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CHAPTER 8: Testing Twisted-Pair Cable
How do you know how fast the signal travels down the cable? All cables
have an inherent characteristic called Nominal Velocity of Propagation
(NVP). This value can be determined by contacting the cable
manufacturer. NVP is the speed of signal propagation through a cable,
expressed as a percentage of the speed of light in a vacuum (refer to
Figure 8-11). Using the NVP value and the time to receive the returned
TDR pulse, the SCAN-LAN VI calculates the cable length.
Figure 8-11. Cable Nominal Velocity of Propagation.
To obtain the most accurate Length measurement possible, you must
calibrate your test equipment to the cable you are testing. Follow the
procedure described in
Chapter 10, Calibration
.
NOTE
It is normal for some variation in the measured length of wire pairs
within the same cable. This is due to the difference in construction of
the individual pairs. Length variation from pair to pair will be greater for
longer cable lengths. Significant differences (more than a few percent)
may indicate a fault in a pair.
= 300,000,000 m/s
Datacom Technologies
NVP =
speed of a signal in a cable
speed of light
X 100%
speed of light
speed of a signal in a cable
= 300,000,000 * NVP m/s
Summary of Contents for SCAN-LAN VI
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