Apparatus for determination of the location of a fault in communications wires

An improved instrument for determining the distance to an open fault in a communications line, the instrument taking advantage of the predictable relationship between the current discharge rate in a reference capacitor and resistor and the current discharge rate in a faulty line whose capacitance is a function of the distance to the fault. Provisions are made to keep the meter readings within the usable scale of the meter of the invention by allowing the operator to select from among a range of driving signal frequencies to suit the distance to the fault. Calibration may be made by reference to an internal capacitor or by means of measuring a known length of open circuited line of the type to be tested. Provision is also made to provide for "tuning out" "dirty" short circuits.

FIELD OF THE INVENTION 
The invention relates to portable open fault meters used in field work for 
determining the location of open circuit faults in telephone wire pairs, 
cables and the like. 
BACKGROUND OF THE INVENTION 
As best understood, prior art portable open fault instruments have been of 
at least two types; those which utilize a constant current or a constant 
voltage fed through a resistor to the capacitance of an unknown length of 
cable (or wire pair) where the time to charge that capacitance to a 
predetermined threshold level is measured and used as an indication of the 
value of the capacitance, and systems which operate as a capacitance 
bridge with the unknown capacitance inserted in the test circuit as one 
leg of that bridge. 
Analog meters which utilize the capacitance of the cable or wire pair under 
test to transfer a precision generated frequency therethrough, as in a 
coupling capacitor between amplifiers, may have been used in a laboratory 
environment to make comparable measurements. 
Neither system handles "dirty" open circuits well. A "dirty" open circuit, 
as is well known in the industry, is one which is somewhat less than an 
infinite impedance open circuit. Many of the problems encountered in the 
field are of that nature; there is a finite impedance in the so-called 
"open circuit" which acts to the detriment of transfer of useful 
communications signals down the line. 
The prior art instruments which have been available to the industry for 
portable field test work have tended to be expensive, of large volume and 
relatively heavy, some being on the order of fifteen pounds in weight. 
SUMMARY OF THE INVENTION 
These and other problems with prior art open fault meters are overcome by 
means of the improved circuits of the instant invention which allow for a 
low cost, reduced size and weight, battery operated open fault meter which 
may be easily used and calibrated in the field by one of relatively low 
skill in the art of finding open faults in communications circuits. 
The circuit of the instant invention utilizes a single semi-precision 
capacitor for calibration of the meter. The test signal is a symmetrical 
square wave which is selectable for fundamental frequency. That selection 
determines the range of the meter. Determination of the distance from the 
meter to the open fault is made by comparing the calibrating capacitance 
value to the capacitance value of the open pair or shielded cable by means 
of the calibrated meter movement reading. 
The meter movement is used to integrate a portion of the discharge current 
from the calibration capacitor over a one-half cycle period of the input 
test signal. 
It is, therefore, an object of the invention to provide a small, portable, 
light weight field test instrument for determining the actual location of 
an open circuit fault in a communications cable or wire pair. 
It is another object of the invention to provide a capacitance measuring 
system for portable field use which may be used to determine the location 
of an open fault in a communications cable or wire pair, such system 
providing for accurate, repeatable measurements without the use of any 
highly precision components. 
It is still another object of the invention to measure the capacitance of a 
communications cable or wire pair having an open fault at an unknown point 
in the cable or wire by charging the capacitance of the cable or wire 
through a first charging network having a relatively fast time constant 
and then discharging that cable or wire capacitance through a second 
discharge network which has a longer time constant and by comparing the 
integrated discharge current to that of a known capacity. 
It is yet another object of the invention to compare the integrated 
discharge current of a known length of a particular wire or cable type to 
that of an unknown length of the same type of wire pair or cable and to 
establish the length of the unknown wire pair or cable by means of the 
comparison. 
It is still another object of the invention to provide a means of "tuning 
out" the detrimental affect of a "dirty" open circuit in a wire pair or 
cable line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
Referring first to FIG. 1, it may be seen that the preferred embodiment of 
the circuit of the invention comprises signal generator 10, divider 
network 12, driver circuit 14, and calibration network 16. Power source 18 
provides power for the rest of the circuit of the invention. 
Signal generator 10 further comprises ceramic resonator Y1 which may be a 
CSB480E device as manufactured and sold by Murata Erie, 1148 Franklin 
Road, S.E., Marietta, Ga. 30067. R1, a one megohm resistor is connected 
across Y1. C1 and C2, 100 pf capacitors are connected between ground and 
each end of Y1, respectively. U1A, one section of a CD4069UB hex inverter 
manufactured by RCA, is connected at its input terminal (pin 1) to the 
ungrounded end of capacitor C1 and to one terminal of ceramic resonator, 
Y1, and at its output terminal (pin 2) to the ungrounded end of capacitor 
C2 and to the other end of ceramic resonator Y1. 
