Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~3;~
: 1
TELECOMMUNICATIONS LINE INTERFACE CIRCUITS
This invention relates to telecommunications line ;nterface
ci~cuits.
In line interface circuits for two-wire and four-wire
telecommunications lines, e.g. telephone lines, it is common to
provide a transformer in view of its~desirable common mode signal
rejection and ground isolat;on characteristics. In telephone
appllcations such lines~usually must be able to conduct a substantial
; direct currentj typically up to about 60mA, which also flows through aprimary winding of the transformer. In addition, a line terminating
impedancej typically of 600 to 900 ohms, is reflected from the
secondary to the primary winding of the transformer to match the
impedance of the line.
To ach;eve a desired low cut-off frequency of 50Hz or less,
the primary winding of such a transformer must provide an inductance
of several Henries, necessitating a large number of turns of the
primary winding even using a ferrite core transformer. To avoid
magnetic flux saturation of the transformer core as a result of the
direct current flow;ng through this large~number~of turns, the
transformer must be physically large, and consequently expensive.
The~transformer size also creates a significant problem~in trying to
provide compact arrangements of many line interface circuits.
An object of this invention, there~fore, is to~provide an
improved line lnterface clrcuit which reduces such disadvantages.
Accordlng to one aspect this invention provides a
telecommunications line~interface circuit for coupling a
telecommunications~line to~a transmit line and a receive line,
compri~sing~ a~first ~transformer~having~a first;winding~for coupling
;to~the~'telecommunicati~ons line~and havlng a second~winding; a first
30~ ampl~ifier~having~an~input for coupling to the recéive line and having
an~output coupled to~the second winding of the first transformer and
providing a low impedance termination thereof; a second transformer
having a first wi~nding for coupling~to the telecommunications line and
having~a~second winding; a'second~ampl~i~ier having an input coupled to
~ the~second winding of the second transformer and an~output ~or
; coupling to the transmit line;~ and means interconnec~ing the first
', '' .
,
~ 3~
windings of the first and second transformers for conducting a direct
current on the telecommunications line through said first windings.
In such a line interface circuit, the second winding of the
first transformer is terminated by a low ~close to zero) impedance
provided by the output of the first amplifier, whereby only a
relatively small impedance, arising primarily from the resiskance of
;` the second winding of the first transformer9 is reflected from this
- second winding to the f;rst winding. Cons~equently, a significant part
of the ~typically 600 to 900 ohm) terminating impedance for the line
~; 10 is constituted by the reslstance of the first winding of the first
` transformer. To this end, this first winding of the first transformer
conveniently comprises resistance wire. Other windings of both
transformers may similarly, if desired, comprise resistance wire, or
may comprise copper wire as is conventional in transformer technology.
Reference is directed in this respect to Canadian patent
application serial No. 568,524 filed June 3, 1988 and entitled
"Subscriber Line Interface Circuit and Transformer Therefor". The
term "resistance wire" is used herein to mean wire which, for the same
cross-sectional size and shape, has a greater resistance per unit
length than copper wire.
Applied to a two-wire telecommunications line, preferably the
first windings of the first and second transformers are connected in
series, the circuit further comprising a balance impedance coupled
between an input of the second amplifier, acting as a summing node for
transhybrid signal cancellation, and either the output of the first
amplifier or the receive line. In the latter case, the circuit may
include a third amplifièr having an input coupled to the second
winding of the second transformer and having an output, and an
impedance coupled between the output of the third amplifier and an
lnput of the fi~rst amplifler; the impedance in this arrangement serves
to increase, in an easily controllable manner, the a.c. impedance
which the line interface circuit presents to the two-wire
telecommunications line without increasi~ng the d.c. resistance
presented by the line interface~c;rcuit to the telecommunications
:~ 35 line.
: ~ :
1~3~2
To facilitate providing a balanced interface circuit for a
two-wire telecommunications line which is balanced with respect to
ground, preferably the first winding of one of the first and second
transformers comprises two substantially equal winding halves, and the
; 5 first winding of the other of the first and second transformers is
connected between said winding halves and in series therewith. The
interface circuit can be made even more fully balanced if the first
winding of the other of the first and second transformers als~
comprises two substantially equal winding halves, coupled in series.
