ELECTRONIC HYBRID AND HYBRID REPEATER WITH BRIDGE CIRCUIT

CIRCUIT HYBRIDE ET REPETEUR HYBRIDE AVEC CIRCUIT EN PONT

English Abstract

Abstract Of The Disclosure
A hybrid bridge circuit has a feed amplifier and an aux-
iliary resistor connected in series with the output of the re-
ceive amplifier between the amplifier and a node of the bridge
so that signals from the receive amplifier are transmitted through
the two-wire line port in the bridge to the auxiliary resistor
and a resistive leg of the bridge, and signals from the two-wire
line port and the adjacent resistive leg of the bridge and the
node V2 between the compensating network in the bridge and the
adjacent resistive leg of the bridge. a differential transmit
amplifier differentially receives inputs from the nodes V1 and
V2 so that the differential amplifier produces a partially equalized
output signal corresponding to the difference between the voltages
at the nodes V1 and V2. The resistance values R1 and R2 of the
two resistors legs of the bridge, and impedances ZC and ZL
of the compensating network and the two-wire line in the bridge,
satisfy the equation <IMG> to maximize the transhybrid loss,
and the resistor R2 adjacent the compensating network is sub-
stantially larger than the resistor R1 adjacent the two-wire
line to minimize the power losses in the compensating network
and the resistor R2. This hybrid bridge circuit can be used
in repeaters or terminating devices in communication networks.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED AND DEFINED AS
FOLLOWS:
1. A hybrid bridge circuit comprising:
a two-wire line port circuit coupled to one leg
of said bridge, and a compensating network in an
adjacent leg of the bridge on opposite sides of an a-c
ground, said compensating network having a frequency
dependent impedance which approximates the impedance
characteristic presented by the two-wire line to the
bridge, balancing resistors R1 and R2 in the other two
adjacent legs,
a receive amplifier and an auxiliary resistor Rs
connected in series with the output of the receive
amplifier between the amplifier and the node between
the resistors R1 and R2 for presenting an impedance to
both positive-going and negative-going signals from
the two-wire line so that all signals are transmitted
to both the node V1 between said two-wire line and the
adjacent resistor R1 and the node V2 between said
compensating network and the adjacent resistor R2,
with signals from the receive amplifier being
transmitted to the two-wire line through said
auxiliary resistor Rs and the resistor R1 in the
bridge, and a transmit amplifier connected to nodes V1
and V2.
2. A hybrid bridge circuit as set forth in claim
1 wherein said transmit amplifier is a differential

amplifier differentially receiving inputs from said
nodes V1 and V2 so that the differential amplifier
produces a partially equalized output signal corre-
sponding to the difference between the voltages at
said nodes V1 and V2.
3. A hybrid bridge circuit as set forth in claim
1 wherein said compensating network and said two-wire
line have impedances ZC and ZL, respectively, with
the values of R1, R2, ZC and ZL satisfying the
equation <IMG> to maximize the transhybrid loss,
and the resistor R2 adjacent said compensating network
is substantially larger than the resistor R2 adjacent
the two-wire line to minimize the power losses in said
compensating network and said resistor R2.
4. A hybrid bridge circuit as set forth in claim
1 wherein the leg of the bridge forming the two-wire
port circuit includes a capacitor, and wherein the
termination impedance formed by the combination of
said capacitor, the resistor R1 adjacent said two-wire
line and said auxiliary resistor Rs is about the same
as the impedance of the two-wire line, and the major
portion of the resistance portion of said termination
impedance is provided by said auxiliary resistor Rs.
5. A hybrid circuit as set forth in claim 4
wherein said termination impedance comprises a
resistance of about 600 or 900 ohms and a capacitance
of about 2.5 microfarads.
16

6. A hybrid bridge circuit as set forth in claim
1 wherein said compensating network has a
frequency-dependent impedance which approximates the
impedance characteristic presented by the two-wire
line to the bridge.
7. The hybrid circuit as set forth in claim 1
wherein said two-wire line port circuit is
transformer-coupled to one leg of said bridge.
8. A hybrid repeater comprising a pair of hybrid
bridge circuits as set forth in claim 1 with the out-
put of the transmit amplifier of each bridge circuit
connected to the input or the receive amplifier of the
other bridge circuit to form a closed repeater loop.
9. A hybrid repeater as set forth in claim 8
which includes equalizing means for completing the
equalization of the output signals from the transmit
amplifier of each bridge circuit.
10. A hybrid repeater as set forth in claim 8
which includes amplifying means for boosting the
amplitude of the output signals from the transmit
amplifier of each bridge circuit.
11. A hybrid repeater as set forth in claim 8
wherein said compensating network and said two-wire
line in each bridge circuit have impedances ZC and
17

