Note: Descriptions are shown in the official language in which they were submitted.
1048669
The present invention relates to a circuit for the measurement
of transmission quality of telephone subscriber loops to verify the ;
serviceability of the loops, and more particularly to apparatus and method
enabling said measurements to be performed at a location central to a
plurality of said subscriber loops without involving participation by persons
remote from the central location.
The maintenance of outside plant equipment has always been a
significant cost to operating telephone companies. Various centrally located
maintenance equipments have been developed for testing of subscriber loops.
Typically these tests consist of a direct current or an alternating current
. test signal being placed across the subscriber loop and an analysis of the
loading and/or reflection effects the loop imparts to the test signal. A DC
signal can be used to test loop capacitance and insulation when the subscriber
equipment is in an on-hook condition. In combination with a known loop
termination an AC signal is used to verify voice band serviceability of the
1 oop .
Recently a trend has become apparent in that some subscriber
equipment is now customer owned. Often, a malfunction in customer owned
equipment results in a request for the operating company to check or repair
the telephone line. In such circumstances field personnel are often
required to journey to the far end of the subscriber loop to disconnect
the subscriber equipment in order that the operating condition of the subscriberloop be verified without interference from the subscriber equipment. This has
proven to be quite unnecessary and costly when it becomes apparent that the
fault is in the subscriber owned equipment and not in the operating telephone
company's transmission facility. Hence it is economically advantageous for
an operating company to be able to verify loop serviceability without
requiring the participation of field personnel.
The present invention provides a subscriber loop test device
for the testing of subscriber loop serviceability at an associated switching
, facility using DC and AC signals simultaneously. The test device includes an
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oscillator means which causes cyclic variations in an energizing current
being caused to flow therethrough. A connection means is connected between
the oscillator means and a subscriber loop adjacent an associated subscriber
equipment. The connection means is responsive to a predetermined voltage
being maintained on the subscriber loop to connect the oscillator means
across the loop and thereby cause AC modulation of the loop current. The
magnitude and modulation characteristic of the loop current at the switching
facility is an indication of the serviceability of the subscriber loop.
An additional feature includes contact means for isolating
the subscriber equipment from the subscriber loop for the duration of a
subscriber loop test. This carries an advantage in that the subscriber
equipment is prevented from interfering with the test.
The present invention also provides a method of testing a
subscriber loop circuit at an associated switching facility. The method
includes the steps of providing a test device at the end of the loop circuit
remote the switching facility, for drawing an energizing current and for
generating a predetermined frequency signal in response to a predetermined
voltage. The predetermined voltage is applied to the subscriber loop and
loop serviceability is verified by detecting the predetermined frequency signal
at the switching facility.
Example embodiments of the invention will now be described
with reference to the accompanying drawings in which:
Figure 1 is a block schematic diagram of a subscriber loop
test device;
; Figure 2 is a block schematic diagram of a subscriber loop
test device similar to the device shown in figure l; and
Figure 3 is a block schematic diagram of a subscriber loop
test device incorporating some of the features of the devices illustrated
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in figures 1 and 2.
Each of the figures in the drawings includes some common or
similar groups of circuit elements and hence for the purposes of this
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1~48~69
description similar circuit elements are identified by the same or similar
reference labels. Each figure includes a test circuit which is connected
to a subscriber loop via subscriber loop terminals 12 and 13, and is connected
to a subscriber equipment via subscriber equipment terminals 16 and 17. The
subscriber loop is normally connected to its associated switching facility
(not shown). In the event that loop serivceability is to be determined,
the subscriber loop is connected to circuitry, at the switching facility,
for activating the test circuit and evaluating its response.
In figure 1, the circuit includes a tone detector 100, and
a resistor 5 connected in series with a voltage source 60 between transfer
contacts 4a and 4b. Leads of a subscriber loop 1 extend toward a subscriber's
remote location, and a test circuit 10 is connected between the leads of the
subscriber loop 1 and a subscriber equipment 9 via the subscriber loop
terminals 12 and 13 and the subscriber equipment terminals 16 and 17. A
protector element 8, for example a carbon block or a gas tube, is connected
across the subscriber loop 1, as is standard practice, to protect the loop
and the associated equipment from lightning damage and the like.