~; lO Applied to a four-wire telecommunications line, preferably the
first windings of the first and second transformers are center-tapped
windings arranged for coupling each to a respective pair of wires of
the four-wire telecommunications line, the means interconnecting the
first w;ndings comprising a connection between center taps of the
f;rst w;nd;nys.
According to another aspect th;s ~nvention provides an
interface circuit for a two-w;re telecommunications line, comprising:
first and second transformers each hav;ng first and second windings,
;~; the first windings of the first and second transformers being coupled
in series with one another for connection across the two wires of a
two-wire telecommunications line; a rece;ve path for coupling a
receive line to the second winding of the first transformer and for
terminating this winding with a low impedance; a transmit path for
coupling the second winding of the second transformer to a transm;t
l;ne; and a balance impedance coupled between the transm;t path and
the rece;ve path.
According to a further aspect this invent;on provides an
interface c;rcuit for a four-wire telecommunications line, compr;sing:
first and second transformers each hav;ng a center-tapped first
30 ~ windlng and a secoad w;ndlng, the first windings o~ the first and
second transformers being arranged for coupling each to a respective
; pa;r of wires of a four-wire telecommunications line; connection
means~between the center taps of the first windings~; a first amplifier
having an output coupled to the second winding of the~first
tran~sformer and prov;ding a low impedance termination thereof, for
; supplying s;gnals via the f;rst transformer to the pair of w;res of
~ ~ the four~-w;re telecommunications line~coupled thereto; and a second
::
~ 3 ~33~
amplifier having an input coupled to the second winding of the second
transformer for deriving signals via the second transformer from the
pair of wires of the four-wire telecommunications line coupled
thereto.
The invention also provides apparatus comprising: a
telecommunications line comprising two wires for conducting a direct
current and carrying an a.c. signal thereon; a transformer having a
first winding, coupled to the two wires for concluctlng said direct
current, and a second winding; and an amplifier having an output
~10 directly coupled to ~he second winding and providing a low impedance
termination thereof, for supplying an a.c. signal via the transformer
to the telecommunications line. At least the first winding of the
transformer preferably comprises resistance wire for providing a
predetermined resistance.
Correspondingly, the invention also provides a method of
interfacing a telecommunications line comprising two wires carrying a
direct current, comprising the steps of: coupling a first winding of
a transformer to the two wires to conduct said direct current;
terminating a second winding of the transformer with a low impedance
output of an amplifier; and supplying a signal via the amplifier and
the transformer to the two wires.
The invention further provldes a method of interfacing a two-
wire telecommunications line comprising two wires carrying a direct
current in opposite directions, comprising the steps of: coupling
first windings of first and second transformers in series between the
two wires to conduct said direct current therebetween; terminating a
second winding of the first transformer with a low impedance output
of a first amplifier; supplying a s;gnal from a receive line via the
first amplifier and the first transformer to the two-wire
; telecommunications line; coupling a second winding of the second
transformer via a second amplifier to a transmit line for supplying to
the transmit line a signal received via the two-wire
telecommunications line; and coupl;ng a component of the signal from
; the receive line t~o the second~amplifier for substantially cancelling
from the signal supplied to the transmit line signal components from
the~receive line.
::~
3 ~33 Z
The invent;on will be further understood from the following
` description with reference to the accompanying drawings, in which:
Fig. 1 schematically illustrates a known form of two-wire
telecommunications line interface circuit;
Fig. 2 schematically illustrates a basic form of a two-wire
telecommunications line interface circuit in accordance with an
embodiment of this invention;
Fig. 3 schematically illustrates a preferred form o~ the two-
wire telecommunications line interface circuit of Fig. 2;
Fig. 4 schematically illustrates i two-wire telecommunications
line interface circuit in accordance with another embodiment of this
invention; and
Fig. 5 schematically illustrates a four-wire
telecommunications line interface circuit in accordance with a further
embodiment of this invention.
