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and/or connectorized, VF deviations can begin to affect the cable’s overall VF.
As long as quality installation practices are used per the cable and connector
manufacturer’s recommendations these will be minimal. Older cable installations,
depending on application and location, can suffer from wear and tear, abuse,
moisture intrusion, etc. All of which will slow a cable’s VF in spots and produce
TDR distance variations. As discussed in Section 2, the TDR determines
distance on a cable by multiplying time by the VF to obtain distance to marked
events on the cable. If the reflections are delayed, even slightly in one spot on
the cable, all the distance measurements after that spot will be off by a factor of
that delay.
All that being said, it’s best to find a way to verify a cable’s velocity either by
measuring a sample length or measuring a known physical distance to the end or
an event on the cable and using that obtained VF. Ways of accomplishing this
will be covered later in this section under “Advanced Features.”
The basic methods for setting a cable’s VF is by selecting the cable type from the
TDR’S’s Cable List or entering the cable’s parameters manually. This will set the
TDR’s VF. All time based measurements of reflections as marked by the cursors
will be converted to distance using this VF. In figure 3-4 the Cursor 1 is marking
the open end of the cable at 57ft 1 inch (17.4m) based on a VF of 68.5c. If the
VF is set correctly, that distance will be correct. If the TDR’s VF is set too slow
the cable’s trace will appear shorter than its physical length. If the TDR’s VF is
set too fast the cable’s trace will appear longer than its physical length.
Figure 3-4
NOTE: “Jacket length” is a term used to indicate a cable’s cut length or actual
physical length. “Electrical length” is used to define a cable’s TDR measured
length based on a specific velocity. When a cable cut to jacket length has
multiple conductors (i.e. twinax or triax cable), the different coax paths may have
slightly different electrical lengths if the same VF is used to measure each coax
conductor. The percent of distance variation depends on VF variations between
them.