than some of those available to date.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved line driver arrangement.
It is also an object of the present invention to provide an improved method of operating a line driver.
It is also an object of the present invention to provide an improved line driver arrangement and method of operating the same which results in less wasted power dissipation.
Accordingly, the present invention provides a line driver for use with an associated load such as a transmission line, the line driver comprising a hybrid circuit, a signal portion and a termination portion connected to an output of the hybrid circuit for load matching, the termination portion including a termination impedance which is supplied in use by the hybrid circuit with a termination current, wherein said termination portion further comprises a back termination transformer, a first winding of the transformer being coupled in series with said termination resistance, a second winding of the transformer being coupled in parallel with said termination impedance so that a portion of the voltage dropped across the termination portion is dropped across the first winding to thereby cause the power dissipated in said termination impedance to be less than the power available to the signal transmission portion for signal transmission to the load, e.g. to an associated transmission line. To provide input impedance matching to the associated load (transmission line), a feedback circuit is provided to feedback current from the signal portion to an input of the hybrid circuit.
The present invention may also provide a line driver for use with an associated load such as a transmission line comprising a termination portion which includes a termination impedance and which is supplied in use by an associated amplifier, preferably in the form of an operational amplifier (op-amp), with a termination current for load matching, wherein said termination portion further comprises a back-termination transformer which is coupled with said termination impedance so that a current is injected into the termination portion via a winding of the back termination transformer, whereby the power dissipated in said termination impedance is less than the power available to a signal transmission portion for signal transmission to the associated load, e.g. to the associated transmission line.
The power dissipated in the termination portion compared to the power available in the load (transmission line) is one of at least 10% less, at least 20% less, at least 30% less, at least 40% less, or at least 50% less,
A back termination current may be induced in a winding of said back-termination transformer and may be fed back
Alcatel
Nguyen Khai M.
Tokar Michael
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