Referring to Fig. 1, there is illustrated a known form of
interface circuit 10 for a two-wire telephone line having a floating
direct current path. The two-wire line comprises tip and ring wires T
and R respectively carrying a direct current Idc which is typically in
20~ the~range of 18 to 60mA, and has an a.c. impedance of 60Q to gO0 ohms
which is~matched~by the line interface circuit. The line interface
circuit 10 compr;ses a transformer 12 having a split primary winding
14, with two equal halves which are coupled between the tip and ring
wires T and R of the line and are coupled together via a resistor 16
for passing the current Idc, and an a.c. bypass capacitor 18, and a
secondary winding 20, with a 1:1 turns ratio between the primary
winding 14 and the secondary~winding 20. The circuit 10 further
comprises a hybrid circuit 22, having~a terminating~impedance 24 which
is connected to~the secondary winding 20, for coupling signals to a
30~;~ tran~smlt line~26 and~from a receive line 28. The terminating
impedance 24 of the hybrid~circuit 22 is reflected across the primary
winding 14 of the transformer l2 to match the line impedance.
; For acceptable~performance of such a line interface circuit
; with telephone~signals, the circuit must provide~a -3dB lower cut-off
frequency~f of 50Hz or less. This necessitates that the primary
winding 14 have an~inductance of at least R/(2~f), where R is the line
impedance. For R=900 obms and f=SOHz, th;s primary winding inductance
12~?3~332
must be at least 2.86 Henries. In order to prov;de such an
inductance, thP primary winding 14 must have a large number of turns.
In order to avoid saturation of the corè of the transformer 12 by the
current Idc flowing through this large number of turns, the
transformer 12 must be physically large and relatively expensive;
typically the transformer must have dimensions of the order of
4cm x 3.5cm x 2.5cm and a volume of the order of 35cm3. Mounting such
transformers on printed circuit boards, which are arranged side by
side in parallel as is common in telecommunications equipment,
I0 necessitates a relatively large spacing between circuit boards, and
hence leads to undesirably large equipment sizes.
F;g. 2 illustrates, using references similar to those of Fig.
1 where applicable, a generally basic form of a two-w1re line
interface circuit 30 in accordance with this invention. As in Fig. 1,
the two-wire line in Fig. 2 comprises tip and ring wires T and R
balanced with respect to ground and which may carry a loop current Idc
in the range of 18 to 60mA. The line interface circuit 30 comprises
two transformers 32 and 34, an optional resistor 36, a balance
impedance represented by a resistor 38 but which may also include
complex impedance components such as capacitors, and transmit and
receive signal amplifiers 40 and 42 respectively, the former having a
feedb~ack resistor 54. These components and their interconnections are
further described below. The transformers 32 and 34 are ferrite core
transformers, types RM8 and RM4 respectively, as descr;bed further
below, and in the drawings dots adjacent the transformer windings
indicate the senses of the windings in conventional manner.
In the line interface circuit 30 of Fig. 2 the transformer 32,
; like the transformer 12 in the circuit of Fig. 1, has a primary
winding 44 which is split into two equal halves, and a secondary
; 30 winding 46. Each winding has not only an inductive component but also
a resistive component, these components being represented
schematically in Fig. 2 by an inductor and resistor connected in
series. Similarly, the transformer 34 has a primary winding 48 and a
secondary winding 50 each having an inductive component and a
resistive component as represented schematically in Fig. 2.
The two halves of the primary winding 44 of the transformer 32
are bifilar wound from insulated resistance wire, and for example
~L~93~3~
comprise 2 by 500 turns of 40 AWG type MWS-60 alloy resistance wire,
providing each half of the primary winding with a resistance of 335
ahms, for a total primary winding resistance of 670 ohms, and a
- primary winding inductance of 0.25H ~Henry). Tlle secondary winding 46
of the transformer 32 can comprise 2000 turns of 40AWG copper wire,
providin~ an inductance of lH, a resistance of 310 ohms, and a
primary:secondary turns ratio for the transformer 32 of 1:2.
