Pulse or digital communications – Cable systems and components – Transformer coupling
Reexamination Certificate
1998-06-16
2002-06-04
Deppe, Betsy L. (Department: 2634)
Pulse or digital communications
Cable systems and components
Transformer coupling
C379S398000, C333S017300, C333S032000
Reexamination Certificate
active
06400772
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns adjusting the output impedance of a line interface to accommodate the impedance of a transmission line with taps for the efficient coupling of power to the transmission line. More particularly, the invention concerns a line interface and a method for matching the output impedance of a transceiver with the input impedance of a transmission line with taps. More particularly, the invention concerns the efficient output of a maximum amount of power to the transmission line.
High bit rate digital service lines (HDSL) are used to transmit digitized information over standard telephone lines. In this and other telephony applications, the transmission line is commonly referred to as a “loop”. A common problem with full-duplex communication over a loop with taps is inefficient power output to the loop due, to an impedance mismatch at the interface between the loop and a transceiver.
One or more taps may exist at various locations on a loop. These taps, which are provided for future connection to the loop are typically unloaded. Loop impedance Z
loop
is a function of the location and number of taps in the loop. A tap located near an end of the loop can significantly affect the value of Z
loop
. Accordingly, if due to the presence taps, the value of Z
loop
differs from the value of Z
out
which characterizes an output impedance driving the loop, signal power transferred to the loop will be reduced.
The ANSI T1E1.4196-006 and ETS1 ETR 152 standards define ten test loops for a transceiver. The loops are designated as one through ten. Because of taps, the loop impedance of some of the loops, for example the loops designated two, seven, and nine, are known to be “problematic” at the customer premises (CP) side of a loop. The remaining loops (one, three-six, eight, and ten) are deemed “non-problematic” because they exhibit loop impedances that are relatively unaffected by taps. In this regard, Z
loop
for one ANSI loop configuration (say, loop
2
) may be in excess of 110 ohms, while the loop impedance for one of the problematic loops may be about 20 ohms. Consequently, a transceiver designed to drive the first loop will very inefficiently drive the problematic loop. Table 2B in the appendix lists the input impedances of loop
6
and loop
2
at several frequencies f
i
of interest. Table 2C in the appendix lists the input impedances of loop
7
and loop
9
at several frequencies of interest. Loop
6
is an example of a loop without taps. Loops
2
,
7
, and
9
are examples of loops with taps close to the CP side of the loops.
FIG. 1
illustrates a circuit commonly used as an interface to a loop
25
. A shortcoming of this circuit is that the output impedance Z
out
of the interface is matched to only one value of input impedance Z
loop
of the loop
25
.
For the circuit shown in FIG.
1
:
Z
out
=2
RL
(
n
2
)+
Rtr
≈135 ohms,
where,
RL=RL
1
=RL
2≈16.7 ohms,
n≈2, (with “n” being the ratio of the number of turns N
2
of the secondary of the transformer T to number of turns N
1
of the primary of the transformer T), and
Rtr≈1.4 ohms, (with Rtr being the winding resistance of the primary side of the transformer T).
Generally, Z
loop
=R+jX. In the circuit illustrated in
FIG. 1
, for the case when the loop has no taps, for frequencies f>100 kHz, X equals about 0, R equals about 108 ohms, and consequently Z
loop
equals about 108 ohms. For frequencies f<100 kHz, X equals about −1/jwc, R equals about 108 ohms, as a consequence Z
loop
may be greater than about 135 ohms. Thus, for the case of no taps connected to the loop, 135 ohms is a reasonable estimate of the value of Z
loop
for frequencies both greater than and less than 100 kHz. A simplified schematic diagram of the relationship between Z
out
and Z
loop
is illustrated in FIG.
2
. For the circuits of
FIGS. 1 and 2
, Z
out
=Z
loop
≈135 ohms, and the transfer function T(s)=Z
loop
/(Z
out
+Z
loop
)=½. Therefore, when there are no taps on the loop, the impedance matching between Z
out
and Z
loop
is good, and consequently the output power is maximum, frequency performance distortions are minimum, and there is no phase shift. A further benefit is good echo cancellation in received signals via operation of a hybrid
30
.
FIG. 3
illustrates the transformer T and the loop
25
of the circuit of
FIG. 1
, but with a tap
45
in the loop near the CP end of the loop. For the circuit illustrated in
FIG. 3
, in the frequency band of interest, which is about 80 KHz to about 400 KHz, the value of Z
loop
is complex and, for example, will drop to about 20±j20 ohms. Z
out
remains equal to about 135 ohms. Consequently, the impedances no longer match and power output to the loop is reduced. A simplified schematic diagram of the relationship between Z
out
and Z
loop
in this case is illustrated in FIG.
4
. The transfer function T(s) is as follows: T(s)=20±j20/((135+(20+j20)). As a result of the poor impedance matching, the output power drops by a factor of about ten, frequency performance distortions are relatively high, there is phase shift distortion, and the echo of the transmitted signal is inadequately removed from signals received from the loop. Thus, the performance of the circuit of
FIG. 1
is maximally efficient without taps in the loop, because, due to the fixed values of R
1
and RL
2
, the circuit is optimized for only one value of Z
loop
. In practice, the value of Z
loop
may vary from one loop to another, and there is a need for an interface that automatically matches its output impedance Z
out
to the impedances of various loops, for example the ten loops defined by the ANSI standard.
A collateral problem with full-duplex communication over a loop is that the transmitted signal's echo (TE) becomes mixed with the signal received from the loop. Line interfaces typically have a line coupling transformer, for coupling the signal to be transmitted into the loop. The line coupling transformer will typically have a first winding and a second winding, with the second winding being connected to the loop. A voltage referred to as V
echo
is present at the first winding. V
echo
consists of an aggregate of both a received signal and TE. TE may be considerably larger than the received signal. The received signal, therefore, may be significantly corrupted by TE. It is desirable to remove TE from V
echo
in order to produce a signal that consists of only the received signal. TE is commonly removed from V
echo
with a subtractor, which subtracts an estimate of TE from V
echo
.
The circuit of
FIG. 1
also subtracts an approximation of TE from V
echo
, thereby reducing the amount of TE coexisting with the received signal. To accomplish this, the signal to be transmitted is tapped after the power amplifiers and is input to a hybrid
30
. Ideally, the output of the hybrid is an accurate replica of TE, which is subtracted from the aggregate of the received signal and TE.
V
echo
(amplified at
35
) and the output of the hybrid are input into a subtractor
40
, where the output of the hybrid is subtracted from the output of the amplifier
35
. As a result of the subtraction, TE is removed from the amplified V
echo
signal to the extent that the output of the hybrid is an accurate replica of TE. However, the output of the hybrid will be an accurate replica of TE only when the input impedance of the loop Z
loop
, equals the value of Z
loop
used for the design of the hybrid.
SUMMARY OF THE INVENTION
An objective of this invention is to provide a line interface apparatus that automatically matches the output impedance Z
out
of the apparatus to the loop impedance Z
loop
of one of several loops with taps that may be coupled to the line interface apparatus, in order to provide for efficient power transfer. Secondary objectives are the reduction of nonlinearities and cancellation of echo signals. The line interface apparatus is for coupling a signal to a loop with taps for tran
Deppe Betsy L.
Gray Cary Ware & Freidenrich
RC Networks
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