25
Frequently Asked Questions
A:
No. The accuracy of the reading is dependent on the
setting of the VOP. While the nominal setting for general
testing is -
66-
, the VOP for that kind of cable is
-56-
. To
improve the accuracy of length measurements for that or
any cable, change the VOP as shown in the instructions in
the Velocity of Propagation section.
Q:
Why does the length reading sometimes change a small
amount when I open and short the far end of a test cable?
A:
There are two causes. The first is that this is a characteristic
of the measurement technique used in almost all low and
medium cost cable length test tools.
In the case of the TS90 Cable Fault Finder, the variance occurs
in only a few cable types and both readings are within the
specified accuracy of the instrument. The second cause
occurs when the cable is coiled, as in a box or on a spool. The
magnetic field caused by the TDR signal itself couples across
to other parts of the cable and changes the characteristics of
the reflections.
Q:
Why, on some cables, does the number displayed on the
readout jump between 2 or 3 different values?
A:
As the TDR signal travels down a cable, it loses some of its
strength. At some point, the noise on the cable has an
amplitude similar to the reduced strength TDR signal and
will influence the measurement results. The tester’s software
filters out many of the noise related variations in the
displayed length, but some variations do get through.
