Fluke 990DSL, Руководство пользователя, Страница: 200 / 242

990DSL
Users Guide
6-18
3. Run the TDR Pair 1 test on the known good pair.
4. Align the reflection cursor on the exact leading edge of the pair’s positive
“open” reflection.
5. Adjust the VOP until the “Delta C” (
Δ
C)
distance equals the known
length.
6. Use this value of VOP for testing the faulty pairs.
Double-ended Testing
To further minimize errors during TDR testing, it is advisable to test from both
ends of the cable. If the sum of the “Delta C” (
Δ
C
) distances to the same fault
(from testing from both ends) does not equal the known section length, adjust
the VOP factor and re-test from both ends. When the sum of the “Delta C”
distances equals the section length, you have found the exact location of the
fault.
Another method entails mathematically deriving the exact fault distance
(without having to adjust VOP and re-test multiple times). However, this
method requires that the exact section length be known. To obtain an exact
distance, do the following:
1. Test the faulted pair from both ends and record both “Delta C” distances
to the
same fault (D1 and D2).
2. Calculate the Correlation Factor (CF) by dividing the known section
length (L) by the sum of the two distances: CF = L / (D1 + D2)