Note: Descriptions are shown in the official language in which they were submitted.
~9S3 E;~2
The invention relates to line interface apparatus for
coupling a communication line with a communica-tion s~stem~ ~ore
particularly, the invention is a line circuit having positive feed
resistances in combination with a neg~tive impeclance circuit.
Telephone loops which are associated with a central offtce
(CO) telephone facility are usually terminated at the CO by a line
circuit. The line circuit is typically required to provide a terminating
impedance of about 900 ohms for voice frequency signals on the loop. The
line circuit is also required to supply energizing direct current for
operation of the loop. This is typically arranged for by two similar
resistors of about 200 ohms, each being connected in series with one of
the tip and ring leads of the loop and a CO battery of about 50 volts.
Usually, it is not permissible to employ resistors of values greatly
exceeding 200 ohms, because, typically, the energizing current is required
to be not less than about 20 milliamperes for CO loops having resistances
up to about 2000 ohms, including the resistance of an OFF HOOK telephone
set. Thus the source impedance of the energizing current is usually close
to 400 ohms, significantly lower than the required a.c. impedance of 900
ohms. Therefore, separate circuits or devices are usually required to
20 meet both the a.c. terminating impedance and the d.c. energizing current
requirements. The line circuit usually must meet stringent requirements
as to permissible levels of unbalance to ground, for example less than -60
decibels, and tolerance of longitudinal induction of up to 20 milliamperes
rms per lead. The line circuit must also be able to survive high voltage
surges of at least 500 volts without darnage to itself or apparatus in the
CO. All of these requirements are well satisfied by many prior line
interface circuits, which typically include a transformer for coupling
a.c. signals between the loop and the C0. The transformer usually
includes tip and ring windings being connected in series with respective
tip and ring feed resistors and may include a d.c. Flux cancellation
winding as for example is disclosed by V,V. Korsky in United States patent
No. 4,103,112 issued on July 25, 1978, whereby the size and cost of the
transFormer is reduced. Many newèr line clrcuits of intended lesser cost
have been developed through reducing bulk of various circuit components
and by utilizing newer devices including semiconductor components to
replace the transFormer. Many of these newer developments in line
circuits ha~ie not been without penalties such as requirements for battery
voltage boosters, or failure to meet all of the typical line interface
circuit operational requirements, or are not yet cost competitive with
present widely used line interface circuits.
A line circuit in accordance with the invention includes tip
and ring feed resistances for connection in series between a source of
energizing power and tip and ring leads of a communication line whereby
the sum of the ohmic values of the tip and ring feed resistances
substantially determines an energizing current Feed resistance for the
communication line. An a.c. negative impedance circuit is connected
20 across the tip and ring kerminals and provides in combination with the
feed resistance, an a.c. termination impedance For the communication line.
In one example an energizing current feed resistance of 400 ohms in
parallel with an a.c. impedance of -720 ohms provides an a.c. terminating
impedance of about 900 ohms for voice frequency signals on the
communication line. A suitable negative impedance circuit includes both
passive and active circuit devices. The negative impedance is preferably
coupled to the communication line through a transformer having a winding
which is a.c. coupled across the tip and ring terrninals. Trans~ormer
coupling has been found to be advantageous in that the negative impedance
is substantially isolated from the communication line at ~/ery lo~l
frequencies approaching d.c. and balanced impedances between the tip and
ring leads to ground are inherent, providing the tip and ring Feed
resistances are matched to each other, Furthermore, a blocking capacitor
in series with the winding prevents high or steady state currents from
flowing in the transformer windings. This is one factor in providing an
economical miniature transformer in the line circuit.
The invention provides a method for terminating a -telephone
line appearance in a communication system. An energizing loop current is
supplied to the telephone line at the telephone line appearance via tip
and ring feed resistors having similar ohmic values5 the sum of which
substantially defines a direct current feed resistance~ An impedance
circuit having a negative ohmic value is a.c. coupled across the telephone
line appearance whereby a positive a.c. terminating impedance is defined
by a summation of reciprocals of the feed resistance and said negative
ohmic value, being divided into unity.
