CommScope Trunk & Distribution Cable - Manual (Page 14)

Impedance/TDR (Time Domain Reflectometer) Testing
Impedance testing using a TDR is a quick and straightforward method for finding the distance from the test point
to any fault (shown as an impedance mismatch) in the cable.
Impedance tests should be made from both ends of the cable to ensure finding the correct distance to fault. The
correct velocity of propagation (Vp) for the test is essential - an incorrect Vp will cause a proportional
measurement error in the distance to the fault. The Vp for QR cable is 88%. The Vp for P3 cable is 87%.
Impedance tests should be done per the standards set forth in SCTE-TP-006. Impedance testing will require a
time domain reflectometer (TDR). Methods of operation will vary for each TDR; however, these are general
guidelines for using one:
1) Set the velocity of propagation for 88% for QR or 87% for P3,and the impedance reference for
75 ohms.
2) Adjust the display for a sharp, clear baseline, and position the leading edge to a convenient
starting point or graticule.
3) Set the pulse width as recommended by the TDR manufacturer.
4) Attach the test lead (coaxial cable test leads are preferred) to the cable under test.
Connectors should match the impedance of the tested cable.
5) Adjust the display and control settings to show the entire length of the cable. The control settings
can be adjusted to allow precise measurement of the distance to any impedance mismatch.
Operator proficiency and equipment capability are critical factors in making consistent and
precise measurements.
End-to-end impedance tests are very beneficial in an evaluation of post-installation plant
performance. The TDR traces should be kept with the as-built drawings along with the reel
numbers of the installed cable.
2.5 Handling and Testing
Impedance/TDR Testing