Tempo Fitness Cable Scout TV90, Руководство по эксплуатации

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Time-Domain Reflectometer Concepts
There are several basic concepts that will help you to accurately locate and
interpret the waveforms you will see on the TDR. You can use these concepts
to help identify the waveforms you are analyzing.
Velocity of Propagation (Vp)
A cable’s velocity of propagation (VoP or Vp) specification is simply a
measure of how fast a signal travels in the cable. It is typically expressed as
a percentage of the speed of light. For example, a cable with a Vp value of
0.85 indicates that the signal is traveling down the cable at 85% of the speed
of light. Since a time-domain reflectometer (TDR) is really making measure -
ments in the time domain, the distance accuracy of the TDR is dependent
upon having the correct Vp value. (Refer to “Coax Cable Vp Table” near the
end of this manual.)
Pulses travel at different velocities on different cables just as an object •
travels at different speeds through different thickness of liquids.
Vp varies between cable types, sizes, and manufacturers, and the type of •
insulation and cross section geometry of a cable will affect the velocity of
a pulse.
Identifying the correct Vp for the cable being tested is imperative to have
•
accurate distance measurements.
Cable Impedance
Cable impedance is made up of resistance, inductance, and capacitance
inherent in a cable. Reflected pulses are caused by impedance changes.
TDRs can measure reflections caused by series impedances from several •
hundred ohms down to a few ohms.
TDRs can also measure reflections caused by shunt impedances up to •
several hundred ohms.
Proper Cable Termination
Cable TV requires a 75 ohm termination on all cable ends, taps, and ter -
minations. When properly terminated, the ability to distinguish faults from
normal occurrences in a cable is greatly enhanced. The 75 ohm termination
absorbs the TDR pulse reflection normally caused by an open cable. (Refer to
“Properly Terminated Cable” under the “Sample Waveforms” section of this
manual.)