Fluke DSP-2000 - Manual (Page 126)

DSP-100/2000
Users Manual
7-14
Nominal Velocity of Propagation (NVP)
NVP is the speed of a signal through a cable relative to the speed of light. In a
vacuum, electrical signals travel at the speed of light. In a cable, signals travel
slower than the speed of light. Typically, the speed of an electrical signal in a
cable is between 60% and 80% of the speed of light. Figure 7-8 shows how the
NVP percentage is calculated.
Signal Speed
in Cable
300,000,000 m/s
NVP =
Speed Pulse Travels in Cable
Speed of Light
=
X 100%
300,000,000 NVP m/s
ENTER
TEST
FAULT INFO
EXIT
SAVE
1 2 3 4
OFF
PRINT
MONITOR SETUP
SMART
REMOTE
SPECIAL
FUNCTIONS
SINGLE TEST
AUTO
TEST
DSP-2000 CABLE ANALYZER
gc34f.eps
Figure 7-8. How NVP is Calculated
NVP values affect the limits on cable length for Ethernet systems because
Ethernet operation depends on the system’s ability to detect collisions in a
specified amount of time. If a cable’s NVP is too low or the cable is too long,
signals are delayed and the system cannot detect collisions soon enough to prevent
serious problems in the network.
NVP and Length Measurements
Length measurements depend directly on the NVP value entered for the selected
cable type. To measure length, the test tool first measures the time it takes for a
test pulse to travel the length of the cable. The test tool then calculates cable
length by multiplying the travel time by the signal speed in the cable.
Because the test tool uses the length measurement to determine cable resistance
limits, the NVP value also affects the accuracy of resistance measurements.