The test circuit 10 includes a charge storage means consisting
of a capacitor 21 and a resistor 20 connected in series across the subscriber
loop terminals 12 and 13, the junction between the capacitor 21 and the
resistcr 20 being a discharge terminal 19. A diode 34, a relay 30 and a
make contact 32 are connected in series between the terminals 12 and 13. A
silicon controlled rectifier (SCR) 24 includes gate, anode and cathode
electrodes 23, 25 and 26 and is connected in aiding current flow relationship
to the junction between the relay 30 and the make contact 32. A resistor 27
is connected in series between the cathode electrode 26 and the subscriber
loop terminal 13. A breakdown device, in this case a gas tube 22, is
connected between the gate electrode 23 and the discharge terminal 19. A
variable impedance 44, in this case a relaxation oscillator, includes a
capacitor 42 connected in parallel with PNPN diode 41, sometimes referred
to as a Diac*, connected in series with a resistor 43. The variable impedance
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1G1 48669
is connected across the loop terminals 12 and 13 via a make contact 31.
In operation the high voltage source 60 is conveniently
provided by the 130 volt DC source associated with the switching facility
and typically used as a voltage source for the supervision of coin telephone
service. To initiate a test the transfer contacts 4a and 4b are operated
to apply +130 volts across the subscriber loop 1. In the switching circuit 10
the capacitor 21 slowly charges via the resistor 20. Eventually the --
breakdown potential of the gas tube 22 is reached and the gas tube 22 fires to
discharge the capacitor 21 via the discharge terminal 19 through the gate
electrode 23 and the resistor 27. This discharge causes the SCR 24 to turn
ON in a well known manner which in turn causes current to flow in the relay 30
via the diode 34. The contacts 31 and 32 of the relay 30 are thus closed,
locking the relay 30 in the operate condition and connecting the relaxation
oscillator (41 - 43) across the subscriber loop via the contact 31. The
impedance of the relaxation oscillator varies dynamically in a well known
manner to induce cyclic fluctuations into the loop current. These cyclic - -fluctuations in the loop current are detected by the tone detector 100. Hence
the DC and AC continuity of the subscriber loop 1 is substantially verified.
If the subscriber loop is unserviceable for example, when
either one or both leads of the loop 1 are severed, or when the side of the
loop to which the high voltage is applied is short circuited, or when the
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subscriber equipment is ~a~ loading the loop, the test circuit 10 fails ;
to function. This is consequently evidenced by a lack of detection by the
tone detector 100. Lack of detection will also occur when the AC shunt
impedance of the loop is too low thus causing the voice continuity to be
below a required minimum even though DC continuity may appear satisfactory.
However, in this case the test device is activated and functioning but the
impedance variationsintroduced by the test device are severely marked by the
AC shunt impedance. As the +130 volt source is usually grounded on one side
there is at least one condition in which the loop will appear to be functional
when it is not. The high voltage source 60, in most switching offices, is
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1~48669
usually grounded on one side. If a lead in the subscriber loop, which is
connected to the ground side of the high voltage source 60, is also shorted
to ground, this ground short will have no effect upon the test and hence will
go undetected. However, this can be overcome by using an additional tone
detector and a resistor, similar to the tone detector 100 and the resistor 5,
connected to the other side of the subscriber loop. Serviceability of the
subscriber loop is verified in this case by detection occuring simultaneously
in both tone detectors.
Referring to figure 2, a subscriber loop includes leads la
and lb which are each in series with break contact portions of transfer
contacts 4a and 4b respectively. A + or positive 130 volt source or other
convenient high voltage is connected to either of the transfer contacts 4a
and 4b via a transfer contact 7 and a resistor 5. High and-low tone
detectors 101 and 102 have inputs connected to the junction between the
resistor 5 and the transfer contact 7. A test circuit lOa is connected
via subscriber loop terminals 12 and 13 to the leads la and lb respectively.