The invention also provides a line feed circuit for
terminating a two wire communication line with a predetermined d~co feed
resistance and with a predetermined a.c. impedance at a frequency in a
voice band of frequenciesO The line circuit includes tip and ring
terminals for connection to the communication line and first and second
power terminals for connection to a source of energizing power. Tip and
ring resistors are connected between respective ones of the tip and ring
terminals and the first and second power terminals, The tip and ring
resistor are of similar ohmic values and in summation provide the
predetermined d.c. feed resistance. A transformer includes first and
second ~indings in cornbination with a core structure. A capacitor is
connected in series with the first winding across the tip and ring
terminals to provide an inductive capacitive series circuit being resonant
at a frequency below the voice hand of frequencies~ A negative impedance
coupling circui-t is connected at an end of the second windiny for coupling
a.c. signals to the communication line via the first and second windings
and whereby said predetermined a.c. terminating impedance is effected
across the tip and ring terminals.
An example embodiment of a line feed circuit and the design
and structure of two examples of negative impedance circuits for the line
feed circuit are discussed with reference to the accompanying drawings in
which:
Figure 1 is a block schematic diagram of a general form of a
line feed circuit in accordance with the invention;
Figures 2a - 2f are schematic diagrams of negative impedance
circuit units useful for illustrating design methods for defining
impedance functions required in a negative impedance circuit used in
figure 1;
Figure 3 is a schematic diagram of one example of a
conceptual form of a negative impedance network useful in a line feed
circuit as illustrated in figure 1;
Figure 4 is a schematic diagram of another example of a
conceptual form of a negative impedance network useful in a line feed
circuit as illustrated in figure 1;
Figure 5 is a more detailed schematic diagram of a line feed
circuit similar to the line feed circuit illustrated in figure 1 and in
accordance with the invention;
8~
Figure 6 is a schemakic diagram of a negative impedance
coupling circuit useFul in the line feed circuit illustrated in figure 5,
and in accordance with the conceptual form illustraked in figure 3;
Figure 7 is a schematic diagram of a negative impedance
coupliny circuit useful in the line feed circuit lllustrated in figure 5,
and in accordance with the conceptua', form illuskrated in figure 4; and
Figure 8 is a graphlcal representation of a t~pical return
loss performance of a line feed circuit in accordance with the invention.
The general form of the line feed circuit illustrated in
figure 1 includes a tip terminal 1 and a ring terminal 2 being connected
to a communication line9 typically a telephone subscriber loop having a
loop impedance ZL. The loop impedance ZL represents a loop impedance
value of tip and ring conductors in series with a typical telephone
station set being in an OFF HOOK state of operation. Power terminals 3
and 4 are connected across a source of direct current labelled BATTERY
such that an energi~ing loop current is supplied to the loop impedance ZL
via feed resistances 5 and 6, The ohmic values of the feed resistances
are typically about 200 ohms each and are sufficiently matched to achieve
in operation, at least a minimum predetermined common mode rejection
value. An a.c~ coupler 10 is connected across the tip and ring terminals
1 and 2 and includes a port connected to a negative impedance circuit 19.
Hence an effective feed resistance measured at the tip and ring terminals
is the sum of the values of the feed resistances 5 and 6. However an
effective feed impedance in the voice band range of frequencies is a
result of the feed resistance taken in shunt with the ohmic value of the
negative impedance circuit 19. Thus in one example a desired -feed
impedance of about 900 ohms is obtained wi-th -720 ohms in shunt wikh 400
ohms~
S
;
Referring briefly to figure 5 a preferred exarnple of the
a.c. coupler 10 is illustrated and includes a trans-Former haviny a primary
winding 10a being connected in series with a capacitor 9 between the tip
and ring terminals 1 and 2. The previously referred to port is provided
by a secondary winding 10b. In this arrangement, there are t~"o basic
problems which make it difficult to achieve a commercially acceptable
design of the line feed circuit. Firstly if the negative impedance 19 is /
assumed to be a simple -720 ohm resistance at voice frequencies, the
inductance of the transformer winding 10a and the capacitance of the
capacitor 9 must be of such high values that the bulk of these two
components would render the line circuit too costly. Therefore, a simple
-720 ohm resistance at voice frequencies would not be a suitable form of
the negative impedance 19 in figure 5. Secondly, it is well known that
circuits containing negative impedance elements tend to be unstable, such
instability usually being demonstrated by either of undesirable circuit
oscillation or d.c. latch up. Occurrence of either of these instability
functions of course negates the intended negative impedance functions of
the negative impedance element.
Basic principles for designs of negative impedance circuits
which are adapted to avoid these two basic problems are discussed with
reference to figures 2a - 2f.
