Note: Descriptions are shown in the official language in which they were submitted.
~Z~i53
This invention relates to a method and apparatus
for measuring a telephone line and more particularly to one for
deriving thirteen distinct parameters from the resultant tip and
ring current and charge flow monitored during three successive
states of applied voltage.
Background of the Invention
During the operation of a telephone system, it is
often necessary to test the conditions of a telephone line,
particularly those referred to as subscriber loops which connect
terminal sets to a central office. In the past, this has been
accomplished by connecting various configurations of test
equipment to the line to separately determine each of the wanted
parameters. An extensive analysis would involve measuring at -
least thirteen separate parameters including any spurious a-c voltage,
d-c voltage, leakage resistance and line capacitance between the
tip and ground, ring and ground, and tip and ring of the subscriber's
loop, as well as the frequency of the spurious a-c voltage. It will
be readily apparent that determining each of these parameters
separately in order to evaluate the state of the subscriber loop
; 20 was a long and tedious operation.
United States Patent No. 4,028,507 invented by `
Richard Scott Hoppough, and issued June 7, 1977, discloses an
improved apparatus for measuring the various parameters, which
entails forcing the tip and ring to several different sets of
potentials with relation to ground, and then measuring the
resultant currents flowing therethrough. From these measurements, -
the various parameters can then be calculated. However with this
arrangement separate sets of d-c and a-c voltages must be applied ~ -~
to measure the respective leakage resistances and the line
reactances (primarily capacitiveJ. In addition, filters are
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required to reject any spurious a-c signals on the line during
the a-c measurements.
Statement of the Invention
.,
It has been found that by connecting only three
suecessive d-c voltage states to the line through known tip and
ring resistors (rather than directly to the line) and monitoring
the resultant transient charge and steady state current
therethrough, that suFficient information can be obtained to
derive the thirteen different parameters of the subscriber loop
detailed above, thereby eliminating the separate application of
a-c signals for the reactance tests. In a typical applicàtion
this can be achieved in a period less than three seconds, the
majority oF the time being required for the line to settle
(in order to measure the transient charge) when the voltage ~ -
state on the line is altered. Thus the measurement time is
directly related to the leakage resistance and the capacitance
of the loop.
In accordance with the present invention there is
prov;ded a test set for measuring a telephone line having tip
.
and ring leads, wh;ch comprises means for separately measuring
during a first state, spurious currents flowing from tip to ground
through a first reference resistor, and ring to ground through a ;
second reference resistor. During a second state9 the test set
includes means for measuring the transient charge and the
steady state current flowing through the first and second
resistors when connected in series with f;rst sources of d-c
voltage between tip anc! ground, and ring and ground respectively.
During a third state, the test set includes means for measuring
the transient charge and the steady state current flowing through
3~ the first and second resistors when connected in series with
- 2 -
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second sources of d-c voltage between tip and ground, and ring and
ground respectively, the second sources having a different voltage
ratio than the first sources.
In a particular embodiment both sides of the loop
are grounded through the reference resistors for measurement of
the spurious currents. A low d-c voltage is applied to both sides
of the loop through the resistors during the second state, and
in the third state the low voltage is removed from one side.
Each state results in two values: a transient charge flow as
a result of the change of state, and a steady state current flow
after the transient. The currents~'give the spurious voltages and
leakage resistances which may be present, while the transient
charges give the capacitance values.
The invention also encompasses a method of measuring
these parameters which includes individually measuring and
recording the spurious currents flowing through individual
reference resistors connected from each side of the line to ~round,
then individually measuring and recording the transient charge
and the steady state current flowing throug~ the individual
reference resistors connected in series with first sources of d-c
voltage from each side o~ the line to ground, and then individually
measuring and recording the transient charge and the steady state
current flowing through the individual reference resistors
connected in series with second sources of voltage from each side
of the line to ground, the second sources having a different
voltage ratio than the first sources; and thereafter calculating
the various parameters directly from the recorded measurements.
Brief Description of the Drawin~s
An example embodiment of the invention will now -
be described with reference to the accompanying drawings in which:
~ ' '.