ZL, respectively, with the values of R1, R2, ZC
and ZL satisfying the equation <IMG> to maximize
the transhybrid loss, and the resistor R2 adjacent
said compensating network in each bridge circuit is
substantially larger than the resistor R1 adjacent the
two-wire line to minimize the power losses in said
compensating network and said resistor R2.
12. A hybrid repeater as set forth in claim 8
wherein the leg of the bridge forming the two-wire
port circuit includes a capacitor, and wherein the
termination impedance formed by the combination of
said capacitor the resistor R1 adjacent said two-wire
line and said auxiliary resistor Rs in each bridge
circuit is about the same as the impedance of the
two-wire line, and the major portion of the resistance
portion of said termination impedance is provided by
said auxiliary resistor Rs.
13. A hybrid repeater as set forth in claim 8
wherein said compensating network in each bridge
circuit has a frequency-dependent impedance which
approximates the impedance characteristic presented by
the two-wire line to the bridge.
18

Description

~L -
The present invention relates generally to hybrid circuits
of the type used in telephone systems and, more particularly,
to hybrid circuits suitable for use in telephone repeaters which ~:
are provided in telephone transmission lines to compensate for
cignal degradation.
Hybrid circuits are used in communication networks as
terminating devices or as an interface between a bidirectional
two-wire line and individual unidirectional sections of a Eour-
wire line. In telephony, for example, bidirectional signals
may be carried over a two-wire line in a subscriber loop, but
must be split into separate transmit and receive unidirectional
signals at the central of~ice. Various forms of hybrid circuits
have been developed to accomplish these functions.
The most commonly used hybrid circuit is a magnetic element
known as a hybrid transformer, and comprises a multi~winding
transformer having a two-wire port, separate transmit and receiv~
ports, and a balance network port. The wlndings are arranged
so that signals imposed on the two~wire port are coupled to the ~;
transmit port, and signals imposed on the receive port are coupled `
to the two-wire port but not to the transmit port. Hybrid trans-
formers have been in use Eor many years, but suffer ~rom the
disadvantages of comparatively large size, high cost, and a lim-
itation on packaging density.
Qther types of hybrid circuits that have been proposed
.
and/or used include various bridge circuits and various 'lactive"
circuits utilizing operational amplifiers.
One common use of hybrid circuits is in "repeaters",
which are devices interposed at spaced intervals along a telephone
line to compensate for signal attenuation in the line by boosting
or amplifying the signal. A typical repeater comprises a loop
:

formed by two hybrid circwits with the receive port of each hybrid
circuit connected to the transmit por-t of the other hybrid circuit,
and with the two-wire ports of the two hybrids forming the input
and output terminals of the loop for connection to the transmission
line. One of the requirements for repeaters used with nonloaded
lines is that they "equalize" the signal being amplified. Because
of tbe charateristics of the complex impedance of a transmission
line, the signals transmitted through a nonloaded line are atten-
uated differently at different frequencies; typically, the high ~^
frequency components of the signal are attenuated more than the
low frequency components. Consequently, when the signal from
a nonloaded line is amplified in a repeater, it is desirable
to amplify the high frequency components more than the low frequency
components to compensate for the uneven attenuation oE the dif-
ferent components of the signal in the line, thereby "equalizing"
the signal.
When equalization is required in a repeater, the amount
of gain that can be achieved in the repeater for the total signal
is reduced. First of al1, regenerative feedback with~in the re-
peater loop must be avoided because~such ~eedback introduces
instability in the; form of~"singing". To avoid regenerative
feedback, the total gain of the repeater cannot exceed the losses
therein. Thus, since the high frequency components of the signal
must be amplified more than the low frequency components to achieve
equalization, the total gain that can be achieved in repeaters
for nonloaded lines is limited to the maximum gain that can be
tolerated for the high frequency components wit~out causing re-
generative feedback. Unfortunately, it is the high frequency
components of the signal -- those that~must be amplified the
most -- that are primarily responsible for the singing and other
deleterious results of regenerative feedback. So the need for ~ -
.
--2--
.~ .

