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TDR900
Operating Manual
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User Manual
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Specifications
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Megger TDR900, CFL535G User Manual

WWW.MEGGER.COM
$9.95
Time Domain Reflectometers ­Applications
The word “Megger” is a registered trademark
TABLE OF CONTENTS
Testing in the Work Center ................................................................................6
Moisture in Twisted Pair .....................................................................................7
Locating Bridged Taps ........................................................................................8
Locating Splits and Re-splits ..............................................................................10
Upgrading Cable Plants for xDSL .....................................................................11
Locating and Removing Load Coils ...................................................................12
Finding Repeat Troubles ...................................................................................12
Cellular ............................................................................................................13
Track Down Illegal Electrical Taps .....................................................................14
Low Voltage Networks .....................................................................................15
Mining .............................................................................................................17
Railroad ............................................................................................................18
Outside Broadcasting .......................................................................................18
Urban Heating and Cooling ..............................................................................18
TIME DOMAIN REFLECTOMETER - TECHNOLOGY REVIEW
Two side-by-side conductors that are separated by an insulator (as in a cable) have a characteristic impedance between them. If the distance between the conductors does not change, this impedance does not change. If the distance between them increases, the impedance goes up. If the distance between them decreases, the impedance goes down. Time Domain Reflectometers (TDRs) use simple transmission line theory and pulse reflection principles to detect these impedance changes along a cable. The TDR transmits high frequency electrical pulses that travel through the cable until a change in characteristic impedance is encountered. Depending on the nature of the impedance change either all or part of the transmitted pulse will reflect back to the TDR.
Figure 1- Time Domain Reflectometer Basic Operation
A change in a cable’s characteristic impedance will cause one of two types of reflections: positive or negative.
n Positive reflections are caused by increases in impedance as the conductors go
farther apart.
n Negative reflections are caused by decreases in impedance as the conductors get
closer together.
As shown in Figure 2, the reflections are translated by the TDR to traces which can be interpreted to indicate that certain “events” such as opens, shorts, splices, etc. are in the cable. But all traces follow these two basic rules stated above and are displayed on a TDR as shown in Figures 2 and 3.
www.megger.com Time Domain Reflectometers 1
<0 ft
893 ft>Gain = 0
VOP=0.670
LESS
CABLE
Figure 2 – Trace of an Open – Positive Reflection- Conductors Moving Apart
<0 ft
VOP=0.670
LESS
CABLE
Figure 3 – Trace of a Short – Negative Reflection – Conductors Moving Closer Together
MORE
CABLE
MORE
CABLE
ZOOM SETUP
ZOOM SETUP
519.1 ft
Cursor
893 ft>Gain = 0
519.1 ft
Cursor
Knowing that these events are in the cable is beneficial but to be really helpful, we need to know WHERE they are in the cable. Above, we discussed that a TDR sends pulses along the cable that are reflected back when they hit a change in impedance. The TDR times how long it takes for the reflections to get back to the unit. It knows how long the pulse has been gone and how long it has been travelling. TDRs are like an arithmetic word problem that asks “if you leave Chicago and travel for two and a half hours at 50 miles an hour, how far have you gone?” Because the TDR knows how long the pulse has been gone, if we could tell it how fast these pulses and their reflections travel along the cable, it would be able to calculate the distance from the TDR to the impedance change. We are able to do this. But to complicate it, the TDR’s pulse travels at different speeds in different types of cables.
2 Time Domain Reflectometers
Fortunately we can tell the TDR how fast the pulses and their reflections travel in various cables. This speed is usually stated as a ratio of the speed of the pulse in the cable divided by the speed of light in a vacuum. This ratio is called the Velocity of Propagation (VoP). If the VoP is 0.80, the speed of the pulse is 80% of the speed of light in a vacuum or 0.8 x 186,000 miles per second. The manufacturer of the cable states the VoP. Some VoPs by cable type are:
n Communications coax – (with some exceptions) between 0.80 and 0.90
n Twisted pair
n 22 gauge filled – 0.66 n 22 gauge air-core – 0.67 n 24 gauge filled – 0.62 n 24 gauge air-core – 0.67
n Coax transmission line – 0.99
If we can see the length markings or measure it to get the length of a section of cable, we can work backwards with a TDR and calculate the VoP. (This gets a bit tricky with twisted pair because the conductors are longer than the cable.)
The TDR now has enough information to calculate where the “event” is. Like our trip from Chicago, it knows how long the pulse and its reflection travelled and it knows how fast that pulse was going. The TDR merely has to do the arithmetic, a LOT of arithmetic.