Sections B and C of inverters U1 are connected in series with the output of 
U1A and perform a buffering and amplifying function for the output of 
signal generator 10. The signal generator configuration shown in FIG. 1, 
reference numeral 10, provides an output signal with a fundamental 
frequency of approximately 480 kHz to feed divider network 12. The output 
signal from signal generator 10 is a symmetrical square wave having a 
positive phase 40 and a negative phase 42 as shown in FIG. 4. 
Output terminal (pin 6) of integrated circuit U1C is connected to the input 
terminal (pin 14) of frequency divider U2. Dividers U2, U3, U4 and U5 may 
all be Part Number CD4018B integrated circuit frequency dividers, as 
manufactured by RCA. U2 is a frequency divider connected so as to divide 
by 6 by means of an external connection between output terminal (pin 6) 
and input data terminal (pin 1). Dividers U3, U4 and U5 are each connected 
as divide by 10 circuits by means of the external connection between each 
output terminal (pins 13) and each data input terminal (pins 1), 
respectively. As may be seen in FIG. 1, the output of U2 is connected to 
the input of U3, the output of U3 is connected to the input of U4 and the 
output of U4 is connected to the input of U5. 
The output from pin 6 of divider U2 is connected to pin 7 of section B of 
switch S1. The output of divider U3 at pin 13 is connected to pin 6 of 
switch section S1B. The output at pin 13 of U4 is connected to pin 5 of 
switch section S1B. And, similarly, the output of divider U5, pin 13, is 
connected to pin 4 of switch section S1B, one section of multi-section 
switch, S1. Wiper 20 of switch section S1B is the output of divider 
network 12 and is connected to the input of driver circuit 14. 
Driver 14 comprises six power amplifiers U6A-U6F connected in parallel. U6 
is an RCA integrated circuit Part Number CD4069UB comprising six inverter 
amplifiers, all of which are utilized in the preferred circuit embodiment. 
The input terminals of inverters U6A-U6F (pins 1, 3, 5, 9, 11 and 13, 
respectively) and the output terminals (pins 2, 4, 6, 8, 10 and 12, 
respectively) are connected in common. The common output terminals of all 
sections of U6 are connected to the bases of NPN transistor Q1 and PNP 
transistor Q2. 
The collector of transistor Q1 is connected to regulated voltage source V+. 
The emitter of transistor Q1 is connected to one end of 10 ohm resistor 
R3. The other end of resistor R3 is connected to one end of 100 ohm 
resistor R4 and to wiper 22 of switch section S1E. The other end of 
resistor R4 is connected to the emitter of transistor Q2. The collector of 
transistor Q2 is connected to ground. U6 (all sections) and transistors Q1 
and Q2, together with resistors R3 and R4 comprise driver network 14. Vss 
(pin 7) of U6 (not shown) is connected to the regulated power supply 
voltage, V+, within power source 18. (The connection is not shown in order 
to preserve clarity in the drawing.) 
Calibration circuit 16 includes capacitor C5, a 4.0 microfarad metallized 
polycarbonate capacitor with a 1% tolerance rating. The negative end of 
capacitor C5 is connected to pins 3 and 4 of switch section S1C. The 
positive end of capacitor C5 is connected to pins 3, 4, 5, 6 and 7 of 
switch section S1D, to one end of capacitor C6, a 39 picofarad capacitor, 
to one end of 1000 ohm resistor R9, to the junction of resistors R3 and R4 
and to wiper 22 of switch section S1E. The other end of capacitor C6 is 
connected to ground. The other end of resistor R9 is connected to ground. 
The positive terminal of meter, M, is connected to wiper 24 of switch 
section S1D. The negative terminal of meter M is connected to pin 2 of 
switch section S1C and to one end of R8, a ten turn precision 
potentiometer having a maximum resistance equal to 1000 ohms. Wiper 26 of 
potentiometer R8 is connected to the junction of R4 and the emitter of Q2, 
part of driver 14. Wiper 28 of switch section S1C is connected to ground. 
Pin 2 of switch section S1D is connected to V++, the unregulated power 
source, through 62K ohm resistor R5. 
