Note: Descriptions are shown in the official language in which they were submitted.
1111586
Specification:
The invention relates to a circuit arrangement for an electronic
subscriber feed with loop current supervision, ringing connection and hybrids
for a transition from two- to four-wire operation with mutually symmetrically
connected current generators for communication facilities, such as private
branch telephone exchanges, especially.
Heretofore known feeding arrangements for telephone subscriber
stations with central battery or current source are formed of feed coils,
transformer and relay arrangements which, in telephone systems with one
feeder, respectively, to each subscriber station, require a great expenditure
of both space and money and, because of the close disposition of coils and
transformers, foster or promote crosstalk. To reduce the mechanical outlay
or expenditure, it has become known, for example, from the German Published
Prosecuted Application DT-AS 1 199 827 which was published on September 2,
1965 in the name of Telefunken Patentverwertungsgesellschaft mbH, to use
current generators.
Other feeding arrangements employ solid-state components, which
eliminate choke coils and transformers and largely preclude crosstalk
(German Published Non-Prosecuted Application DT-OS 20 20 527 which was
publlshed on December 23, 1970 in the name of International Standard Electric
Corp.).
Such feeding arrangements may be combined with ringing current
feed-in and a hybrid, and through loop current supervision, the initiation
of the further switching of a connection by operation of a grounding key of
a subscriber station may be evaluated, besides triggering the establishment
of the connection, current stabilization during the operation of the ground-
ing key being ensured. This arrangement for subscriber feeds is useful,
preferably for house exchange equipment, as it does not fully meet the hybrid
attenuation requirements for long loops.
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It is accordingly an object of the invention to provide a circuit
arrangement for an electronic subscriber feed with a hybrid, which permits
the use of long loops and is affected relatively little by tolerances of
components.
With the foregoing and other objects in view, there is provided,
in accordance with the invention, a circuit arrangement for an electronic
subscriber feed with loop current supervision, ringing connection and
hybrid for a transition from two to four-wire operation with mutually
symmetrically connected current generators for communication facilities
such as branch telephone exchanges, the current generators comprising, in
a two-wire branch of the hybrid, operational amplifiers having respective
transistors operatively connected therewith, said operational amplifiers
being operative as pure voltage followers and having inverted inputs through
which modulation of the loop current is controllable, dynamic self-balancing
of said current generators being increased by amplification of a modulation
voltage applied to said two-wire branch in an incoming line branch of a
four-wire branch of the hybrid, a differential amplifier operatively connect-
ed with said operational amplifiers and having an output connected to an
outgoing line of said four-wire branch and an input, a transistor having a
collector connected to said input of said differential amplifier, and means
for applying, with the loop open, a loop signal through said transistor
connected to said input of said differential amplifier for controlling said
differential amplifier so as to obtain a large hybrid attenuation in a
manner that no a-c voltage reaches said output of said differential amplifier,
said hybrid comprising a complex line balancing network connected in said
four-wire branch and an input impedance connected in said two-wire branch,
said input impedance being selected from a group consisting of a real
resistance and a complex impedance.
In accordance with another feature of the invention, the input
of the differential amplifier is inverted and an amplifier is pro-
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vi~ed for applying a countervoltage to the inverted input of thedifferential amplifier, with the loop closed, so as to obtain a
large attenuation.
In accordance with a further ~eature of the invention, the
circuit arrangement includes voice wires connected to the opera-
tional amplifiers and having an external termination, the comple~ ¦
line balanclng netwokr having a frequency response correspondin~
to the ~requency response of the external termination o~ the
v~ice wlres.
In accordance with an added feature of the invention, an R-C
component connected in shunt with the complex line balancing net-
work is also included.
In accordance with an ad~itional feature of the invention, the
circuit arrangement includes voice wires connected to the opera- ¦
tional amplifiers and having a load connected thereto, the complex
input impedance a~d the complex line balancing network for the
complex internal generator impedance having a frequency response
corresponding to the load connected to the voice wiresO
In accordance with yet another feature of the invention, the
circuit arrangement includes an incoming line branch connected
to the operational amplifiers, and means cooperating with the
operational amplifiers for converting an unbalanced input voltage
applied to the incoming line branch to a balanced voltage at the
voice wires.