The amplifier 42 is a differential amplifier acting as a
unity-gain buffer for coupling a signal received via the receive line
o ?8, connected to a non-inverting input of the amplifier 42, to the
secondary winding 46 which is connected between an output of the
amplifier 42 and ground. As the amplifier 42 has a low output
impedance, its output constitutes a virtual ground for a.c. signals,
whereby the secondary winding 46 operates in a short-circuited mode in
which its winding reslstance, multiplied by the square of the
transformer 32 turns ratlo from the secondary to the pr~mary, is
reflected at the primary winding 44 of this transformer. Thus there
is an impedance of 310*(~2=77.5 ohms reflected at the primary winding
44 from the sécondary winding 46. This forms with the primary winding
inductance of 0.25H a -3dB lower cut-off frequency of
77.5/(2*~*0.25)=49.3Hz. ~
The primary winding 44 of the transformer 32 is connected
between the wires T and R~, as for the transformer 12 of Fig. 1.
- However, as the secondary winding 46 is terminated by the low output
impedance of the amplifier 42, it can~not be used for producing a
signal voltage for the transmit line 26 as in Fig. 1. In Fig. 2,
thereforej the two halves of the primary winding 44 are coupled
together via the primary winding 48 of the transformer 34 in ser;es
with the optional resistor 36. ~he secondary winding 50 of the
30 ;~ transformer 34 is connected between ground and an inverting input of
the~transmit amplifier 40, which is a differential amplifier having a
non-inverting input which is grounded and an output which is connected
; d to the transmit line 26. The feedback resistor 54 is connected
between the output and the inverting input af the amplifier 40. The
balance impedance 38 is~connected between the output of the amplifier
42 and the inverting input of the amplifier 40 to provide for
~ 3~33~
transhybrid cancellation of signals at the signal summing node
constituted by the inverting input of the amplifier 40.
The primary winding 48 of the transformer 34 comprises 112
turns of 40 AWG copper wire providing a resistance of 35.5 ohms and an
inductance of 2mH, and the secondary winding 50 comprises 448 turns of
40 AWG type MWS-60 alloy resistance wire providing a resistanc2 of 30
ohms and an inductance of 32mH, with a primary:secondary turns ratio
of 1:4. The secondary winding 50 is terminated in a low ~mpedance by
the virtual ground at the inverting input of the amplifier 40, and
consequently the secondary winding 50 provides at the primary winding
a reflected impedance of 30*(1/4~2=1.875 ohms.
The optional resistor 36 provides a resistance which is
selected to pad the total impedance presented to the line wires T and
R to match the impedance of the line, in this case 900 ohms. This 900
ohm impedance is made up by the following contributions as discussed
above:
Resistance of primary winding 44: 670
Impedance reflected from secondary winding 46: 77.5
Resistance of primary winding 48: 35.5
Impedance reflected from secondary winding 50: 1.875
Padding resistance 36: 115.125
Total: 900 ohms
Obviously, the impedances provided by the transformer windings
could be increased to eliminate the need for the padding resistance
36, ;f des;red.
In the line interface circuit 30 of Fig. 2, the loop current
Idc of up to 60mA flows through the primary winding 44 of the
transformer 32 and through the primary winding 48 o ~ he transformer
34. Because the inductance of the primary winding of the
transformer 34 is very low, this current Idc can be accommodated by
the small RM4 core of this transformer without saturation. The RM)3
core of the transformer 32 is also able to accommodate this current
Idc flowing through the primary winding 44, without saturation,
; 35 because the magnetic flux generated;by this current is reduced,
relative to the flux in the transformer 12 of Fig. 1, due to the
relatively reduced number of turns of this primary winding.
3~3~2
Viewed alternatively, it can be seen that in the line
interface circuit 30 of Fig. 2 the line terminating impedance is
provided to a large extent by the resistance of the primary winding
44, and to only a small extent by impedance reflected from the
secondary winding 46, in contrast to the full 900 ohm terminating
impedance 24 in Fig. 1. Consequently, for the same lower cut-off
frequency of about 50Hz, the primary winding 44 can have a much lower
inductance than the winding 14 of Fig. 1, and hence can have fewer
turns, creating proportionally a much smaller magnetic flux for the
same loop current Idc and consequently allowing a much smaller
transformer to be used without saturation.