e~u~lization in a repeat~r seve~ely restricts t~.~ total signal
gain ~hat ~an be ~ttained.
It is a primary object of this invention to provid~ an
imp~oved hybrid circuit ~hat permits increased gain within a
repeater for nonloaded lines while achieving equalization across
the ~ntire freguency band of the signal~ In this conn~ction,
~ related ob~ect of 'che inv~n~ion i ~o p~ovide such an improved
hybrid circuit that r~duces the amoun~ o~ e~u~lization requir~d
within th~ repeat~r loop ~tself.
It ~ anothe~ o~j~ct o~he invent~on to provid~ su~h
~n ~mproved ~ybrid circui~ that provides psrtial e~ualization
within the hybrid circuit it~el.
It is still another object of this invention to provide
~n i~prov~d hybrid.circui~ of ~he foreyoing type tha i~ suit~ble
for use ln a variety o~ general pur~ose applicat~ons, including
terminating devices and two-wi~e/two-wire or two-wire/~o~r-wlre
~epeaters.
.:
A fu~ther object o~ thl~ invention ~s to provide such
an improYed bybrid circuit that can be e~onomically produced ~:
at a relatively low ~ost c~mpared with other comm~rcially a~ailable :
hybrid ~ircuits.
A still further object of ~he inven~ion is to provide
sucb an improved hybrid circui~ that has relatively low power
losses so that a large n~mber of She hybrid circuits can be densely
packaged in ~ small spac~ without beat dissipat~on proble~sO ~:
: Yet anothe~ objeot o~ the invention is to provide ~uch
an i~proved hybrid cir~uit which is transformer-isolated to avoid
: pro~lems of longitudinal ~ c~ ~o that it.can be u~ed on both
b~l~nc~d and unba~anc~d l~nes.
--3--
.
- ~ .
: ,

q~:27
More particularly, there is provided:
A hybrld brldge clrcuit comprising:
a two-wire line port circuit coupled to one leg
Or sald bridge, and a compensating network in an
adJacent leg Or the bridge on opposite sides of an a-c
ground, sald compensatlng network having a ~requency
dependent impedance whlch approxlmates the impedance
characteristic presented by the two-wlre line to the
bridge, balancing resistors Rl and R2 in the other two
ad~acent legs,
a receive ampllfler and an auxiliary resistor Rs
connected in series with the output o~ the receive
amplifler between the ampli~ier and the node between
the resistors Rl and R2 for presenting an impedance to
both positive-going and negatlve-Koing signals from
the two-wire line so that all slgnals are transmitted
: to both the node Vl between said two-wlre llne and the
ad~acent resistor Rl and the node V2 between sald
compensatlng network and the adJacent reslstor R2, : :
with ~lgnals from the receive ampllfier being:
transmlt:ted to the two-wire llne:through said ~
j`~ auxiliary reslstor Rs and the resiBtor Rl in the ~ :
brldge 9 and a transmit ampll~ler connected to nodes Vl
and V2. .
Other objects and advantages of the invention will be
apparent from the following detailed ~escription and the accom- :
panying drawings, in which: :
-3a-

