Operating Instructions
1502C MTDR User Manual 1–23
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ac 3.000 ft
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Stored
Waveform
VIEW STORE
Current
Waveform
VIEW INPUT
Difference
VIEW DIFF
Figure 1–21: Current Waveform Center, Stored Waveform Above, Difference Below
Notice the VIEW INPUT waveform is solid, VIEW DIFF is dotted, and VIEW
STORE is dot-dash.
There are many situations where the VIEW DIFF function can be useful. One
common situation is to store the waveform of a suspect cable, repair the cable,
then compare the two waveforms after the repair. During repairs, the VIEW
INPUT, VIEW DIFF, and VIEW STORE waveforms can be used to judge the
effectiveness of the repairs. The optional chart recorder can be used to make a
chart of the three waveforms to document the repair.
Another valuable use for the VIEW DIFF function is for verifying cable integrity
before and after servicing or periodic maintenance that requires moving or
disconnecting the cable.
The VIEW DIFF function is useful when you want to see any changes in the
cable. In some systems, there might be several reflections coming back from
each branch of the network. It might become necessary to disconnect branch
lines from the cable under test to determine whether a waveform represents a
physical fault or is simply an echo from one of the branches. The STORE and
VIEW DIFF functions allow you to see and compare the network with and
without branches.
Two important things to be observed when using the VIEW DIFF function:
HIf you change either the VERT SCALE or DIST/DIV, you will no longer be
comparing features that are the same distance apart or of the same magnitude
on the display. It is possible to save a feature (e.g., a connector or tap) at one
distance down the cable and compare it to a similar feature at a different
distance by moving the
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POSITION and n
oPOSITION controls.
HWhen this is done, great care should be taken to make sure the vertical and
horizontal scales are identical for the two waveforms being compared. If
either the stored or current waveform is clipped at the top or bottom of the
display, the difference waveform will be affected.