Figure 1 is a block and schematic circuit diagram
of a telephone line connected to a test set for measuring the
line;
Figure 2 illustrates typical current waveforms
developed across the reference resistors during the three applied
states, when no spurious a-c signals are present;
Figure 3 illustrates a typical steady-state current
on either the tip or ring lead when a spurious a-c component is
present, and the a-c component of that current after having been
rectified; and :
Figure 4 illustrates the transient current flow ~ -
when a step function d-c voltage is applied to one side of the
line during the presence of a spurious a-c component.
Description of the Preferred Embodiment
Figure 1 illustrates the test set connected to the
tip (t) and ring (r) of a telephone line 10 (o~ten referred to
as a subscriber loop) which is terminated in a conventional
telephone set 11. When the set 11 is on-hook, it generally
appears as about a 0.45~F capacitor Cp across the line 10, which .
is primarily contributed by a d-c isolating capacitor in series ~::
with a ringer (not shown) in the set 11. Shown in dotted form
is the leakage resistance Rtg and line capacit~nce Ctg in series :
with spurious a-c and d-c voltages Vact9 and ~e~ from tip (t)
to ground (g); leakage resistance Rrg and line capacitanJce Crg - .
in series with spurious a-c and d-c voltages Vacr9 and~from ring (r)
to ground (g); and leakage resistance Rtr and line capacitance Ctr .
from tip (t) and ring (r) o~ the line 10. Since the capacitor Cp
is in shunt with the capacitance Ctr the total tip to ring
: capacitance will equal Ctr = Cp + Ctr. Illustrated in lumped
form is the series res-istance Rt and Rr of the line 10. Normally
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this resistance is small (<3000~) compared to any leakage resistance
(>>50000Q) of the line lO and consequently has little effect on the
resistance measurements. Also the line resistance does not affect
the accuracy of the capacitance measurements, since these calculations
which take place only after the transient voltages have subsided,
are based on the total charge flow and not on the charge flow for
a preselected period of time.
In the following description of the test set,
corresponding elements and waveforms for both t and r will be
identified by the same reference numeral or character followed
by the distinguishing reference character t or r. However, only
the reference numeral will be referred to except where it is
necessary to distinguish between the two elements. Also1 the
location in Figure 1 of the waveforms illustrated in Figures 2, 3
and 4 is indicated by corresponding reFerence characters. In
addition, all switches which are illustrative only, are connected
concurrently to their respective contacts bearing identical contact
numbers, under control of timing control circuitry, as will be
explained in greater detail hereinafter.
Both t and r of the line 10 are coupled through input
reference resistors Rit and Rir (each typically lOOK~) to switches 13
which control the three different voltage states applied to the
line 10. During each of the three measurement stages, the current `
and transient charge flow through the reference resistors Rit and Rir
is obtained indirectly by monitoring the voltage thereacross. The `
balance of the test set will manifest itself from the fol10wing
description of its function and operation.
During the initial stage, the resistors Rit and Rir
are both connected to ground g to enable measurement of any spurious
a-c or d-c voltages Yac or Vdc on the line 10. Initially, with all
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switches in position 1, spurious a-c voltages developed across the
resistors Rit and Rir respectively are coupled through capacitors 14,
a-c/d-c converters 15 and switches 16 to respective voltage/frequency
converters 17. The use of v/f's 17 permits simply counting
(integration techniques -for obtaining capacitance measurements).
As explained hereinafter, the d-c measurements (yielding spurious
d-c voltages Vdctg and Vdcrg, leakage resistances and capacitances)
are made with the v/f converters 17 coupled directly to the line 10.
The a-c measurements (spurious a-c voltages Vactg and Vacrg) are
made by coupl;ng the v/f converters 17 through the d-c blocked
a-c/d-c converters 15 to the line 10. ~ ~ -
The outputs from the capacitors 14 are also fed
to positive zero crossing detectors 20. If there is a detectable
spurious a-c component present on the line 10 then the sample
period ~ will be that between two positive-going zero crossings
of the a-c component. All d-c measurements are synchronized with
integral multiples of cycles of the spurious a-c component to give
a-c rejection.