With the characteristics described above, the transformer 32
can have a size of about 2cm x 2cm x 1.78cm with a volume of about
7.1cm3, and the transformer can have a size of about lcm x 1cm x lcm
wlth a volume of abouk lcm3, glving a total volume of 8.1c~3 or less
than one quarter the volume of the transformer 12 of Flg. 1. In
particular, such transformers are not only smaller and less expensive
than the transformer 12 of Fig. lj but also enable adjacent printed ;
circuit boards on which the transformers are mounted to be spaced
apart by significantly reduced distances, resulting in much more
compact equipment than is possible with the line interface circuits of
Fig. 1.
Fig. 3 iilustrates a preferred form of the line interface
circuit 30 of Fig. 1; similar references are used to denote similar
.:
components, and only the differences from Fig. 2 are described below.
In the circuit 30 of Fig. 3, the primary winding 48 of the
transformer 34 is split into two equal halves 48a and 48b, and the
padding resistor;36 is similarly split into two equal resistors 36a -~
and 36b,~which are connected in series between the two halves of the
30~ primary winding 44 of the transformer 32~to prov~ide a fully balanced
arrangement. A~central junction between the series resistors 36a and
36b~is~grounded via a relatively high impedance resistor 52. The
balance impedance 38 of Fig. 2 is consti~tuted in Fig. 3 by a series-
connected resistor 38a and capacitor 38b.~Fig. 3 also illustrates
feedback resistors~54 and 56 for determining the gain of the
~ ~ ampll~f~iers 40 and 42 respectively,~a~nd coupling capac1tors 58, 60 and
: :
~ 2'~ 32
resistors 62, 64 associated with the transmit and receive lines 26 and
28.
It should be appreciated that the order of series connections
of the components 44, 48a, and 36a and 44, 48b, and 36b can be changed
arbitrarily, for example to be as illustrated in the line interface
circuit of Fig. 4 as described below. In addition, it should be
appreciated that instead of completing a loop for the current Idc as
described and illustrated, the resistors 36a and 36b could instead be
connected to ground and -48 volt terminals of a d.c. supply for
supplying loop current to the line wires T and R, again as described
below for the circuit of Fig. 4.
In the line interface circuits of Figs. 2 and 3, the line is
terminated with a d.c. resistance which is of generally similar
magnitude to the a.c. impedance with which the line is termlnated.
However, in certain sltuations it is desirable to terminate the line
with a relatively high a.c. impedance, for example 900 ohms, and with
a significantly lower d.c. resistance, for example 440 ohms or less.
Fig. 4 illustrates a modified form of line interface circuit which
facilitates this. Again, similar references are used in Fig. 4 to
denote components similar to those of Figs. 2 and 3, and only the
modifications are described below.
In the line interface circuit, referenced 70, of Fig. 4, d.c.
loop current flows between a -48 volt source and ground via the
resistor 36b, one half of the primary winding 44 of the transformer
2S 32, the winding half 48b of the primary winding of the transformer 34,
the ring wire R and the tip wire T of the two-wire line, the winding
half 48a, the other half of the primary winding 44, and the res;stor
36a. The two halves of the winding 44 may each have a resistance of
39.6 ohms, the winding halves 48a and 48b may each have a resistance
30~ o~ 6 ohms, and the resistors 35a and 35b may each have a resistance of
174.5 ohms to provide a total resistance of 440 ohms for d.c. on the
line. The resistors~36a and 36b may comprise thick film and PTC
resistors, electrically connected in series and thermally coupled with
one another, as described in Jakab U.S. Patent No. 4,467,310 issued
August 21, 1984 and entitled "Telephone Subscriber Line Battery Feed
Resistor Arrangements".
;3~3
11
The receive signal path from the line 28 to the secondary
wind;ng 46 of the transformer 32 in the line interface circuit 30 is
substantially the same as for the circuit 30 of Fig. 3. For the
transmit signal, the amplifier 40, with its feedback resistor 54, has
its output coupled to the transmit line 26, its non-inverting input
grounded, and its inverting input acting as a summing node for
transhybrid signal cancellation in a similar manner to that of Fig. 3.
