TDR and Cable Length Meters

TDR and Cable Length Meters

Cable length meters and TDR’s (time domain reflectometer) are very similar but do have differences. A cable length meter simply determines spooled length on uninstalled cables or wires.

A TDR finds cable length plus where along the length of the cable there are faults. Sometimes a cable may not have a complete break. The TDR locates these faults

Cable Length Meters

Cable length meters and TDR’s (time domain reflectometer) are very similar but do have differences. A cable length meter simply determines spooled length on uninstalled cables or wires. A TDR finds where there are faults in cables and will also determine the length of the cable.

A cable length meter is used to determine how long spooled cable or wires are from end to end. Some cable length meters feature an option for temperature compensation since the temperature indirectly affects the relevant wire. In order to perform an exact and reliable measurement by means of the cable length meters the temperature needs to be compensated first. That can be performed with a variety of methods. In some of the meters the temperature is measured by an internal sensor. The collected temperature will then be incorporated into the calculation of length and caliber of the relevant cable. To get the cable length meters adjusted to the temperature, it is necessary to place the device for at least two hours near the wire that needs to be measured. Another option is to set the temperature compensation manually and the surrounding temperature of the wire is set by the user.

Things to consider when purchasing a cable length meter

• Wire gauge
• Wire type

Time Domain Reflectometer (TDR)

A TDR (time domain reflectometer) is an instrument that uses time domain reflectometry to locate and characterize faults in wire cables by timing 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.

Time domain reflectometry is how a TDR meter finds the faults in cables or electrical lines by using simple transmission line theory and pulse reflection principles to detect 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.

A change in a cable’s characteristic impedance will cause one of two types of reflections: positive or negative.

• Positive reflections are caused by increases in impedance as the conductors go farther apart
• Negative reflections are caused by decreases in impedance as the conductors get closer together

Learn the theory of operation of TDR's (Time Domain Reflectometers) and some key applications in Telephone/CATV, Electric, and more in a 24 page book from Megger. Click the image or link below to download. Megger Time Domain Reflectometers

What is Velocity of Propagation (VOP)?

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. 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). VOP is a cable specification indicating the speed at which a signal travels down the cable. 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 Velocity of Propagation (VOP) values by cable type

• Communications coax – (with some exceptions) between 0.80 and 0.90
• Twisted pair
  • 22 gauge filled – 0.66
  • 22 gauge air-core – 0.67                               
  • 24 gauge filled – 0.62
  • 24 gauge air-core – 0.67
• 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. The TDR now has enough information to calculate where the abnormality is. If you cannot find the VOP, take a sample measurement by testing a smaller piece of wire and preforming the test on that to determine the VOP.  Check 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. 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.

Why is Gain important?

Gain is another variable that we can set that needs consideration. Gain 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.

A few points to consider when selecting the proper TDR

• This must be done with two conductor wire
• What type of wire are you testing? 
• What is the VOP? Velocity of propagation was described above
• Length of the wire
• Cable impendence
• Graphical or alphanumeric display

What is a copper and fiber cable certification tester?

Certification test tools answer the question, “Does this cable comply with cabling standards? (e.g. TIA/EIA-568-C , ISO/IEC 11801, ANSI/TIA 1152 (copper only), ISO/IEC 61935-1). Certification is the most rigorous of all cable testing. A certification tester makes many types of measurements across predefined frequency ranges and compares the detailed results to standards. The results from these measurements determine if a link is compliant with a category or class of cable. Certification tools are the only tools that provide “Pass” or “Fail” information on the cabling, in accordance with TIA or ISO standards. Cabling manufacturers often require certification before issuing a warranty and enterprises often require certification testing before commissioning an installation.

Who uses a copper and fiber cable certification tester?

• Commercial datacom installers/contractors and enterprise facility managers to ensure that new cabling fully meets performance standards.
• Network owner who wants to check their own and third party installations.

Functions of copper and fiber cable testers

• Verification Testers. Used to determine if the cable is properly connected. Verification test tools perform basic continuity functions (for example, wiremap, toning for copper). These verification test tools sometimes include additional features such as a Time Domain Reflectometer (TDR) or Optical Time Domain Reflectometer (OTDR)  to determine the length of the cabling link or the distance to a break or a short circuit in the link-under-test. They may also support continuity of twisted-pair links as well as copper coaxial cabling links and check coaxial connections. Verification tools may also detect and report that the cable under test is connected to an active device like a hub or switch. Verification tools are ubiquitous, simple-to use, low-cost tools that are often the first line of defense in finding cabling problems. Verification tools are especially valuable as a quick and easy screening tool when performing large-scale installations. They can be used to verify that cables have been correctly wired and terminated, and to find breaks and shorts before the certification tests are performed. Eliminating connectivity problems before the certification tests can save valuable time and can significantly reduce the overall costs of testing.
   
• Qualification Testers.  Qualification is a new category of testers designed to meet the emerging needs of network technicians who need to upgrade to higher network speeds as well as troubleshoot connectivity problems. Qualification testers help determine if the cabling will support technology requirements like fast Ethernet, VoIP, Gigabit Ethernet, etc. This type of tool is sometimes erroneously called a “Speed Certifier”. I you are a network technician, and have undocumented cabling and need to see if it will support your 1000BASE-T network, a qualification tool is the right choice. If you have an existing network and are doing small adds, moves, and changes, or you are setting up a temporary network and just need to qualify it for a specific network technology, a qualification tool is a good option.
   
• Certification Testers. Test the cabling to ensure that it meets specific cabling performance standards. A good certification tester is easy to use, but has the ability to quickly do complex and accurate calculations. If you have a mixture of fiber and copper cabling, and often need to test both, certification tools do that best.