The following simplifying assumptions are made
about spurious a-c sources:
(1) That any spurious a-c voltage is periodic.
If ~his assumption ;s not val;d, for example if the a-c voltage
is broadband white noise, then the meter will still make an attempt ~ -
to produce a mean a-c voltage read;ng but will not be able to
make any other measurements since synchronization o-f the sample
period will be ;mposs;ble.
(2) That any spurious a-c voltages on both the tip
and ring are due to a slngle source. Separate sources on tip and
ring of similar amplitu~es but of differing frequencies will result
in a situation similar to the above. -
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~ s a result of assumption 2, the loop 10 may be
reduced to one of two equivalents for a-c measurement: one where
the spurious a-c sources on the tip and ring are in-phase and the
other where they are out-of-phase. The phase 0 is detected by
comparing the outputs of the positive zero crossing detectors 20
in a phase detector 21. The tip to ring a-c potentials may be
calculated: Vactr = ~Vact9 + Vacrg - 2-Vact9 Vacr9 Cos0
The outputs from the v/f converters 17 are fed to
counters 22 which provide at their output, a frequency count n over
a preselected interval as explained hereinafter. In addition an
internal l~lz clock 23 provides a reference output frequency fref `
which is fed to a further counter 24. The counter 24 provides
two outputs: (1) a measure of the sample period T which is the
time between successive zero crossings of the spurious a-c
com~onent Vac as detected by one or other of the positive "O"
crossing detectors 20, which indirectly controls the resetting of
the counter Z4; or (2) if no a-c component is present the maximum
sample period Imax wh;ch is provided each time the counter 24 is filled.
is selected to be longer than the period of the lowest spurious
a~c signal expected to be encountered. Typically Imax = lOOmsec is used.
Either sample period pulses ~t~ Tr or Tmax are coupled
by a switch 30 and/or an OR gate 25 to a computer 26 which also
receives the counter and phase outputs nt, nr and ~ as shown in
;` order to perform the various calculations as determined by the
formulas given in TABLES I and II at the end of this description.
The switch 30 is initially connected to the output
of detector 20t. ~owever, if no spurious a-c signal is detected
the information is conveyed to the computer 26 which controls the
connection of the switch 30 to the output of detector 20r. This `-
permits a spurious signal on either the tip and/or ring to control
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the resetting of the counters 22 and 24.
In addition the computer 26 provides the reset signals
~or the counters 22 and 24 each time a control signal is received
from the OR gate 25. The computer 26 also outputs the calculated -
values to a record and display unit 279 as well as provides control
signals to the various switches. Once the measurements for switch
positions 1 are completed, the switches 13 and 16 are moved to ~ -
positions 2 under control o~ the computer 26 for measurements
of any spurious d-c voltages. The switches 13 and 16 are then moved
to position 3. Here an internal voltage source Vs (typically Vs = 50v)
is applied to both t and r through the resistors Rit and Rir
respectively, and another set of d-c voltage measurements is taken ~-
as well as the magnitude of the transient charge flowing through
the resistors Rit and Rir.
When a steady state condition is reached (as determined
by two successive measurements of equal magnitude) the ring r is
again connected to ground through resistor Rir as indicated by switch
positions 4 and a further set o~ d-c and charge Flow measurements -
is taken. It is to be noted that resistor Rir need not necessarily ;~
be connected to ground but may be connected to a voltage source o~
; a different magn;tude. The important th;ng is that the ratio of
the two voltages applied to the resistors Rit and Rir be different
for positions 3 and 4.
Once this is completed sufficient in~ormation has
been fed to the computer 26 to initially obtain the derived
mea~urements shown in the TABLES and ultimately the 13 calculated
parameters. However only three different voltage states have been
applied to the resistors Rit and Rir~ In positions 1 and 2,
both resistors are connlected to ground during position 3, both
resistors are connected to Vs, during position 4 Rir is again
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connected to ground while Rit continues to be connected to source Vs.
