Abstract:
A constant current termination is provided for cable locating tones on communication and control cables that may be buried or placed in underground duct structures. The constant current termination limits the current on each branch of the cable to that required for cable location, thus ensuring that branches furthest from the tone source have adequate current for location purposes. The termination is the same for each branch, regardless of its position along the cable system. This eliminates the need to calibrate and recalibrate termination distances for a cable on installation and when branches are added. It also allows the location of damaged cables where the tone signal strength on a damaged branch is less than that for which the termination was designed.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of cable location and more particularly to the location of hidden or underground cables using a tone signal applied to the cable. 
   BACKGROUND 
   Telephone, cable television and other communication and control cables are often buried or placed in underground duct structures. For this type of cable placement, the most significant cause of cable outages is from dig ups by contractors. In an effort to minimize inadvertent dig ups, “call before you dig” programs are heavily promoted. The operating company must then be able to quickly and accurately locate and mark the buried cable. 
   Methods have been developed and are in commercial use which place a locating tone or signal in the form of an oscillating current at a predetermined frequency on the cable armour or shield. A special receiver with magnetic field detecting coils is used to sense the tone current travelling along the cable. The strength of the received signal is directly proportional to the magnitude of the tone current in the cable sheath directly below the receiver. 
   The transmission circuit for the tone current is formed by the metal armour or shield and insulated by the plastic cable jacket from earth which forms the return conductor. The circuit is basically a coaxial transmission path with the insulated cable armour forming the inner conductor and the surrounding earth forming the outer conductor. 
   The tone current must be present on all segments of a cable at a level greater than the minimum current dictated by the receiver sensitivity. This requires a termination at the end of the cable to draw at least the minimum amount of current. A distribution cable typically has a number of branch cables which must also draw enough tone current for cable locating. Since the current is heavily attenuated by the cable, the terminations near the source will draw a much higher current level than the distant terminations. To compensate for this, most installations use terminators with different signal load impedances for near, middle and far terminations. In addition to the inconvenience of using different impedances for different terminations, the known systems require recalculation and replacement of the terminators when an additional branch is connected. 
   Where a cable is damaged, the current level may fall below the minimum, making it difficult or impossible to locate the damaged cable. 
   SUMMARY 
   The present invention simplifies the terminator selection and mitigates problems created by the addition of branches and by cable damage. 
   According to one aspect of the present invention there is provided a tone locating system for a cable installation having a backbone cable, a plurality of branch cables, splices coupling the branch cables to the backbone cable and tone conductors along the backbone and branch cables, the tone conductors being connected at the splices, the locating system comprising:
         a tone source connected to the tone conductor of the backbone cable at an inner end of the backbone cable;   a plurality of terminations connected to the respective tone conductors at ends thereof remote from the tone source and the splices, each termination comprising:
           a load impedance connected to the respective tone conductor and to a tone signal return path; and   
           an active component responsive to variations in a voltage between the respective tone conductor and the return path to vary the magnitude of the load impedance to maintain a substantially constant current through the load impedance.       

   The invention thus simplifies the terminator selection and installation by replacing all of the different fixed load terminators of the prior art with a single device. A termination according to the invention draws only enough tone current to ensure location of the cable. It is not affected by the signal strength. This has the additional benefit of allowing the location of a damaged cable when the tone signal level is below that for which a fixed terminator would have been designed. If branches are added later, the termination loads do not have to be recalculated and replaced as with fixed terminators. 
   In a preferred embodiment of the present invention, the termination circuit has an input terminal for connection to the tone conductor of the cable and an output terminal for connection to a ground return path. A lightning protection device, e.g. a gas tube surge suppresser, a MOV or both, bridges the two terminals. A high pass filter is connected in series with the lightning protection to block low frequencies used by either equipment on the same cable conductor. Also in series with the lightning protection and the high pass filter is a band stop filter for filtering induced mains frequency signals. The signal thus processed is delivered to a rectifier, the output of which is connected to a series circuit including the load resistor and a variable impedance, which is in the preferred embodiment the drain to source path of a field effect transistor (FET). The gate and source of the FET are connected across the load resistor. The FET regulates the gate—source voltage and therefore the current draw of the load resistor. A high frequency bypass filter bridges the source and drain terminals of the FET to prevent ringing since the FET may turn on and off very quickly around the current limit with very large input currents. A zener diode is connected in parallel with the load resistance to prevent damage to the FET from input surges. 
   Other embodiments of the invention are possible using other forms of current limiting circuit, for example a voltage regulator based circuit. 
   According to a further aspect of the invention there is provided a method of providing a controlled signal current on a cable having opposite inner and outer ends and a signal conductor along a cable between the inner and outer ends, said method comprising:
         applying an electrical signal to the signal conductor adjacent the inner end of the cable;   providing a resistive termination at the outer end of the cable, connecting the signal conductor to a signal return path;   monitoring the electrical signal at the termination; and   maintaining a substantially constant electrical signal current at the termination by varying the resistive termination in response to variations in the electrical signal at the termination.       