Power source 18 comprises battery 30, having a grounded negative terminal 
and a positive terminal which is connected to wiper 32 of switch section 
S1A. Battery 30 may comprise eight conventional, size AA dry battery 
cells. Pins 2-7 of switch section S1A are connected in common to the 
positive terminal of capacitor C3, to unregulated power output terminal 
V++ and to the input terminal (pin 3) of voltage regulator U7. Voltage 
regulator U7 may be Part Number LM317L as manufactured by National 
Semiconductor Corporation. The negative terminal of capacitor C3 is 
connected to ground. 
The output terminal (pin 2) of regulator U7 is connected to one end of 1000 
ohm resistor R6, to the positive terminal of 10.0 microfarad capacitor C4, 
and to the regulated output terminal of power source 18, V+. The other end 
of resistor R6 is connected to one end of 3900 ohm resistor R7 and to the 
voltage control terminal (pin 1) of regulator U7. The other end of 
resistor R7 is connected to ground. This completes the description of the 
circuit of the invention. 
OPERATIONAL THEORY 
Signal generator 10 produces a square wave signal at pin 14 of U2. This 
signal is divided down in fundamental frequency from 480 kHz to 80 kHz, 8 
kHz, 800 kHz and 80 Hertz by dividers U2, U3, U4 and U5, respectively. 
Wiper 20 of switch section S1B is used to select the desired operating 
frequency. Of course, it will be recognized that pin 1 position of switch 
S1 is the OFF position, there being no power connected to the circuit of 
the invention in that position. Meter M has the usual screwdriver 
adjustment provision for meter zero control with no power applied. This 
adjustment is accomplished with switch S1 in the OFF position. 
The pin 2 switch S1 position is the battery test position. Battery 30 is 
connected through switch section S1A to voltage regulator U7, which 
supplies all circuits with regulated power. However, note that in this 
battery test mode, switch section S1B is connected to an open pin 2 and 
there is no drive provided to or from drive circuit 14. The battery is 
thus tested with all circuits drawing quiescent current. Meter M is 
connected through resistor R5 to the unregulated V++ terminal and to 
ground through switch section S1C, pin 2. R5 is selected to provide a 
somewhat higher than midrange meter reading for battery test. The meter 
may be appropriately marked for such a test. 
When switch S1 is moved to the pin 3 position, the circuit of the invention 
may be calibrated. Divider network 12 provides a 80 Hertz output to drive 
circuits 14 in this switch mode. Capacitor C5 is charged during the 
positive half cycle 40 (see FIG. 4) square wave input signal excursion. 
(FIG. 2 shows the equivalent circuit of the charge current path during 
this half cycle.) NPN transistor Q1 is turned on and PNP transistor Q2 is 
turned off by the positive excursion of the input signal on the bases of 
the transistors, Q1 and Q2. Capacitor C5 is charged with current conducted 
through transistor Q1 and 10 ohm resistor R3, through capacitor C5 to 
ground. The charge path has a relatively fast time constant because of the 
low impedance of resistor R3 and heavily conducting transistor Q1. Meter M 
is essentially out of the circuit because transistor Q2 is turned off 
(only a very small leakage current flows through transistor Q2 and 
resistor R4). 
During the negative going half cycle 42 of the input signal, capacitor C5 
partially discharges. (An equivalent circuit of the discharge current path 
is shown in FIG. 3.) A portion of the discharge path is through meter M 
which is somewhat damped in its action by its own movement inertia. Meter 
M, therefore reads an average current value over the whole input cycle of 
the signal from driver 14. R8 is used by the operator to set meter M to 
read a value commensurate with the type of cable or other communications 
line to be tested with the instrument of the invention. Table I, below, 
indicates settings for the 640 ohm, 200 microampere meter M which has been 
employed in the preferred embodiment of the invention. 
TABLE I 
__________________________________________________________________________ 
MUTUAL CAITANCE 
CABLE CAITANCE 
CONDUCTOR TO MEASUREMENTS 
OR TYPE SHEATH MEASUREMENTS 
(multiply meter readings by 
__________________________________________________________________________ 
2) 
0.1 Microfarad/mile 
141 106 
0.092 153 115 
0.083 170 127 
Aerial drop wire 
N/A 92 
IW (inside wire) 
N/A 129 
__________________________________________________________________________ 
It should be noted that the figures of this table are derived from actual 
measurements of known lengths of each of the cable or wire pair types. It 
is useful to understand that a known length of cable of a given type may 
be measured in the LINE TEST mode of operation of the instrument of the 
invention and, then, the CALIBRATION mode may be entered to set the meter 
M to the same reading. (See below.) In that way, calibration of meter M 
may be reset precisely, even in the field, and even if capacitor C5 is not 
accurately known in terms of its capacitance. 