The instantaneous output frequency f of the
voltage/frequency converters 17 is proportional to the
instantaneous current i through the resistors Ri. The transfer
function of the converters 17 is therefore: f = ~ i + fO where
K is a gain term and fO is the output frequency of the converters 17
when i = 0. Also if t be a measure of time and n be a count of
cycles of f, the instantaneous frequency f is then given by
f = dn/dt. By rearrangement, the current i and the charge flow q .
can be calculated in terms of n and t as follows: .
i = l/k (dn/dt - fO)
q = ri dt = l/k [~dn - fo~dt]
For a finite measurement sample periocl I the charge '-
flow is given by: q ~ r idt = l/k(N - fOI)
The mean value i of the current is given by:
1 = 1/T~ idt = l/k(N/T - fO)
T
where N = r dn, i.e. the count accumulated during the period T.
The frequency fs of the spurious a-c voltage is
then f = 1/~.
Figure 2 illustrates typical charge ~low and steady
:~ state currents which flow through the reference resistors Rit and
Rir during the various measurement intervals when a spurious d-c ~ :
voltage of different magnitude is present on both the tip and ring.
If this voltaye were not present then the currents itl and irl would
be zero during the initial interval when the switches were in . :~
positions 1 and 2~ The transient charge flow Qt and Qr during -
states 2 and 3 results from the change in voltage applied to both
sides of the line 10. This also results in a different steady
. state current it and ir during states 2 and 3.
: 30 An à-c component resulting from the presence of a
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spurious a-c voltage on the line 10 is shown superimposed on a
steady-state current i in Figure 3. The charge flow q for one cycle
of the spurious signal is also indicated. The equivalent current
output from the a-c/d-c converter 15 is also illustrated in the
bottom portion of Figure 3.
Figure 4 illustrates the transient charge that flows
following a change of state p-l to p (e.g. from state 2 to state 3).
The transient is considered to be complete when consecutive samples
show no detectable mean current change thus: im 1 = im.
The transient charge flow is as follows:
m-2 _ m-2
Q 1 q - lm-l 1 1
This is not necessarily the total charge in the
capacitance but is related to it. Hence the capacitance values
can be determined directly from $he measurements by applying
corrections for leakage resistance as shown in the accompanying
TABLES. It should be noted that the change of state is synchronized
with the period T under control of the computer 26. The leakage
resistance values are calculated from the stable, post-transient
current values ip which can be readily derived from the formulas
giv2n above.
Initially, the derived measurements shown in TABLES I
and II are determined from the information fed to the computer 26
during the respective switch positions. These measurements can
then be used to derive the intermediate calculations as shown, as
well as the desired 13 calculated parameters which are subsequently
fed to the record and d;splay un;t 27.
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TABLE I
. . . . . ._
SWITCH DERIVED INTERMEDIATE CALCULATED
POSITIONS M~ASJREMENT' CALCULATIONS PARAMET~RS
1 lac tl Vactg = lac tl Rit
ac rl VaCrg = lac rl Rir
l tr ~ tg rg tg rg
0 f = 1 /T
. ... _ .
. 2 ltl Vdc = i Ri
Vdcrg = ~ lrl Rir ~ :
~rl Vdctr = Vdctg - Vdcrg
_ . . _ . __ . . _ .... _l , .. . . .
3 1t2 t2 1t2 ltl VtrZ
_ I . . Vtg2 ~ Vtg3 Vtr3
; lr2 r2 r2 ~rl ItrIt3 Vt 3
s Vtg2 = Vs ~ It2 Rit r
(known) Vrg2 = Vs - Ir2 Rir V - V Vtr2
tr2 tg2 rg2 rg2rg3 Vtr3
. _ _ rg Vtr2
.~ 4 1t3 I - i Ir2Ir3 Vtr3
r3 Ir3 1r3 irl
Vrg3 0 ~ Ir3 Rir .9~ ~ ~2 V
tr3 = Vtg3 ~ rg3 ~
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LLJ ~ C~l
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I O~ ~ I cr I
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