In accordance with yet another ~eature of the invention, the
circuit arrangement includes means for evaluating the loop compris- ¦
ing resistancesconnected to outputs of the operational amplifiers~
In accordance with a concomitant feature Or the invention, the
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circuit arrangement inclu~es voice wires connected to the opera-
tional ampliriers, and balancing resistors respectively associated
with the operational amplifiers and directly connected to the
voice wires.
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Other features which are considere~ as characteristic for the
inventior arL~ set forth in the app~nded claim.~.
.1lthou~h the invention is illustrate~ an~ described herein a~
embodled ~n circuit for an electron~c subscriber fee~, it is
nevertheless not intende~ to be llmited to th~ details shown,
since various mod1fications and structural changes may be made
therein without departing from the spirit of the invention and
within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, how-
ever, together with additlonal ob~ects and advantages thereof
wlll be best understood from the following description when read
in connection with the single figure of the drawlng which is a
diagram of a circuit arrangement for two alternate embodiments
of the invention and which shows only the features essential
to the lnventionc
Referring now to the drawing, it is noted that the supply system
or feed is formed of two current generators which are disposed
symmetrlcally to each other and for which operational amplifiers
Jl and ~2 each in con~unction with a transistor~ are used. The
advantage of the operational amplifiers Jl and J2 instead of
transistors, for example, in a Darlington arrangement, is in the
independence of the loop current of transistor parameters such as
the base-emitter voltage, for example, which can fluctuate sharply.
The loop current is consequently determined exclusively by the
voltages at the resistors Rl, R2, which are transm~tted to the
terminals on the emitter side of the loop resistors R3 and R4,
respectively, via the operational ampli~iers Jl, J20 In this
regard, the operational amplifiers Jl, J2 operate as pure voltage
followers, so that the potentials at the positive inputs of the
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amplifiers Jl, J2 an~ the potentials at the loop resistance R3,
R4 are equal. The symmetrJ of the arrangement is ensured by the
balancing resistors R5, R6, which are directly connecte~ to the
voice wires A, B without the intermediary of any diodes. Through
the direct connection, the tolerances of the protective resistors
R7, R8, which serve to protect the feed against voltages on the
wires of the subscriber loop or connected lines, are also equal-
ized.
Modulation of the closed-circuit loop current occurs via the
inverted inputs of the amplifiers Jl, J2. The associated re-
sistances of R9, Rll and R10, R12 determine the rat;io of the
modulation voltage to the moaulation current. If the resistances
of R9 to R12 are equal, the modulation voltage at the capacitors
C2, C3 is transmitted to the loop resistors R3, R4 with the
gain 1. With increasing gain of the modulation arrangement by
appropriate dimensioning, the a-c voltage gain of the positive
inputs of the amplifiers Jl, J2 and, thereby, the ~ynamic self-
or automatic balance are also improved.
For evaluating the loop, the inverter resistors R13, R14 are
connected to the outputs of the amplifiers Jl, J2. Without these
inverter resistors R13, R14, the same voltage wou1d be present at
the loop resistors R3, R4, when the subscriber loop is open as well
as closed, which could prevent evaluation of the loop. With in-
creasing ratio of the resistance values o~ the inverter resistors
R13~ R14 to those of ~he loop resistors R3, R4, the evaluation
threshold for the loop signals also increases.
The amplifier J6 has a ga~n 1 and serves for inverting the
signals coming from the preceding amplifier J4 and J~, respect-
ively, for modulating the voice wire B.
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In order to achleve a large hybrtd attenuation if the subscriber
loop is open, the differential amplifier J3 is driven into limit-
ing via the transistor T by the loop signal at the point S, so
that no a-c voltage can reach the point G of the outgoing branch
of the hybridO
As a~orementioned, two circuit variants or embodiments of the
hybrld arrangement are shown in the figure, both of which are
described hereinafter in greater detail.
The first circuit variant has a real lnput resistance and there-
~ore a real internal generator resistance and a
complex line balanclng network whereby me;.sured transn~ssion values
are obtained which correspond to conventional inductive hybrids.