It is known that a negative resistance is created by a
circuit as illustrated in figure 2a~ The circuit includes an opera~ional
amplifier and resistors R1, R2 and RS. Typically resistors R1 and R2 each
have a much higher ohmic value than that of the resistor RS. In such a
case an impedance Ri as between a point A and ground in figure 2a is
approximately as defined by the following equation:
Ri = -RS R1/R2 (1)
i82
Complex impedances with negative elements are created by a circuit as
illustrated in figure 2b, where in contrast to the circuit in figure 2a
the resistor R1 is replaced by an impeddnce Z1. An irnpedance between a
point B and ground is approximately as definerl by the following equation:
Zi = -RS Z1/R2 (2)
Figure 2c illustrates a specific example wherein the impedance Z1 in
figure 2b is provided by a capacitor C1 connected in serles with the
resistor R1, In this example~ a resulting impedance between a point C and
ground is substantially defined by the negative resistance Ri from
equation (13, taken in series with a negative capacitance Ci as
approximately defined by the following equation:
Ci = -C1 R2/RS (3)
As a matter of review, it is well known that a negative capacitance is
similar to an inductance in that a current component lags a voltage
component of an applied alternating signal. However the negative
capacitance differs in that a magnitude of impedance (voltage to current
ratio) varies in direct proportion with frequency for an inductance but
varies in inverse proportion for a capacitance, whether or not the
capacitance is negative or positive. In figure 2d a circuit similar to
the circuit in figure 2b is illustrated, however in contrast to figure 2b
the impedance Z1 is a three terminal impedance for example similar to a
resistor capacitor network illustrated in figure 2e. Another negative
impedance circuit including two complex impedances Z1 and Z2, ls
illustrated in figure 2f. In the circuit of -Figure 2f an impedance
between a point F and ground is approximately defined by the following
equation:
Zi = 1/[Z2 - R2/RS Z1)] (~)
~Læ~
The circuit units illustrated in figures 2a - 2f are selectively used as
building blocks to achieve a desired complex negative impedance functior
in conjunction with other circuit elements.
Negative impedance circuits useful in line feed circuits
sirnilar to that illuskrated in figure 5 are illustrated in conceptual
forms in figures 3 and ~. These negative impedance circuits are
represented by passive resistor and capacitor circuit elements some of
which are indicated to be negative characteristic functioning components
by an associated label NEG.
These conceptual forms were derived by combining known
methods of circuit analysis, synthesis, and stability testing followed by
numerical trial and error methods implemented by means of computer
simulation. The negative elements have been realized in practical form by
using the circuit units as described in relation to figures 2a - 2f. The
practical realizations are illustrated in figures 6 and 7. The negative
impedance coupling circuit in figure 6 is exemplary of a practical
realization of the conceptual form illustrated in figure 3. The circuit
is made up of three circuit units 20, 21 and 22 being connected in series
as shown and having a negative impedance node 7 and an input node 8 for
receiving voice frequency signals for coupling to a subscriber loop. The
negative impedance coupling circuit in figure 7 is exemplary of a
practical realization of the concPptual form illustrated in figure 4. In
contrast to the circuit in figure 6, the negative impedance coupling
circuit in figure 7 has been found to be more stable in a wide variety of
line feed circuit tests and thus is decribed in more detail.
The negative impedance coupling circuit in figure 7 includes
four circuit units 30, ~0, 50 and 60. A series combination of clrcuit
8~
units 50 and 40 is coupled in parallel with the circuit unit 60 to the
circuit unit 30. This is somewhat similar to the arrangernent illustrat,ed
in figure 2f wherein the series combination of the circuit units 50 and 40
provides a function corresponding to Z2 and the circuit unit 60 provides a
function corresponding to Z1. Each of the circuit units 30, 40, sn and 60
includes circuit elements, each of which is labelled with an identifier
having d respectively corresponding tens digit. Each of the identifiers
having a units digit 1 designates an operational amplifier having an
output and having inverting and non-inverting inputs. Each of the
identifiers having a units digit 2 designates a resistor being connected
between the output and the non-inverting input of the corresponding
operational amplifier. Each of the identifiers having a units digit 3
designates a resistor being connected between the output and the inverting
input of the corresponding operational amplifier, and each of the
identifiers having a units digit 4 designates a capacitor being connected
between the output and the inverting input of the correponding operational
amplifier. Each of these capacitors 34, 44, 54 and 64 provide
compensating negative feedback for the respective operational amplifier to
prevent high frequency oscillation. The effect of these capacitors on
circuit operation is otherwise substantially insignificantD In the
circuit unit 30, a resistor 36 is connected between the non-inverting
input of the operational amplifier 3î and a resistor 37 is connected
between the inverting input and ground. In the circuit unit 40, a
resistive capacitive network is provided by a resistor 45 and a capacitor
47 connected in series between the inverting input of the operational
amplifier ~1 and ground. The junction between the resistor 45 and the
capacitor 47 is connected to the non-inverting input of the operational
amplifier 51 in the circuit unit 50O Also in the circuit unit 50, a
resistive capacitive network is provided by a capacitor 55 and a resistor
56 connected in series between ground and the inverting input of
the operational amplifier 51. In the circuit unit ~0, a resistive
capacitive network is provided by a capacitor 65 and a resistor 66
connected in series between the invertlng input of the operational
amplifler 61 and an input node 8, corresponding to the input node 8
illustrated in figure 5. Reslstors 28 and 29 are connected in series
between ground and the non inverting inputs of the operational amplifiers
31 and 41 and provide a voltage divider having a tap at the junction of
the resistors 28 and 29 which provides a negative impedance node 7,
corresponding to the node 7 in figure 5.