   According to another aspect of the invention there is provided a method of providing a controlled signal current on each of a backbone cable with inner and outer ends and a signal conductor from the inner end to the outer end and a plurality of branch cables with respective inner and outer ends and with the inner ends spliced to the backbone cable, each of the branch cables having a signal conductor spliced at the inner end of the branch cable to the signal conductor of the backbone cable, the method comprising:
         applying an electrical signal to the signal conductor at the inner end of the backbone cable;   providing resistive terminations at the outer end of the backbone cable and at the outer end of each branch cable, connecting the signal conductor to a signal return path;   monitoring the electrical signal at each termination; and   maintaining a substantially constant electrical signal current at each termination by varying the resistive termination in response to variations in the electrical signal at the termination.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, which illustrate an exemplary embodiment of the present invention: 
       FIG. 1  is a representation of an exemplary cable topology showing termination and ground leakage currents; 
       FIG. 2  is a graph showing voltage and current draw waveforms; 
       FIG. 3  is a circuit schematic of a termination according the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to the accompanying drawings  FIG. 1  illustrate a cable system  10  that includes a backbone cable  12  and branch cables  14 . Each of these cables has a core  16 , a metallic armour  18  surrounding the core and a plastic material outer jacket  20 . The branch cables  14  are connected to the backbone cable at splices  22 . In the illustrated embodiment, the armour  18  of the cables is connected at the splices to serve as electrically connected tone conductors. 
   At an inner end of the backbone cable is a tone source or transmitter  24  that applies a tone signal or oscillating current of a predetermined frequency to the tone conductor. This is transmitted down the conductor to a termination  26  at the outer end of each of the cables. 
   The transmitter  24  generates the tone and transmits it on the cable system including the backbone cable  12  and the branch cables  14 . The terminations, cable faults and cable capacitance to ground attenuate the amplitude of the signal. The amplitude of the signal is important because the current that flows generates a magnetic field which is radiated around the cable. This radiated field is not blocked by the surrounding soil and is readily detectable several meters away. The locating receiver has a coil that is excited by the magnetic field and converts the field back into an electrical signal. 
   Signals other than the locate tone may be induced on the cable, from low frequency power line harmonics to broadcast radio frequency signals. The noise level, sensitivity of the receiver and the maximum buried depth of the cable set the minimum required tone current. The Biot-Savart law establishes the relationship of the magnetic field intensity (H), tone current (I) and cable depth (r). The factor a 0  is a constant. This equation is simplified for DC current, but the relationship is the same. 
       H   =       I     2   ⁢   π   ⁢           ⁢   r       ⁢     a   0           
 
   In normal practice, the minimum locating current specified by the receiver sensitivity for cable depth of one to two meters is 5 ma. This assumes typical ground conditions and noise levels. For extra safety margin, the minimum locating current on each segment should be 10 ma. Thus in  FIG. 1 , currents i 1 , i 2 , i 3 , i 4  and i 5  should be 10 ma. Some current, designated i 6 , i 7  and i 8  in  FIG. 1  will be drawn by the cables&#39; capacitive coupling to ground, especially with the higher frequency tones. Since the branches off the main cable may be very short and not have much capacitance, this current cannot be relied upon for locating all segments. When designing the tone source, this current has to be added to the maximum permissible fault current plus the number of terminations times the minimum locating current. The termination must draw the minimum locating current to ensure that no segment between the source and termination will carry less than the minimum current. In the example illustrated in  FIG. 1 , the tone source must supply current equal to i 1 +i 6 . Current i 1  is in turn the summation of the remaining currents i 2  through i 5  as shown in the drawing. 
   The load of each of the terminations  26  must draw the minimum required location current regardless of the input voltage or tone frequency. The electrical schematic of each termination is illustrated in  FIG. 3 . As shown in that drawing the termination has input terminals  28  and  30 . Terminal  28  connects to the tone conductor of the cable while terminal  30  is connected to a ground return path. A lightning protection surge suppressor  32  is connected to the incoming signal wires to protect the termination from lightning. 
   The output terminal of the surge suppressor leads to a high pass filter  34  in the form of a large capacitor C 1 . Other equipment in the cable system may be connected to the cable sheath which is acting as the tone conductor and may apply a signal current thereto. This equipment generally operates to apply signal currents at very low frequencies, much below the frequency of the locating tones. The high pass filter  32  prevents interference with the other functions of the tone conductor so that, the termination will not discharge the signal currents of the other functions on the cable. 
   In series with the surge suppressor  32  and the high pass filter  34  is a band stop filter  36 . The tone conductor typically has induced AC voltages from power lines at significant amplitudes relative to the amplitude of the tone current. These induced voltages are also discharged by the termination and add to the drawn current. If the induced voltages are large enough, they cause the current to limit at the minimum locating current. If a tone signal arrives at the termination with the current already limited, there will be no current draw at the tone frequency. If current is not drawn at the correct tone frequency, the locating receiver will filter away the signal from the current that is drawn from the induced voltages and will not be able to find the cable. The band stop filter  36  includes an inductor L 1  and a capacitor C 2  connected in parallel. The inductance and capacitance are calculated as follows: 
       freq   =     1     2   ⁢   π   ⁢     LC             
 
At the design frequency, normally 60 hz or 50 hz depending on the local mains frequency, the impedance of the inductor is equal and opposite to that of the capacitor. The currents are 180° out of phase and cancel each other out. For lower frequencies, the inductor shorts out the capacitor and for higher frequencies the capacitor shorts out the inductor.
 
