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TDR Tutorial and Riser Bond TDR Product Review

Waveform Analysis

A great variety of waveforms may be encountered. This is due to the various applications, and electrical and environmental characteristic differences found in the wide variety of cables that exist today. Following are examples of some waveforms which may be encountered when viewing a traditional waveform TDR.


TDR Trace

A reflection with the same polarity indicates a fault with OPEN (high impedance) tendencies. The reflection shown at the second cursor is a COMPLETE OPEN.


TDR Trace

A reflection with the opposite polarity indicates a fault with short (low impedance) tendencies. The reflection shown at the second cursor is a DEAD SHORT.


TDR Trace

The middle reflection at the second cursor is a PARTIAL OPEN followed by a COMPLETE OPEN (end of the cable). The more severe the fault, the larger the reflection will be.


TDR Trace

The middle reflection at the second cursor is a PARTIAL SHORT followed by a COMPLETE OPEN (end of the cable). The more severe the fault, the larger the reflection will be.


TDR Trace

Due to attenuation (cable loss), the reflections caused by each of these equally spaced yet identical taps are progressively smaller. A larger reflection (second cursor) beyond a smaller reflection may indicate an undetermined or faulty tap.


TDR Trace

Two sections of coaxial cable with a barrel connector shown at the second cursor. The amount of reflection caused by the connector is directly proportional to the quality of the connector and connection.


TDR Trace

An unpowered line extender amplifier will cause a major reflection a the amplifier. The TDR signal will stop at the amplifier, but other reflections (ghost images) may appear beyond the amplifier.


TDR Trace

Coaxial taps (both indoor and outdoor) will cause reflections along the waveform. The quality and value of each tap determines the amount of reflection.


TDR Trace

A splitter or directional coupler can be identified although accurate measurements are difficult due to multiple reflections. The second cursor identifies the splitter. The two reflections following are the ends of each of the two segments.


TDR Trace

A high resistance joint or splice will produce an "S" shaped reflection. A high impedance upward reflection will be followed by a lower impedance downward reflection.


TDR Trace

A properly terminated cable will absorb the TDR signal resulting in no reflection. Faults prior to the termination may show up as reflections along the waveform. If a terminator causes a reflection, the termination may be bad.


TDR Trace

Testing through to an antenna usually results in an "S" shaped reflection, although reflections can vary greatly depending on the antenna.


TDR Trace

Testing tower cables with antennas can be challenging due to energy induction from high RF areas shown on this waveform. Stepping through various noise filter settings will result in a "cleaner" waveform.


TDR Trace

A water soaked cable will display a waveform with a downward slope indicating the beginning of the water and an upward rise at the end of the water . Generally, the area in between the two reflections will appear "noisy".


TDR Trace

A wet section of cable is clearly seen on this waveform.


TDR Trace

A bridge tap will appear as a negative or downward reflection followed by a positive reflection (caused by the end of the tap). Due to the multiple reflections, it is difficult to analyze results when testing through multiple taps.


TDR Trace

Crossed or split pairs: On twisted pair cable, a split may appear as an upward or downward reflection. The resplit will appear as a reflection of the opposite polarity.


TDR Trace

A telephone load coil will cause a high impedance upward reflection (similar to a complete open). A TDR signal generally will not test past a load coil.


TDR Trace

A telephone build out capacitor causes a low impedance downward reflection (similar to a short) followed by a smaller positive reflection.


TDR Trace

Mechanical inner-connectors (known as bullets) connecting sections of broadcast transmission lines sometimes burn open causing power outages. These bullets as well as other transmission line faults can be detected by a TDR.


TDR Trace

All types of twisted pair and coaxial LAN systems can be tested with a TDR. It is best to test for discontinuities in the cable if there is no traffic or power present. This waveform shows a section of unpowered cable with four transceivers installed.


TDR Trace
Pulse Waveform 1 - Short Pulse Width

Pulse waveforms 1, 2, and 3 illustrate how only one setting can change the way a waveform appears. All three waveforms are of the same cable. Only the pulse width setting of the instrument has been changed.


TDR Trace
Pulse Waveform 2 - Medium Pulse Width

The width of the output pulse is also referred to as the blind spot or dead zone. It is more difficult to "see" a fault when it is contained within the blind spot.


TDR Trace
Pulse Waveform 3 - Long Pulse Width

The larger the pulse width, the longer the blind spot. Note how the output pulse, and thus the blind spot, is progressively larger in samples 1, 2, and 3.



| TDR Principles of Operation | Usage Techniques | Waveform Analysis |
| General | CATV | Telephone | Broadcast | Cellular | 2-Way Radio | Power | LAN/WAN |
| Dual Twisted Pair and Coax TDR | Dual Twisted Pair TDR | Coax TDR | Basic Numeric TDR |


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Copyright ©1993, 1994, 1995, 1996, 1997, 1998 James M. Atkinson
(Some material ©1996 Riser-Bond Instruments, used with permission)