In the line feed circuit illustrated in figure 5 a nega'cive
impedance coupling circuit 19 is provided by a circuit similar to that
illustrated in either of figures 6 or 7. Elements in figure 5 which are
similar to elements in figure 1 have corresponding identifying labels.
The negative impedance coupling circuit 19 is intended to be connected to
a two wire/four wire electronic hybrid circuit (not shownj via the nodes 7
and 8 labelled Tx for transmit and Rx for receive respectively, The a.c.
impedance of the electronic hybrid circuit at the node 8 is preferably
near zero ohms and at the node 7 it is preferably more than ten times the
absolute value of the impedance of the negative impedance coupling circuit
19, and rnore typically is in excess of 1 megohm~ The node 7 is also
connected at one of two ends of the transformer winding 10b. The other
end of the transformer winding 10b is connected to an active ground
circuit. The active ground circuit is not essential but tends to improve
the operation of the line feed circuit when a battery connected across the
power terminals 3 and 4 is used to feed many other line feed circuits in
common, as is usually the practice in a typical telephone exchange~ The
active ground circuit includes a differential amplifier 12 connected in
combination with resistors 13, 14, 17 and 18 and with capacitors 15 arld
16, as shown, across the power terminals 3 and 4. The act-i~/e yround
circuit responds to volce band signals typically referred to as noise
signals which may be present at the battery terminals, to cornpensate for a
crosstalk effcct that these noise signals would otherwise tend to have on
the operation of the line feed circult.
Zener diodes lla and 11b in this example having Zener
voltages of about 18 volts, are connected between ground and respective
ends of the transformer winding 10b to protect the associated circuitry
from transient surges that may from time to time occur on typical
subscriber loop circuits. In one example the transformer in the a.c.
coupling circuit 10 consists of about 1000 turns of ~0 gauge copper wire
in the winding 10a and about 500 turns of the same gauge of copper wire in
the winding 10b, the windings being resident in a ferrite pot core having
diameter and height dimensions of about 1.5 centimeters. Typical values
of the resistive and capacitive components in the negative impedance
~0 coupling circuit illustrated in figure 7 for yielding the required
negative impedance function in the line feed circuit illustrated in figure
5 are listed in the following table:
11
32
Resistor 28~ 50 ohms
29~ lO
32~ 1000
33~ 86
37~o~ - 47
62 ~ o ~100 ll
36~ 8~42 kilohms
42~ 22
Il 43~ 430
ll 45~ 24
52 ~ 10
53~ 60~ 7
56~ 76~2
63~ 16
Il 66~ 124 ll
Capacitor 34~ 68 nanofarads
55~ 10 ll
47~a~ 0~1 microfarads
Il 65~ O~ 1 ll
" 44~o~ D~15 picofarads
Jl 54~ o~ 100
Il 64~ 33 ll
Results of a typical voice band signal test o~ the line ~eed
circuit illustrated in ~igure 5, having a negative impedance coupling
circuit as illustrated in figure 7 are recorded in the graph o~ Figure 8
where the horizontal axis represents frequency in hertz and the vertical
axis represents return loss in decibels. Input return loss is typically
12
measured using a signal source including an a.c~ voltage generat,or
connected in series with a capacitor of about 2.2 microfarads and a
resistor of 900 ohmsO The signal source is connected across the tip and
riny terminals 1 and 2 in combination with a t~pical return loss
measurement -test apparatus, and the input node 8 is connected to ground.
13
