TDR Tutorial and Riser Bond TDR Product Review
APPLICATION #1 Moisture in twisted pair cable.
The greatest percentage of twisted pair problems fall within the moisture-in-the- cable category. How to locate the problem, why one pair may be affected but not another, and how much of the cable is affected are all problems the craft person has to address.
A TDR will find water in the cable. It shows up as a lowering of the cable impedance. Most times, though, it is not possible to accurately tell how wide the water problem is. In the case of flooded cable, moisture cannot migrate inside the cable so it is always a point problem. In un-flooded cable, moisture can migrate and might be located anywhere along the cable. By testing the cable from both ends and recording the distance to faults in all pairs, it is possible to determine approximately how wide the problem is. When testing through water, measurements up to the water in the cable are very accurate. After the water, distance readings may be erroneous because water can change the VOP of cables.
Even though the moisture may be 20 or 30 feet wide, each pair usually becomes impregnated at different points. The range of these points will sometimes indicate how wide the problem is.
The water can seep into the conductors through pin holes in the plastic insulator around the conductors. Water in a multi-paired, non-flooded cable may be several feet wide. When testing each pair, the footage to the problem may read different for each pair. This is because the water has penetrated through the insulating plastic at different points and shorted out the conductors at different footage's.
The location and how wide the water damage ranges is now known. But it is still necessary to locate where the water actually entered the cable. The break in the jacket will not necessarily be within the span of where the water is and will not necessarily show up in the testing. If the break in the jacket is not fixed, the problem will show up again in the future.
If the hole in the jacket happens to be at a high point in the cable, the water will enter through the hole then migrate to a lower point. If the water entry point is not found, it may be necessary to visually inspect the cable. It is also necessary to check the integrity of the sheath. To test the sheath, use it as one conductor and a pair close to the sheath as the second conductor.
As with any test equipment, it is best to learn about the equipment in a controlled environment by creating faults on the cable while in the shop. A word of caution: MAKE SURE THE PROBLEMS ARE REAL WORLD! The physical problems such as opens, shorts, load coils, and bridge taps are easy to duplicate. Water in the cable is harder. But what does a water problem really look like? In the real world, when water causes a problem in the cable, it takes place over a long period of time. Also, the water by itself is not the problem. It is the contamination (for example, salt from the ground and air) that the water is carrying that causes the problem. To simulate the water in the cable problem, make a hole in the cable and immerse the cable in water. To simulate the salts in the ground or air and to speed up the cable deterioration process, add some common table salt to the water. Now, using the TDR, it is possible to see what water in the cable really looks like.
Another example of modeling a field problem in the shop is to test across the pair with a VOM. A reading of less than 100 K Ohms indicates a bad pair. A TDR connected to this pair will usually find the problem. Try to simulate this problem in the shop by simply connecting a 100 K Ohm resistor across a pair. But, now the TDR will not find the 100 K Ohm resistor. Why not?
The field pair with the low resistance will also have a change in impedance caused by moisture in the cable. The VOM is looking only at the resistance; the TDR is looking at the total cable impedance. The total cable impedance includes the resistance, the capacitance, and the inductance. The whole cable and the whole fault are included in the impedance. To simulate the fault with just a resistor is not simulating the whole fault. It is an unfair and unrealistic simulation.
The ohmmeter is not a good way to test cable. It will not tell you the quality of the cable or how far away the problem is. The TDR will. Therefore, when simulating a field problem in the shop make sure the whole problem is simulated.
It is best to do some testing in the shop with some known faults before going into the field and having to interpret the information under the pressure of having to get the cable back on line. As with any equipment, familiarity and experience will build confidence in yourself and the test equipment.
APPLICATION #2 Nullifying Load Coils
Generally speaking, a TDR cannot look through a load coil; the TDR is, by nature, a high frequency device, and the load coil is a low frequency device. A TDR may see a very bad problem while looking through a single load coil if the instrument is close enough to the load coil.
There is a way to nullify the effects of the load coil, but it requires two similarly loaded pairs. Short tip and ring of one pair together and connect to one side of the Model 1205T's output. Short tip and ring of the second pair together and connect to the other side of the same output of the 1205T. The load coils within the cable pairs are nullified by shorting these respective conductors together.
APPLICATION #3 The case of the mysterious splice.
A rural telephone subscriber complained of a noisy telephone line. The noise was traced to the drop . The subscriber has a 2,800 foot two part drop to his home along a country road ditch, through the yard, and into the house. The unused pair was found to be quieter so the customer was switched to the quieter pair. This seemed to solve the problem until a few months later when the customer started to complain again.
Retesting the pairs found the original pair was quieter. Noisy pairs going quiet and quiet pairs becoming noisy led the technicians to suspect water in the cable. Plant records showed the cable was not spliced so it was unknown how and where was the water was getting into the cable.
The cable was tested with a TDR and an unrecorded splice was found. Close examination of the splice showed it to be to ally saturated with water. The cable was re-spliced and the lines were quiet.
APPLICATION #4 Locating Splits using Crosstalk
A split pair in a telephone cable is when one conductor of one pair is switched with one conductor of another pair. Splits can be hard to find because they cause such a slight change in pair impedance. The Model 1205T can greatly help the craftperson find splits by using the crosstalk mode of the instrument. Simply connect tip and ring of one of the split pairs to Line 1 and tip and ring of the other split pair to Line 2 and put the 1205T in the crosstalk mode. The signal will be transmitted on Line 1, down one pair, and, at the point where the pairs are split, the signal will transfer to the other pair and return to the 1205T to be displayed as Line 2... crosstalk. The point of split will easily be seen on the waveform display.
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