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Impedance/tdr (time domain reflectometer) testing – CommScope Trunk & Distribution Cable User Manual

Page 14

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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