The second circuit varlant is equipped with a complex input im-
ped~nce and, consequently, with a complex internal generator im-
pedance and with a complex line balancing network. The dimenslon-
ing of the balancing networks depends upon the type of cable of
the subscriber loop and.upon the internal impedence of the sub-
scriber station.
In the embodiment of the circuit which has a real input resistance
R15, the latter is connected between the polnts 1-1. The connection
o~ the points 2-3 is severed and connectlon of the points 2-4 is
maintained; tn the practical design of the circuit, the ampli~ier
J5 and the llne balancing network N2 as well as the unidentifted
resistor and capacitor associated there~th, are omitted~ The
voltage present at the real input resistor R15 is transmitted via
the differential amplifier J3 to the outgoing branch of the hybrid
arrangement, point G.
The voltage present in the incoming direction at the point K is
transmltted v~a the modulation circuit to the voice wires A, B
s~
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and therefore, ~lso to thq inpuls of the differential amplifierJ30 In order to obtain ~ larg~e hybrid attenuation, a counter-
voltage is applied to t.he inv~ ted input of the di~ferential
amplifier J3 by the amplifier J51, so that there ls no voltage
at the point G.
Under operating conditions iOe. lf the voltage present at the
point K produces a ~requency-dependent voltage on the voice wires
A, B, the ~requency response o~ the line balancing network N
must correspond to that of the external termination o~ the voice
wires A, Bo Since the input resistance R15 together with the
capacitor Cl is also frequency-dependent to a certain extent, the
line balancing network N is shunted by an R-C member, the elec-
trical data or values Or which correspond to those of the input
resistance R15 and the capacitor Cl~
In the embodiment of the circuit which has a complex input im-
pedance, a complex ~nput impedance Nl is connected between the
polnts l-lo Connection of the points 2-3 is maintained and that
of the points 2-4 is severed; the amplifier J51, the complex
balancing network N with the R-C component thereof and the other-
wise unidentified resistor associated therewith being omitted in
the practical construction of the circuit. The circuit arrange-
ment is formed in such a manner that the voltage at the voice
wires A, B is transmitted as an unbalanced or asymmetric voltage
to the point G o~ the outgo~ng branch of the hybrid via the
dif~erential amplifier J3. An unbalanced or asymmetric voltage
at the point K of the incoming branch is transmitte~ to the YoiCe
wires A, B as a balanced or symmetric voltageO The frequency
response of the complex input ir..pedance Nl, as well as the llne
balancing network N2 for the co~plex internal generator impe~ance
corresponds to the load on the voice wires A, Bo
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The current flowing in the line balancing network N2 is converted
by tne amplifier J5 into a control voltage for the current
generators. The current delivered by the current generators to
the parallel circuit consisting of the complex input impedance
Nl and the subscriber stat~on consequently has the same frequency
response as the current in the line balanc~ng network N2. Through
appropriate selection of the gain of the operational amplifiers
Jl, on the one hand, and J2 and J5, on the other hand, the
voltage of the voice wires A, B is exactly equal to the voltage
at the point K. The equality of these voltages ls utilized for
the hybrid function of the arrangement. Wlth equal hybrld re-
sistances R16 to ~21, the voltage at the point K is fed vla the
amplifier J4 to the resistor R17 and counteracts with twice the
amplitude the voltage from the voice wire A, which is thereby
cancelled. Together with the voltage of the voice wire B, a
common-mode signal is delivered therewith to the inputs of the
differential amplifier J3 but is not taken into consideration.
The advantage of the last-described circuit variant is that the
input impedance is matched to the impedance of the connected
line. A telephone system constructed in this manner is very
well matched to a line network. If, in additlon, the input
impedance and the lnternal impedance of the subscriber station
are matched to the line impedance, then good hybrid attenuation
and side-tone reduction, respectively, can be obtained also with
very short loops or connected linesO
The use of operat~onal amplifiers affords the realization o~
the balancing network N2 with physical or spatially small
components, which have an impedance that is many times higher
than that of conventional balancing networks.
_9_
