88
SCAN-LAN Vt
When the reflected signal reaches the tester where the signal was
generated, the timer stops. By knowing the speed of the signal traveling
down the cable, you can determine the length of the 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 Vt 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
.
= 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