Subscriber equipment terminals 16 and 17 are for connection of subscriber
equipment thereto as for example shown in figure 1. A ground terminal 18
is for connection with earth. The test circuit lOa includes two relaxation
oscillators 44a and 44b. One relaxation oscillator 44a consists of a
Diac 41a in parallel with a capacitor 42a, both connected in series with a
resistor 45a, and the other relaxation oscillator 44b consisting of a
Diac 41b in parallel with a capacitor 42b, both connected in series with a
resistor 45b. The values of the capacitor 42a and 42b are different so that
each oscillator will operate at a frequency different from the other. Diodes
34a and 34b are connected to the subscriber loop terminals 12 and 13
respectively and in series opposing relationship via Zener diodes 28a and
28b. The Zener diode 28a resides in series opposing relationship with the
diode 34a and the Zener diode 28b resides in series opposing relationship
with the diode 34b. A relay 30 is connected between a ground terminal 18
and the junction between the Zener diodes 28a and 28b. The relay 30 includes
associated transfer contacts 31a and 31b. The break portion of the transfer
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1~:)4~69
contact 31a is connected in series between the terminals 12 and 16, and the
break portion of the transfer contact 31b is connected in series between the
terminals 13 and 17. The relaxation oscillator 44a is connected between the
ground terminal 18 and the make portion of the transfer contact 31a, and the
relaxation oscillator 44b is connected between the ground terminal 18 and the
make portion of the transfer contact 31b.
In operation, to test the subscriber loop, the high voltage
at terminal 6 is applied via the resistor 5 and the transfer contacts 7 to
one of the transfer contacts 4a and 4b. The transfer contacts 4a and 4b are
10 operated to isolate the leads la and lb from the associated switching facility
(not shown). Assuming the high voltage is applied via the break contact
portion of the transfer contact 7, a current is drawn through the diode 34a,
the Zener diode 28a, the relay 30 to ground via the terminal 18. The relay 30
responds by operating its associated contacts 31a and 31b. Hence the terminals
12 and 13 are disconnected from the terminals 16 and 17 respectively and
interference from any associated subscriber equipment is thus prevented.
The high voltage is applied via the make portion of the transfer contact 31a,
across the relaxation oscillator 44a. The relaxation oscillator 44a operates
in a well known manner to impart an AC modulation component onto the surrent
20 flow in the lead la. This AC component is detected by one of the tone
detectors 101 or 102, and the lead la is verified as serviceable. The high
voltage is then withdrawn from the lead la and applied to the lead lb by
actuating the transfer contacts 7. The relay 30 is now operated via current
flowing via the diode 34b and the Zener diode 28b. The high voltage is
likewise withdrawn from the relaxation oscillator 44a and applied to the
relaxation oscillator 44b via the make portion of the transfer contact 31b.
Operation of the relaxation oscillator 44b causes detection in the other of
the tone detectors 101 or 102, and the lead lb is verified as serviceable.
Referring to figure 3, a test circuit lOb is connected to the
30 leads la and lb of the subscriber loop in place of the test circuit lOa in
figure 2. Also a high pass filter 108 is connected in series with the inputs
of the high and low tone detectors 101 and 102 to facilitate measuremen~
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1~48~9
of AC signal levels at a terminal 106 without interference from DC potentials.
All else, except for the details of the test circuit lOb, is essentially the
same as illustrated in figure 2.
