TDR Tutorial and Riser Bond TDR Product Review
LOCATING MULTIPLE FAULTS
Sometimes a cable contains more than one fault. Multiple faults in a cable can be caused by rodent damage, improper or faulty installation, construction, ground shift, or even structural flaws from the manufacturing process.
If a fault is a complete open or a dead short, the TDR will read only to that point and not beyond. If the fault is not an open or short, the TDR may indicate the fault and that of other faults further down the cable.
In the case of a waveform TDR, the waveform signature of the cable will show all of the discontinuities, both large and small, along the length of the cable.
In the case of a digital numeric TDR, only the distance to the first major fault will be indicated, and not the smaller faults beyond. You may need to test from the opposite end for signs of other possible faults.
When testing cables it is best if the cable is not terminated. A termination can absorb the pulse and no signal will return to the instrument. The TDR's transmitted pulse must be reflected back to the instrument by a fault or the end of the cable. It if is not, the distance calculation cannot be made. In order to minimize confusion and eliminate guesswork, it is best if all other equipment or hardware is disconnected from the cable under test.
Sometimes it is not always practical to disconnect the far end of the cable. However, it is still possible to test. If the cable is damaged, the signal will reflect at the damaged point prior to being absorbed.
As signal energy travels down a cable, some of the signal energy is lost due to the resistance of the cable. This is known as cable loss. Cable loss is measured in decibels (dB). If the transmitted signal energy reaches an impedance discontinuity, some or all of the energy is reflected back up the cable. The ratio of energy transmitted to reflected is known as return loss. Return loss is also measured in decibels (dBRL).
Return loss is a way of measuring impedance changes in a cable. Understanding dBRL is sometimes confusing. This is due to the fact that a large dBRL number means the reflection or fault is small and vise versa.
A large return loss means that most of the transmitted pulse was lost instead of being returned as a reflection. The signal simply continued down the cable or was absorbed by a termination or load on the cable. A small return loss means that most of the transmitted pulse was reflected or returned due to an impedance change caused by a fault or the end of the cable. A complete open or a dead short would reflect all of the signal energy. Therefore, the return loss would be zero.
Mathematically, the formula for finding return loss is:
Where dBRL is the decibels of return loss, V0 is the voltage of the output signal and VR is the voltage of the reflected signal.
Although return loss can be mathematically calculated on most TDRs, Riser-Bond Instruments' waveform TDRs provide automatic dBRL calculations.
TEST FROM BOTH ENDS
It is always best to test a cable from both ends. It can help reduce error in VOP and uncover hidden faults.
A reduction in the TDR pulse strength, caused by attenuation, can make a small fault difficult to see if the fault is a long distance away. By going to the end of the cable and testing in the opposite direction, you place yourself and the TDR much closer to the fault, making it easier for the TDR to locate.
As mentioned previously, a digital TDR cannot "see" a small fault beyond a larger fault. Again, by testing from the opposite end, a second fault may be located which might otherwise remain hidden.
Testing a cable from both ends also assures that no faults are being hidden by a blind spot or dead zone caused by the pulse width.
It is also a good idea to re-test a cable after making a repair. You may be able to locate another problem beyond the first. resting from the opposite direction assures that the entire cable is good.
QUALITY CONNECTION....A MUST!
The importance of a good quality connection cannot be overstated. Riser-Bond Instruments' TDRs are supplied with alligator clip connectors for convenience, but it is best if the cable is adapted to connect directly to the instrument front panel . If testing 50 ohm cable, try to maintain a good 50 ohm environment; if testing 75 ohm cable, try to maintain a 75 ohm environment, etc.
A poor connection can result in a distorted waveform which can mask a fault.
USE COMMON SENSE
Although a thorough understanding of the TDR and its operation is vital to successful troubleshooting, there is never a substitute for good common sense.
If your TDR indicates a distance of 500 feet to a fault, but you notice that a new fence post happens to be at 490 feet, there is a pretty good chance that the fault was caused by the fence post.
If your TDR indicates a minor fault far away, make some adjustments. Adjusting the vertical gain control to enhance the fault, testing using a larger pulse width, testing from the other end of the cable, or moving to a breakout point closer to the cable will all help.
When using a digital numeric TDR, it is even more important to use common sense. Although it is not necessary to interpret all of the waveform information, it is more difficult to know what the digital TDR is "seeing". Many digital TDRs can be interfaced with an oscilloscope which makes the instrument more versatile.
Familiarity develops versatility. The more you use a TDR, the more confident and comfortable you will become, and the more applications you will find for it.
Whether using a digital or waveform type, you soon discover that the TDR is one of the best tools available for locating cable faults fast and accurately.
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