FIGURE 1 i~ a simplified circuit diagram of a hybrid
circuit embodying the invention;
FIG. 2 is a graphical illustration of signal level vs.
frequency at three different nodes Vl, Vr, and V20 as well as
the difference between the signal levels at V1 and v2, when used
with a nonloaded llne;
FIG. 3, appearing on the first sheet of the drawings with Fig. 1, is a
graphical illustration of signal attenuation vs. fre~uency in the circuit of ~IGURE 1
at different values of Rl; R2 and RS and with a nonloaded ~ -wire line; and
FIG. 4 is a circuit diagram of a two-wire/two-wire re-
pea~er utilizing hybrid clrcuit~ embodying the invention.
While the invention will be described in connection with
certain preferred embodiments, it will be understood that it
, . .
- is not intended to limit ~he invention to these par~icular em-
-~ bodiments. On the contrary, it is intended to cover all alterna-
tives, modifica~ions and equivalent arrangements as may be included ~
within the spirit and scope of the invention as defined in the ~ :
appended claims.
Turning now to the drawings and re~errin~ fir ~ to FIGUR~
1, there is shown a hybrid bridge circuit 10 for interfacing .
a two-wire line 11 with a four-wire line comprising a receive
line 12 for ~upplying ~ignals to the two-wire line 11 via an
amplifier 13, and a transmit line 14 ~or receiving signals from
the two-wire line 11 via an amplifier 15~ The basic purpose
of ~he hybrid circuit is to provide a matched impedance (1) to
the bidirectional two-wir~ line 11 from the unidire~tional re~eive
line 12 and g2) to the unidirectional transmit line 14, r~m
the two-wire line, while at ~he same time providing a high degree ~-
of isolation of (1) the transmit line 14 rom the receive line ~:
12 and (2) ~he receive line 12 from the two-wir~ line 12. The
i~ola~ion of ~he ~ra~smit line ~ro~ the receive line is ~m~only
'~
: ~ 4

reEerred to as "~ranshybrid loss", with an infinite transhybrid
loss representing the ideal 100% signal isolation. Good impedance
matching and signal isolation avoid undesirable si~nal "reflections"
and other objectionable interference or degradation of the quality
of the signals transmitted through the hybrid circuit.
In the case of ~he illustrative hybrid circuit 10, isola-
tion o~ the transmit amplifier 15 from signals arriving at the
receive port 20 is achieved by providing a balanced bridge with
a repeating coil 21 (orming a two-wire port) and a compensating
network ZC in adjacent legs of the bridge. The compensating
network ZC is designed to present a frequency-dependent impedance
to signals from the receive port 20 which approxi~ates the im-
pedance characteristic Z~ oE the line 11 presented to the two-
wire port, so that the bridge 10 can be balanced. The node between
ZL and ZC is grounded, and the other two legs of the bridge con-
tain balancing resistors Rl and R2, with signals arriving at
the receive port 20 from the receive amplifier 13 being transmitted
to the two-wire~line 11 via node Vr and the~resistor Rl.
The use of compensatlng networks, o~ten referred to as
"precision balance networksl' or "PBM's", is well known in hybrid
circuits, including hybrid bridges. The nature of the specific
network used in any given application depends on the characteristics
of the two-wire line involved, such as the gauge of the line
and the load thereon. Since these networks are so well known
in the art, no specific examples will be illustrated or described
herein.
To balance the bridge, the values of the parameters are
selected to satisfy the equation z =z so that the differential
signal across the nodes Vl arld V2 which form the transmit port
is unaffected by the transmission of signals from the receive
amplifier 13 to the two-wire line 11. That is, the bridge is

~ 2~
balanced for signals supplied to the receive port 20, and thus
the volta~es Vl and ~2 always change b-y the same in~rement in
response to signal transmission from the receive amplifier 13
to the two-wire line 11. This effectively isolates the input
to the transmit amplifier 15 from signals entering the bridge
from the receive amplifier, providing a high transhybrid loss
in this mode of operation.
When signals are transmitted from the two-wire line 11
to the transmit amplifier 15, the magnitude of the voltage changes
at vl are different rom those at V2 because the bridge is not
balanced for signals received at the two-wire port. ~owever,
good signal isolation is still maintained in this mode of operation
because there is an a-c. ~round provided by the output of the
receive am~lifier 13, thereby effectively isolating the receive
amplifier 13 from signals transmitted from the two-wire line
11 to the transmit amplifier 15.
In accordance with one important aspect of the present
invention, the resistor R2 in the bridge leg adjacent the leg ~;
containing the compensating network ZC is substantially larger
than the resistor Rl in the bridge leg adjacent the leg containing
the two-wire line 11 to minimize the power losses in the compen-
sating network ZC and the adjacent resistor R2 when signals are
transmitted from the receive amplifier 13 to the two-wire line
11. This ensures that most of the available power of the receive
amplifier 13 is utilized in driving the two-wire line 11 rather
than being wasted and dissipated as heat in R2 and ZC which simple
lead to ground. Furthermore, the capacitors in the compensating -
network ZC can be made much smaller than would be permissible
if the resistors Rl and R2 were equal! for example. This reduction
in the size of the capacitors represents a significant cost re-
duction and permits the hybrid circuit to be economically manu-
factured.
. . .
- . . . ~ . , . -
:- : . . . ,. -, . : - .

