Appendix B
70
Greenlee / A Textron Company
LCD
Liquid Crystal Display: a kind of display used in this instrument. Therefore, the terms
LCD
and
display
are often used interchangeably in this manual.
Line Loss
The amount of signal that is absorbed in the cable as the signal propagates down it. Cable
attenuation is typically low at low frequencies and higher at high frequencies, which should be
corrected for in some TDR measurements. Cable attenuation is usually expressed in decibels
(dB) at one or several frequencies. See also “dB.”
Noise
Any unwanted electrical energy that interferes with a signal or measurement. Most noise is
random with respect to the signals sent by the TDR to make a measurement and appears as if
the waveform moves slightly up and down on the display.
Open Circuit
In a cable, a broken conductor does not allow electrical energy to flow through it. These
circuits are also called broken circuits. The circuit is “open” to the air, which appears on the
display like very high impedance.
PW
Pulse Width: the horizontal size of the transmitted pulse, usually measured in nanoseconds.
RL
Return Loss: measures the severity of an event. RL does not account for cable loss in the
measurement.
TDR
Time-Domain Reflectometer: an instrument that sends out pulses of energy and times the
interval to reflections (also called cable radar). If the velocity of the energy through the cable is
known, distances to faults in the cable can be computed and displayed. Conversely, the speed
that energy travels through a cable of known length can also be computed. The way in which
the energy is reflected and the amount of the energy reflected indicate the condition of the
cable.
Velocity of Propagation (Vp)
The speed that electricity travels in a cable is often expressed as the relative velocity of
propagation. This value is a ratio of the speed in the cable to the speed of light. This is always
a number between 0 and 1. A velocity of propagation value of 0.50 indicates that the electrical
energy moves through that particular cable at half the speed of light.
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