It is believed that it would be possible to measure the charging current to 
capacitor C5 rather than the discharge current as is done in the preferred 
embodiment of the invention. That this was not done is a matter of design 
choice. 
When wipers 32, 20, 28, 24 and 22 (hereinafter, "wipers") of selector 
switch S1 sections A-E, respectively, are set to pin position 4, a LINE 
TEST may be accomplished. Test leads are connected between test points TP1 
and TP2 and to cable or wires 44 to be tested or used for calibration. An 
output signal having a fundamental frequency of 80 Hertz, from driver 
circuit 14, U5, pin 13 is applied to the line under test between switch 
section S1E, pin 4 and ground. Any meter reading variation from the 
setting made in the CALIBRATION mode represents a fault condition which 
must be compensated for prior to making a distance test (normally the next 
step). The fault referred to here is the type which is caused by a "dirty" 
open circuit in cable or wire pair 44. It is compensated for by resetting 
calibration potentiometer R8 to obtain the same meter reading obtained 
during calibration of the instrument of the invention. Effectively, the 
fault is "tuned out" by this procedure. For cable 44 lengths of over 2000 
feet, it is not necessary or even desirable to "tune out" sall variations 
from the calibrated meter reading values. Once this LINE TEST procedure is 
completed, the operator may proceed to make a distance test measurement. 
Alternatively, calibration may be accomplished by using a known length of 
cable or wire 44 connected to terminals TP1 and TP2 of the circuit of the 
invention and open circuited at the far end. This is accomplished in the 
LINE TEST mode (pin 3 position of the wipers of selection switch S1), as 
stated, above. Once calibrated for a given wire or cable type and length, 
the circuit is very accurate at all distances within any of its ranges. Of 
course, it will be apparent that other ranges may be accomplished as a 
matter of design choice by changing the range of frequencies provided by 
divider circuits 12. 
To make a distance test for an open fault, the operator selects one of 
three ranges: 0-200 feet (pin 7 position of the wipers of S1), 0-2000 feet 
(pin 6 position of the wipers of S1) or 0-20,000 feet (pin 5 of the wipers 
of selector switch S1). For the 0-200 foot range setting, readings are 
taken directly from meter M; 1.0 microampere being equal to 1.0 foot in 
distance to the open fault. For the 0-2000 foot range, the meter readings 
must be multiplied by 10; 1.0 microampere being equal to 10 feet in 
distance to the open fault. For the 0-20,000 foot range, the meter 
readings must be multiplied by 100; 1.0 microampere being equal to 100 
feet in distance to the open fault. Note that the only difference in the 
internal operation of the circuit of the invention caused by the three 
range settings is the fact that the fundamental frequency utilized is 
changed by a factor of 10 for each incremental change in range. This is 
accomplished by means of operating section B of selector switch S1 
switching driver 14 input to the output of one of U2, U3 or U4 dividers, 
each part of divider network 12. 
It will be well understood by one of ordinary skill in the electronic art 
that the impedance of a capacitor is reduced as the frequency applied is 
increased according to the equation: 
EQU Xc=1/2.pi.FC 
Where 
Xc is the capacitive reactance (impedance) in ohms, 
F is the frequency in Hertz, and 
C is the capacitance in Farads. 
By increasing the applied frequency to capacitance 44 under test (the 
capacitance of the cable or communications wire pair 44), the impedance is 
reduced so that the current flow in meter M is caused to increase. Since 
for longer ranges to an open fault the capacity of the tested line 
increases proportionally with the increasing distance (with an attendant 
decrease in impedance), reducing signal frequency applied compensates for 
the higher capacitance to maintain the effective impedance at a relatively 
constant level. Of course, the reverse is true for shorter distances. The 
combination of higher frequency and lower capacitance or lower frequency 
and higher capacitance work to keep meter M readings within scale. 
Since the frequency used for calibration of meter M and the frequencies 
used for the range measurements are all related by fixed and accurately 
known ratios (determined by the dividing ratios of frequency dividers 
U2-U5 in network 12) calibration may be accurately maintained at any range 
provided for by the dividers of the circuit. The ratios just referred to, 
above, may be submultiples or integral multiples of the signal generator 
10 frequency; the important factor being the fixed and known relationship 
between the various signals in terms of relative frequency. 
While the invention has been particularly shown and described herein with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various other modifications and changes may 
be made to the present invention from the principles of the invention 
described above without departing from the spirit and scope thereof as 
encompassed in the accompanying claims. Therefore, it is intended in the 
appended claims to cover all such equivalent variations as may come within 
the scope of the invention as described.