;~ o3~
In accordance with another important aspect of the in-
vention, an auxiliary resistor i5 connected in series wi.th the
output of the receive amplifier between the ampliier and the
bridge so that signals ~rom the receive amplifier are transmitted
to the two-wire line throu~h the auxiliary resistor as well as
one of the res.istors in the bridge, and signals from the two-
wire line are tFansmitted to the transmit line via both (1) the
node Vl between the two-wire line and the adjacent resistor Rl
and (2) the node V2 between the compensating network ZC and the
adjacent resistor R2. From the transmit port formed by the two
nodes Vl and V2, the signals from the two-wire line are differ-
entially applied via resistors R3 and R4 to the inputs of a dif-
ferential trans~it amplifier so that the transmit amplifier pro-
duces a partially equalized output signal corresponding to the
difference between the voltages produced at the nodes Vl and
V2. Thus, in the illustrative circuit of FXGURE 1 an auxiliary
series resistor Rs ls connected between the output oE the receive
amplifier 13 and the node Vr between the resistors Rl and R2.
Lecause of the presence o the resistor Rs, incoming signals
from the -two-wire line 11 appear at both the nodes Vl and V2,
rather than only node Vl. Without the resistor Rs, incoming
signals ~rom the two-wire line 11 would not appear at the node
V2 because the node Vr between Rl and R2 would be an a-c. ground
by virtue of its direct connection to the output of the receive
amplifier 13. However, by providing the resistor Rs in series
with the node Vr and the output of the receive amplifier 13,
outside the bridge 10, a voltage drop is interposed between the
node Vr and the a-c. ground at the output of amplifier 13, and
thus a portion of the signal from the two-wire line passes through
R2 and ZC to the groonded node between ZC and ZL ~:
.
. .

3~
In the case of a nonloaded line, ~he signals that appear
at both the nodes Vl and V2 require equalization because of the
uneven attenuation at different frequencies. However, because
the transmit amplifier 15 senses only the differential signal
across the nodes Vl and V~, the output of the amplifier 15 is ~ ?
partially equalized. Consequently, when the hybrid circuit is
utilized in a repeater, less equalization is required in the
repeater loop, which permits a higher gain to be attained ln
the loop because the gain for the lower frequency components
of the signal can more closely approach the gain for the higher
frequency components. This equalization feature o~ the hybrid
circuit is also advantageous when the hybrid is used to couple
a two-wire line and a four-wire line, i.e., where gain is not
involved.
The advantageous effect of the resistor Rs on signals
transmitted from the two-wire line to the transmit amplifier
15 can be more clearly understood from the curves shown in FIGS. ~ ~;
2 and 3. FIG. 2 illustrates the voltage levels at the nodes
Vl, Vr, and V2, and the diferential (Vl-V2), as a function of -
frequency, in the circuit of FIGURE 1 with a nonloaded linè.
Without Rs, the voltages at the nodes Vr and V2 would both be
zero because of the connection of the node Vr directly to the
a-c. ground provided at the output of the receive amplifier 13;
in this case, the voltage Vl would be somewhat higher than il~
lustrated in FIG. 2, but the general shape of the voltage vs.
frequency curve would still be the same. Thus, it can be seen
that with a nonloaded line the input signal to the transmit
amplifier, i.e., the slgnal at the node ~1, would be increasingly
attenuated with increasing frequency.
With Rs in the circuik~ ho~ever, a signal from the two-
wire line appears at all three nodes Vl, Vr and V2, with the
;::
_~_
.

