What is a Time Domain Reflectometer (TDR)?

A Time Domain Reflectometer (TDR) is a device used to detect the location of faults in transmission lines and coaxial cables. The TDR sends a low voltage pulsed signal through the transmission line and checks for any reflections seen due to impedance mismatch. If there is no impedance mismatch throughout the line then there will be no reflections, however, if there is a discontinuity in the transmission line at a particular point, some part of the pulsed signal will get reflected back to the TDR. By measuring the time and propagation velocity of the received pulse, the TDR can calculate the exact location of the fault and the nature of the fault ex. open circuit, short circuit or impedance mismatch.

Cable Fault Location:

Where:

D = distance in meters

V_p = velocity of propagation in meters per second

T = transit time from the monitoring point to the mismatch in seconds.

The Nature of Mismatch can be given by:

Where:

ρ = reflection coefficient

E_r = reflected pulse voltage level

E_i = input pulse voltage level

Z_L = load impedance in ohms

Z_o = line impedance in ohms

Examples of typical impedance changes that occur in cables, observed by TDRs