Telephonic communications – Subscriber line or transmission line interface
Reexamination Certificate
1999-03-25
2003-12-16
Iser, Forester W. (Department: 2747)
Telephonic communications
Subscriber line or transmission line interface
C379S398000
Reexamination Certificate
active
06665399
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to line drivers and more particularly to line drivers having a desired output impedance and required to have a high efficiency.
BACKGROUND OF THE INVENTION
A line driver is typically used for sending an electronic signal onto a transmission line type medium, such as a copper twisted pair. To avoid unwanted reflections at the far end of the transmission line, the latter is usually matched, i.e., terminated by a far-end impedance element which presents to the transmission line a resistance equal to the characteristic impedance of the line.
In many situations, the transmission medium is bidirectional, meaning that there is a second transmitter at the far end of the transmission line. In order for the transmission line to be properly matched in the reverse direction, the line must “see” a near-end impedance equal to its own characteristic impedance. The required near-end impedance, known as the output impedance of the line driver, is usually provided by a reference resistor having a resistance equal to the characteristic impedance of the transmission line. In a voltage-mode configuration, the reference resistor is usually placed in series with the line driver, while in a current-mode configuration, one usually opts for a parallel arrangement.
For example, referring to
FIG. 1
, there is shown a conventional voltage-mode line driver comprising a voltage source/amplifier combination, shown in dotted outline at
102
, connected in series with a reference resistor
106
having a resistance R
e
. The reference resistor
106
is connected to ground via a transformer
108
. The transformer
108
generally has two windings with an equal number of turns, one of which is connected to the reference resistor
106
and the other being used for interfacing with a transmission line
110
. The transmission line
110
has a characteristic impedance Z
c
, which is typically in the range of 50 &OHgr; to 600 &OHgr;.
The output resistance of the line driver (denoted R
out
) is defined as the ratio of the voltage V
0
appearing across the line driver side of the transformer
108
to the current I
0
caused by the voltage V
0
when the voltage source/amplifier combination
102
is short-circuited. Upon applying the short circuit, it is seen that the voltage V
0
appears in its entirety across the reference resistor
106
, from which it follows that R
out
equals R
e
. Therefore, in order for the line driver to be matched to the transmission line
110
, R
out
should equal the characteristic impedance Z
c
, which means that R
e
should be set equal to Z
c
.
Unfortunately, this conventional arrangement results in a wastage of power, as half of the energy output by the voltage source/amplifier combination
102
is lost in the form of heat dissipated in the reference resistor
106
. This prevents high-speed modems and other devices that use line drivers from meeting strict power efficiency guidelines. Furthermore, since the amount of circuit card real estate required for a line driver depends on the amount of power that is dissipated, it follows that only a few such drivers can be placed on a circuit card.
A similar scenario occurs in the current-mode dual configuration, now briefly described with reference to FIG.
2
. The current-mode line driver comprises a current source/amplifier combination (shown in dotted outline at
202
) and placed in parallel with the first winding of a transformer
206
. The transformer
206
has a second winding for interfacing with a transmission line
208
having a characteristic impedance Z
c
. Proper termination at the line driver end is provided by a reference resistor
210
having a resistance R
e
and also placed in parallel with the transformer
206
.
The output resistance P
out
of the line driver in
FIG. 2
is defined as the voltage V
0
appearing across the line driver side of the transformer
206
, divided by the current I
0
caused by the voltage V
0
with the current source/amplifier combination
202
open-circuited. By applying this open circuit condition, it is seen that the voltage V
0
appears in its entirety across the reference resistor
210
. Therefore, R
out
is simply equal to the resistance R
e
of the reference resistor
210
. To achieve proper termination at the line driver end, R
out
should be equal to Z
c
and thus R
e
is usually set equal to Z
c
.
Because R
e
is equal to the characteristic impedance of the transmission line
208
, half the energy output by the source/amplifier combination
202
is dissipated in the reference resistor
210
. This causes the above-mentioned disadvantages, namely the inability of a conventional line driver to meet power efficiency requirements and the imposition of an undesirably low limit on the number of devices employing line drivers that may be placed on a circuit card.
A known solution is the use of a smaller reference impedance between the output of the line driver and the transmission line and to use a combination of positive and negative feedback around this reference impedance to achieve the desired output impedance. However, one limitation of this method is that the gain of the line driver is low and an additional stage is required at the expense of efficiency and noise performance. Higher gains are achievable but the output impedance is severely affected by tolerances of the components in the positive and negative feedback loops and in the reference impedance.
The above background has shown that there is a need in the industry to provide a line driver which can have the same output resistance as a conventional line driver while reducing the amount of energy or power dissipated in the reference resistor. Furthermore, it would be advantageous to provide a line driver which would also have an independently specifiable gain.
SUMMARY OF THE INVENTION
The invention may be summarized according to a first broad aspect as a line driver equipped with an amplifier, a transformer, a reference resistor and a feedback circuit. The amplifier has an input for connection to a voltage source and having an output. The transformer has a first winding and a second winding, the first winding having a first end connected to the output of the amplifier and having a second end and the second winding being connectable to a transmission line having a characteristic impedance. The reference resistor has an end connected to the second end of the first winding at a junction point. The feedback circuit is connected to the input of the amplifier, to the output of the amplifier and to the junction point.
In accordance with this first broad aspect of the invention, the reference resistor has a resistance equal to {fraction (1/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 point, for a predetermined value of K.
The resulting output impedance will be equal to K times the reference impedance. At the same time, the voltage across the reference impedance will be reduced by a factor of K, which advantageously reduces the power lost in the reference resistor by a factor of K.
Preferably, the amplifier is an operational amplifier connected in an inverting configuration and the feedback circuit is a resistive network consisting of a first feedback resistor having a first end connected to the output of the amplifier and having a second end connected to the input of the amplifier and also having a second feedback resistor of which a first end connected both to the input of the amplifier and to the second end of the first feedback resistor and of which a second end connected to the junction point. The first feedback resistor preferably has a resistance equal to (K−1) times the resistance of the second feedback resistor.
According to a second broad aspect, the invention may be summarized as a line driver equipped with a transformer, a reference resistor and a feedback circuit. Again, the transformer has
Anderson Carl
Gorcea Dan
Jakab Gyula
Harold Jefferey
Iser Forester W.
Nortel Networks Limited
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