The test circuit 10, in figure 3, includes resistors 20a
and 20b connected in series between subscriber loop terminals 12 and 13 and
the junction between the resistors 20a and 20b is connected to a discharge
terminal 19. Diodes 34a and 34b are connected in series opposing relationship
between the subscriber loop terminals 12 and 13. A relay 30 includes
associated transfer contacts 31a, 31b and 32a and is connected between the
transfer contacts 32a and the junction between the diodes 34a and 34b. The
make portion of the transfer contacts 32a is connected to a ground terminal 18
via a resistor 29. The break portion of the transfer contact 32a is
connected to the anode electrode 25 of a silicon controlled rectifier (SCR) 24,
the cathode electrode 26 being connected to the ground terminal 18 via a
resistor 27. A charge storage means, a capacitor 21, is connected between
the discharge terminal 19 and the ground terminal 18. A gas tube 22 is
connected between the discharge terminal 19 and gate electrode 23 of the
SCR 25. The transfer contacts 31a and 31b and relaxation oscillators 44a
and 44b as described in figure 2 are connected similarly as shown and
20 described in regard to figure 2.
The operation of the circuit of figure 3 in the area of the
relaxation oscillator is similar to that described in conjunction with
figure 2. The operation of the relay 30 and the associated circuitry is
somewhat similar to that described in conjunction with figure 1 except that
in this case the relay is operated by current supplied between one of the
leads and the terminal 18 as in figure 2, rather than by outgoing and
returning currents on both the loop leads, as in figure 1. In the arrangement
illustrated in figure 3, test circuit lOb, has been found to provide more
versatile performance than the test circuits 10 or lOa. The capacitor 21
30 is utilized purely as a store for trigger energy. The RC time constant of
the capacitor 21 in combination with the resistors 20a and 20b is preferably
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1G~4~f~69
quite long so that the capacitor 21 is not rapidly charged by typical high
voltage transients which can occur from time to time on a subscriber loop.
Hence a charge of sufficient potential to cause the gas tube to conduct is
normally attained only in response to a plus 120 volt potential or greater
at the terminal 6 being applied to the lead la or lb for a substantial period
of time for example about 3 seconds. When the gas tube fires to discharge
the capacitor 21, the discharge current is conducted via the gate
electrode 23 causing the SCR 24 to conduct. Current in the relay 30
flows initially via the SCR 24 and the break portion of the transfer -
contacts 32a. The make portion of the transfer contacts 32a is arranged -
to make connection before the break portion breaks connection and thus an
alternate holding current path is established through the resistor 29 before
the current path through the SCR 24 is broken. Once the contacts of the relay
30 are operated, operation of the test circuit lOb is identical to the
apparatus of the test circuit lOa described in conjunction with figure 2.
Each of the foregoing test circuit arrangements described in -
conjunction with the figures 1, 2 and 3 represents a significant departure
from previous subscriber loop test apparatus and methods. Each of these
circuits substantially verifies the subscriber loop serviceability by
requiring DC continuity in order for the test circuit to be activated and
at least a minimum AC transmission quality in order that loop serviceability
be verified. In figures 2 and 3 test circuits with the added versatility of
disconnecting the subscriber equipment have been described. Only in the
case where the subscriber equipment has introduced a permanent ground on
both leads of the subscriber loop will the test circuits lOa or lOb fail to
be operable on a serviceable loop.
In figure 1, the relaxation oscillator 44 operates in a
balanced relation with the subscriber loop. In contrast in figures 2 and 3,
the relaxation oscillators 44a and 44b both operate in an unbalanced
relationship with the subscriber loop. Hence the relaxation oscillators are -
preferably designed to induce a relatively low level of modulation into the
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~48669
energizing current to avoid any significant crosstalk problems with adjacent
subscriber loops.
The circuits in figures 1 and 3 are particularly advantageous
in that they are virtually transparent to normal methods of subscriber loop
testing. For example in one test with the well known type 14 Loop Test Desk
a 100 volt signal is applied to the ring side of a loop through a 100 kilohm
resistor and a 1.2 milliamp meter movement and then alternately applied to
the tip side of the loop. Under these conditions the test circuitslO, lOa
and lOb showed no appreciable leakage and thus had no appreciable effect upon
this standard test. If there is a requirement for a subscriber loop test
requiring higher voltages to be applied to the subscriber loop and at ~he
same time it is desired that the test device remain inactive, this can be
accommodated for example in figure 3 by replacing the gas tube 22 with
another element having a higher threshold voltage.