The balance impedance 38 is in this case constituted by resistors and
capacitors 38a to 38f coupled between the receive line 28 and this
summing node.
In the line interface circuit 70 of Fig. 4, the secondary
winding 50 of the transformer 34 is connected between ground and the
inverting input of a differential amplifier 72, whose non-inverting
input is grounded (so that the inverting input ls a virtual ground)
and whose output is coupled via a gain-determining feedback resistor
74 to the inverting input and via a coupling capacitor 76 and resistor
78 to the summing node, referred ts above, constituted by the
inverting input of the amplifier 40. The output of the amplifier 72
is also coupled, via an a.c. impedance controlling impedance 80,
constituted in Fig. 4 by a resistor 80a and a capacitor 80b in series,
to the inverting input of the amplifier 40 which also acts as a
~` summing node. The impedance 80 serves as described below to control
the a.c. impedance presented by the line interface circuit 70 to the
~ line comprising the wires T and R, so that it can be significantly
; 25 different from the d.c. resistance presented to the line by the
circuit 70.
More particularly, the amplifier 72 produces at its output a
voltage which is dependent upon the (alternating) current flowing via
the l;ne w;res T and R. This voltagej as well as being coupled via
~ the amplifier 40 to the transmit line ?5 to constitute the transmit
s;gnal, is applied via the impedance 80 and the a`mplifier 42 as a
feedback signal to the transformer 32, whereby it increases the a.c.
impedance presented to the line by this transformer in accordance with
the magnitude and characteristics of the impedance 80. The impedance
80, which can be a simple complex impedance formed by the resistor 80a
and capacitor 80b as shown, or a more complicated form of complex
33
12
impedance, or simply a resistance, thus serves to control the a.c.
impedance of the line interface circuit 70.
Although the above described embodiments of the invention
relate to two-wire line interface circuits, the invention can also be
applied to a line interface circuit for a four-wire line, for example
as illustrated for a line interface circuit 90 in Fig. 5.
Referring to Fig. 5, the line interface circuit 90 uses
transformers 32 and 34 as in Figs. 2 and 3 as described above,
together with amplifiers 40 and 429 for coupling signals frQm a first
pair of w;res ~1, R1 to the transmit l;ne 26 and from the receive line
28 to a second pair of wires T2, R2, the two pairs of wires
constituting the four-wire line. Each of the four wires carries a
loop current Idc/2 as shown, a total loop current Idc flowing towards
the l~ne interface circuit via the wires T1, R1, a connecting line 98
from a center tap of the primary winding 44 of the transformer 32 to a
cenker tap of the primary winding 48 of the transformer 34, and away
from the line interface circuit 90 via the wires T2, R2.
As in the case of Fig. 2 as described above, in the line
interface circuit 90 of Fig. 5 the secondary 46 of the transformer 32
is connected between ground and the output of the amplifier 42, and
hence is operated in a short circuited mode whereby its resistance is
reflected at the primary winding 44 of this transformer, the
resistance of which itself contributes as in Fig. 2 to the impedance
presented by the line interface circuit 90 to the wires T2, R2. In
the transmit direction, a signal on the wires T2, R2 is coupled via
the transformer 34 to the inverting input of the amplifier 40, the
output of the amplifier 40 being connected to the transmit line 26
and being coupled via a feedback resistor 94 to the inverting input of
the ampli~fier 40.
It should be appreciated that in the line interface circuit
90, in addition to a size reduction of the transformers for reasons
similar to those described abo~e for the two-wire line interface
circuits, the core size of the transformers 32 and 34 can be further
reduced because the currents Idc/2 flow in opposite directions in the
two halves of the primary windings 44 and 48 of these transformers,
so that the magnetic flux due to these direct currents cancels in each
transformer.
~.2~3~
13
Numerous other variations, modifications, and adaptations may
be made to the embodiments of the invention described above within the
. scope o~ the invention as defined in the claims.
~'
~ 5
:,::; :
'
, :