Does it matter if the event that we are trying to find is very close to the TDR, or far away? It sure does. We can “look” along the entire distance of a very long cable by varying the power, the “width”, of the pulses that the TDR sends out. This is the amount of time that the transmitter is turned on.
We “set” the pulse width. TDRs have varying pulse width settings and modern units, while allowing the operator to vary the pulse width, will automatically set the proper pulse width to best display the first event. The larger the pulse width, the more energy and, therefore, the further the signal will travel on a given cable. A TDR’s distance range is determined by how far the transmitted pulse will travel while still having a detectable reflection returned.
www.megger.com Time Domain Reflectometers 3
Why don’t we just send the most powerful pulse all the time? The width of the transmitted pulse will affect the TDR’s ability to identify reflections. It’s a bit like walking in from the dark and turning on a bright light. The power of the light overpowers our receivers (eyes) and blinds us for a minute. Our eyes adjust by reducing the amount of light that enters our receivers. The TDR’s receivers don’t do this so lower-powered pulses for closer-in events have to be sent. The width of the pulse is sometimes referred to as the dead zone; the distance that the TDR is blind. Figure 4 is a table that relates the typical dead zone at launch to the pulse width in communications coax.
Pulse Width Dead Zone
<1 ns
One billionth
2 ns 3 feet
10 ns 14 feet
100 ns 55 feet
1 µs
One millionth
2 µs 810 feet
4 µs 1,600 feet
Figure 4 – Pulse Width Dead Zones
1 foot
430 feet
We not only have to worry about blind spots at launch (right next to the TDR) but the reflections of events also have blind spots, the size of which are related to the pulse width. The second of two closely-spaced reflections may become masked by the dead zone of the first one.
We’ll consider later what the shape of a trace tells us about an event but for now accept that the signatures displayed in Figure 5 show a twisted pair cable of 3,000 foot length with a splice at 2,000 feet. Signature 1 used a pulse width of 160 ns. Signature 2 used a pulse width of 500 ns and signature 3 used a pulse width of 2 µs. Notice the lengths of the launch dead zones. Also consider the dead zones following the splice. If the splice were a short bridged tap, we wouldn’t see the end of the lateral if we used the 500 ns pulse width.
4 Time Domain Reflectometers
160 ns
500 ns
2 µs
Figure 5 - Pulse Width and Dead Zone
Gain is another variable that we can set that needs consideration. Gain has nothing to do with the physics of what the TDR is doing. Rather, it is a way for us to adjust the trace that the TDR is showing us. It is the degree to which the trace deviates from the horizontal. Remember that a TDR shows us events in a cable that cause the impedance to change at that spot. Consider also that elements that are placed in the cable are meant to exactly match the impedance of that cable so as to minimize the very reflection that the TDR depends on. So, elements such as well-made splices and terminations are, by design, hard for a TDR to see. Modern TDRs adjust the display such that they try to show the trace of the most prominent element. To do this they automatically adjust the gain of the display to nicely show, for example, a major event such as an open. This is all well and good if what we want to see is the open. But, what if we really wanted to see a well-made splice that was somewhere before the open? The impedance mismatch at the open is much more than that at
www.megger.com Time Domain Reflectometers 5
the splice. Therefore, the TDR trace for the splice would only have a small hard­to-see bump, if anything at all. The trace of the open would be a big, easy-to-see curve. In order to see the splice, we would have to increase the gain to the point where the sides of the trace of the open looked like two nearly-vertical lines that go off the top of the screen. The trace of the splice, however, would likely show up as a nice “S” on its side. We didn’t change what the TDR “knew” about the cable. We merely manipulated the display in order to see what the TDR had already found.
TIME DOMAIN REFLECTOMETER - APPLICATIONS
Telephone/CATV
Testing in the Work Center
As with any test equipment, it is best to learn about the equipment in a controlled environment by simulating some known faults before going into the field and having to interpret the information under pressure or having to get the cable back on line. MAKE SURE THE PROBLEMS ARE REAL WORLD! Physical problems such as opens, shorts, load coils and bridged taps are easy to duplicate. Water in the cable is harder to duplicate. 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, simply 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 an ohmmeter. A reading of less than 100 K Ohms indicates a bad pair. A TDR connected to this pair will usually find the problem. However, if you try to simulate this problem in the shop by simply connecting a 100 K Ohm resistor across a pair, 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 ohmmeter 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
6 Time Domain Reflectometers
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