.f~3~
voltage levels at each succeeding node decreasing due to the
successive voltage drops across the resistors Rl and R2. As
in the case of the signal level at node Vl, the signal levels
at the nodes Vr and V2 exhibit increasing attenuation with in-
creasing frequency in the case of a nonloaded line, as illustrated
by the curves Vr and V2 in FIG. 2. However, as can be clearly
seen in FIG. 2, the differential signal (V1-V2) exhibits sub-
stantially less attenuation than the signals at either Vl or
V2r and thus this differential signal is partially equalized.
For example, the signal level at Vl drops from 0.460 at 250Hz
to 0.068 at 4000Hz, and V2 drops from 0.410 at 250Hz to 0.053
at 4000Hz. Both these drops are substantially greater than the
drop in the diferential signal (Vl-V2), which decreases from
0 050 at 250Hz to 0.015 at 4000Hz.
FIG. 3 illustrates signal attenuation as a function of
frequency across the circuit of FIGURE 1 with nonloaded line
(24 gauge, 24000 feet) and the following values of Rl, R2 and
Rs:
Rl R2 Rs
Curve~A 900 ohms 20,000 ohms 0 ohms
Curve B 500 11,111 400
Curve C 300 6,666 600
Curve D 200 3l333 700
Curve E 50 1,111 850
Curve F 20 444 880
Curve G 10 222 890
As can be seen from FIG. 3, the slope of the attenuation
curve diminishes considerably at increasing values of Rs. With
Rs = 0, the attenuation curve drops from ldb at 1000Hz to -9db
at 4000Hz (curve A). With Rs = 800 ohms. the curve drops only ~;
from -34db at lOOOH2 to -37db at 4000Hz, thereby providing an
input signal to the transmit amplifier 15 that is partially equal-
ized. Although the magnitude of the input signal to the transmit
amplifier is reduced as the value of Rs increases, this reduction -
_g_
.
. .

in amplitude does not present a gain problem because the attendant
increase in the transhybrid loss permits additional gain to be
built into the transmit amplifier 15 or one or more additional
amplifiers in series therewith. The net result is that the partial
equalization of the output signal from the transmit amplifier
15 permits the total gain of a repeater u~ilizing this hybrid
circuit -to be increased significantly. For example, whereas
a typical two-wire/two-wire repeater achieves up to lldb of gain
in each direction, a repeater utiliæing the hybrid circuit of
the present invention can achieve gain increases of 2 to 3db
in each direction, which represents an increase in the range
of 1~ to 27~.
The termination impedance presented by the illustrative
circuit to signals arriving from the two-wire line 11 is the
series combination of resistors Rs and Rl plus a capacitor Cl
between the secondary windings of the repeating coil 21. As
explained previously, this termination impedance should substan~
tially match that of the two-wire line 11, which is generally
presumed to comprlse a resistance either 600 ohms or 900 ohms
plus a capacitance of 2.15 mf in most standard telephone systems.
As between Rs and Rl, it is preferred to have Rs form by far
the major portion of the termination impedance in order to minimize
the value of Rl and thereby minimize the impedance and power
loss in the bridge. As Rl is reduced, R2 must be reduced and/or
ZC must be increased in order to satisfy the equation zl=Rz2
because ZL is fixed. It is desirable to keep ZC ~ R2 relatively
small to avoid noise problems at V2, and thus it is preferred
that R2 be reduced as Rl is reduced. However~ it is desired
to keep R2 large in order to minimize power losses in~that side
of the bridge. Furthermore, as Rl and R2 are reduced, the magnitude
of the differential in the signal levels at Vl and V2 diminishes

3.~
(see FIG. 3), thereby reducing the signal-to-noise ratio in the
difEerential signal sensed by the amplifier 15. Consequently,
there is a practical limi-t to how far Rl can be reduced, and
a compromise must be struck in dividing the resistance portion
o the termination impedance between Rl and Rs. A suitable com~
promise is an R1 value that comprises about lO~ of the required
resistance, with Rs comprising the other gO~.
In FIG. 4 there is illustrated an exemplary two-wire/two-
wire repeater for use with a nonloaded line. The repeater comprises
two hybrid circuits of the type shown in FIGURE l, connected
back-to-back to form a closed loop, i.e. t ~he transmit port of
each hybrid circuit is connected to the receive port of the other
hybrid circuit. Thus, referring specifically to FIG. 4, a bridge
30 couples a two-wire line 31 with (l) a receive line 32 for
supplying signals from the repeater to the two-wire line via
a receive amplifier 33 and (2) a transmit line 34 fQr receiving
signals from the two-wire line 31 via a differential ampliier
35. The bridge circuit is identical to that described above
in connection with FIGURE l, including a repeating coil 21, a
compensating network Z~ and balancing resistors Rl and R2, with
an auxiliary resistor Rs connected in series with the output
of the receive amplifier 33 and the node of the bridge between
resistors Rl and R2.
The gain of the differential transmit amplifier is set
by resistors R5 and R6, and the output of this amplifier is sup- ~;
plied via resistor R7 to a frequency equalization network 36. ~ ~
The network 36 comprises an operational amplifier 37 having a ;
capacitor C2 and a resistor R8 connected in parallel in the feed-
back loop of the amplifier to provide the requisite equalization
of signals received from the two-wire line 31 before they reach
the m~ting two-wire line 31' at the opposite end of the repeater.