   In series with the surge suppresser  32 , high pass filter  34  and band stop filter  36  is a rectifier  38 . This is a diode bridge composed of four diodes D 1  to provide a full wave rectification of the AC tone signal applied to the terminals  28  and  30 . The output of the rectifier  38  is connected to a series circuit including a load impedance  40  and an active component  42 . The load impedance  40  is a resistor R 1 , while the active component is a field effect transistor Q 1  with the gate and source terminals connected across the resistor R 1  and its drain terminal connected to the rectifier  38 . The full wave rectifier  38  is employed in this embodiment because the constant current regulator is a DC device and the incoming tones are AC. 
   The constant current regulator works by detecting the current through the load resistor R 1  and limiting the current when it reaches a set threshold. It limits the current by increasing the series impedance of the circuit so that the load resistor R 1  gets less current. The impedance in this case is controlled by the biasing the depletion mode FET Q 1  so that its gate voltage decreases relative to the source voltage as the drawn current increases. This will limit the gate voltage to the gate threshold voltage because any more current would gradually turn off the transistor. 
         I   limit     =       V   gs       R   sense           
 
Since the gate voltage is limited and the load resister is fixed, the drawn current I limit  is limited to:
 
   The voltage V gs  is a specification of the depletion mode FET, so R 1  is chosen to set I limit  to the minimum locate current. 
   The resulting current wave form is shown in  FIG. 2 . It will look like the tone signal with the peaks chopped off because the current increases with the input tone voltage until the set current limit. The current stays at the limit until the tone input voltage comes back down. Thus, with a strong tone signal near the beginning of the cable, the current waveform will approach that of a square wave. A square wave of current is acceptable and is detectable by the tone receiver because the tone receiver locating the cable will filter harmonics and only detect the fundamental frequency. 
   The fourier expansion for a square wave is given by: 
         f   ⁡     (   x   )       =         4   π     ⁢       ∑       n   =   1     ,   3   ,     5   ⁢   …         ⁢           ⁢       1   n     ⁢   sin   ⁢       n   ⁢           ⁢   π   ⁢           ⁢   x     L           =         4   π     ⁢   sin   ⁢       π   ⁢           ⁢   x     L       +       4   π     ⁢       ∑       n   =   3     ,   5   ,     7   ⁢   …         ⁢       1   n     ⁢   sin   ⁢       n   ⁢           ⁢   π   ⁢           ⁢   x     L                   
 
This indicates that the peak amplitude of the first harmonic (n=1) will be 4/ π  times greater than the square wave peak. The peak of the sign wave must then be converted to an RMS value as follows: 
         V   rms     =         V   peak       2       =         4   π       2       =   0.9000           
 
With the tone current being 0.900 of the current limit, the calculations for the current limit will have to be 1/0.9 or 11% higher than the minimum desired locate current.
 
   In use of the termination illustrated in  FIG. 3 , the lightning protector  32  protects the circuit from lightning. The capacitor C 1  blocks DC and passes AC signals including the tone signal. The band stop filter  36  blocks any induced mains frequency currents. The rectifier  38  rectifies the incoming signal because the following current regulator is limited to one polarity. The field effect transistor Q 1  regulates the gate to source voltage across the load resistor R 1  to about 1.77 volts. The minimum locate current of 10 ma results in a maximum current limit set to 11.1 ma from the 11% correction calculated above. The value for the load resistor R 1  is calculated as the gate to source voltage divided by the current limit. The high frequency bypass capacitor C 3  prevents ringing as the FET would turn on and off very quickly around the current limit with very large input tones. The zener diode D 2  clamps the gate voltage to a tolerable limit, say 5 volts, to prevent damage to FET Q 1 . 
   With this circuit tolerances may be quite large for the inductor L 1  and some tuning of the capacitor C 2  may be required to centre the band filter at 60 Hz or 50 Hz as the case may be. 
   While one embodiment of the present invention has been described in the foregoing, it is to be understood that other embodiments are possible within the scope of the invention. For example, various different forms of constant current regulator may be employed. It is for example, possible to produce a regulator for both polarities, thus eliminating the rectifier. Voltage regulator based limiters, can for example, be used. The invention is therefore to be considered limited solely by the scope of the appended claims.