Time Domain Reflectometry (TDR)

TDR is a measurement technique used to determine a cable's length and characteristic impedance and locate faults along the cable.  TDR is sometimes referred to as cable radar because it involves analyzing signal reflections in the cable.

If a Signal traveling through a cable encounters an abrupt change in the cable's impedance, some or all of the signal is reflected back to the source.  The timing, size, and polarity if the reelected signals indicate the location and nature of impedance discontinuities in the cable.

Reflections from Opens

An open, or break, in the cable represents an abrupt increase in the cable's impedance.   The impedance of an open is nearly infinite.  In an open cable, a signal's energy is not dissipated by a terminating impedance, so the signal bounces back towards the source.  This reflection appears at the source with the same amplitude and polarity as the original signal.

Reflections from Shorts

A short represents an abrupt decrease in the impedance between the two conductors in a cable.  A short is caused when the insulation surrounding a cable's wires is damaged, allowing the wires to touch each other,  The result is a near-zero impedance connection between the conductors.

A short also causes signal reflections, but in manner opposite if an open.  In a shorted cable, the signal's energy is not dissipated because the short's impedance is close to zero.  The signal is reflected back to the source, where it appears with the same amplitude but a polarity opposite of the original signal.

Reflection from Other Discontinuities

Reflections are also caused by impedance discontinuities that measure somewhere between infinite and zero impedance.  These discontinuities can be caused by mechanical stress that damages cable wire or insulation without causing a complete open or short.   They can also be caused by cable mismatches and faulty contacts at connectors or punch-down blocks.

A cable fault that has an impedance higher than the cable's characteristic impedance reflects a signal that has the same polarity as the original signal.  If the fault is not a complete open, the amplitude if the reflected signal will be less than the original signal.

If the fault's impedance is lower than the cable's characteristic impedance, but is not a complete short, the reflected signal will have the opposite polarity and less amplitude than the original signal.