Frequency equalization networks of -this type are well known in
the telephony art and need not be described in detail herein.
From the equalization network 36, the equalized signal
is passed through a coupling capacitor C3 to a high pass filter
38 and a low pass filter 39 which pass only that portion of the i~
signal within a preselected frequency range. This frequency
range may vary somewhat for different applications, but in general
repeaters designecl for use in telephone transmission lines pass
signals within a frequency range from about 250Hz to about 3500
or 4000E~z. This frequency range generally encompasses all signals ~ ~
of .interest, and thus any signals outside this frequency range : ~:
are preferably rejected to avoid interference with ~he desired
signals. The particular circuitry employed in the high pass
and low pass filters 3~ and 39 is well know to those familiar
with the telephone art and need not be described in detail herein.
As can be seen from~the circuit diagram in FIG. ~, the illustrative
filters 38 and 39 each include an operational amplifier in associ.a~
tion with various passive components.
From the low pass filter 39, the signals~are fed through
a resistor R9 to a receive amplifier 33' associated with a second
hybrid bridge circuit 30'. A tee o~ three resistors R10, Rll
and R12 in the feedback loop of the amplifier 33' is set to control
the gain of the amplifier, and an auxiliary resistor Rs' is con-
nected in series with the output of the amplifier 33' and the
node between resistors Rl' and R21 in the bridge 30'. As in
the case of the first hybrid bridge circuit 30, the second hybrid
bridge circuit 30' is identical to the bridge circuit lQ described
above in connection with FIGURE 1.
The nodes Vl' and V2' of the bridge 301 are differentially
applied to the transmit ampl.ifier 351 which produces a partially
equalized output signal that is fed through a frequency equaliza-
' ~ -

~ r~
tion ne-twork 36l, a high pass filter 38', and a low pass filter
39' to the receive amplifier 33 of the first brid~e circuit 30
It can be seen that the signal path from the transmit amplifier
35' of the bridge circuit 30' to the receive amplifier 33 of
the bridge circuit 30 is identical to the signal transmission
path aLready described above leading from the transmit ampllfier
35 of the bridge circuit 30 to the receive ampliier 33' of the
~ridge circuit 30'. Thus, it can be seen that the two hybrid
bridge circuits 30 and 30' together with the two frequenc~ equal-
ization networks and filters form a closed repeater loop, with
the two-wire line ports of the two bridge circuits forming the
terminals of the repeater.
It will be understood that the various resistance networks
in the repeater shown in FIG. 3 can represent o~e of a plurality
of pads which can be selected by appropriate switch settings,
thereby providing a "full prescription" repeater.
As can be seen from the foregoing detailed descrip~ion,
the improved hybrid circuit provided by this invention permits
increased gain within a repeater for nonloaded lines while achieving
equalization across the entire frequency band of the signal.
This result is achieved by providing partial equalization within
the hybrid circuit itself, thereby reducing the amount of equaliza-
tion required within the repeater loop. This hybrid circuit
is suitable for use in a variety of different purpose applications,
including terminating devices and two-wire/two-wire or two-wire/four-
wire repeaters. The circuit has relatively low power losses
so that a large number of the circuits can be densely packaged :
in a small space without heat dissipation problems, and the circuit
is transformer-isolated to avoid problems of longitudinal balance
so that it can be used on both balanced and unbalanced lines.
Furthermore, the hybrid circuit can be economically produced
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at a relatively low cost compared with o~her commercially available
hyhrid circuits.
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