Design And Implemented Measurement Methods
Implemented Measurement Methods
Time Domain Reflectometer RI-407 User Manual Page 9 of 67
6. Design And Implemented Measurement Methods
6.1. Implemented Measurement Methods
6.1.1. Time Domain Reflectometry (TDR) Method
The instrument uses a Time Domain Reflectometry method (TDR), which is based on the
phenomenon of a partial reflection of electromagnetic waves by the impedance irregularities in the
line. When applying the TDR method a rectangular probe pulse generated by the pulse generator is
sent into the line. Probe pulse propagating through the cable line, completely or partly reflected from
the fault area in the line (impedance irregularities) and returns to the receiver input. Reflection
waveform is observed on the screen and allows to determine the type of the fault (see Table 6-2) and
the distance to it. The reflected pulses are returned to the device after a certain time from the moment
of sending the probe pulse.
Knowing the speed of electromagnetic wave propagation along the line, and the time delay of
the reflected signal (measured by the device), we can calculate the distance to the impedance
irregularity.
here,
L – Distance to the impedance irregularity, m.
v – Propagation speed of electromagnetic wave in the line, m/µs;
PF – Propagation Factor, c/v
VoP – Velocity of Propagation , (v/c)*100%
td – Measured time delay of the reflected signal, µs;
с – Speed of light is equal to 299,8 m/µs;
Impedance irregularities are the result of violations manufacturing technology of cables as
well as a consequence of mechanical and electrical hazards during the construction and operation of
the cable lines. Any cable device (couple, split, joint, Pupin coil, etc.) cause irregularity of the line
impedance.
