Glossary
1503C MTDR User Manual
Glossary–3
A protective coating on an electrical conductor that will not readily allow electrical
energy to flow away from the conductive part of the cable or circuit. Insulation is
also called dielectric. The kind of dielectric used in a cable determines how fast
electricity can travel through the cable (see Velocity of Propagation).
The short term error or uncertainty in the clock (timebase) of a TDR. If the timing
from sample to sample is not exact, the waveform will appear to move back and forth
rapidly.
An acronym for Liquid Crystal Display. It is the kind of display used on this
instrument, so the terms display and LCD are often used interchangeably.
rho ( ) is the reflection coefficient of a cable or power delivery system. It is the ratio
of the voltage reflected back from the cable or circuit due to cable faults or an
impedance mismatch at the load, divided by the voltage applied to the cable.
Millirho are thousandths of one rho. Rho measurements are often used to judge how
well the cable is matched to the load at the other end of the cable. If there is an open
circuit in the cable, nearly all the energy will be reflected back when a pulse is sent
down the cable. The reflected voltage will equal the incident pulse voltage and rho
will be +1. If there is a short circuit in the cable, nearly all the energy will be
delivered back to the instrument through the ground or return conductor instead of
being sent to the load. The polarity of the reflected pulse will be the opposite of the
incident pulse and rho will be –1. If there is no mismatch between the cable and the
load, almost no energy will be reflected back and rho will be 0. In general, a load
or fault with higher impedance than the cable will return a rho measurement of 0 to
+1, and a load or fault with a lower impedance will return a rho measurement of 0
to –1. The scale for rho measurements is determined by the height of the incident
pulse. A pulse two divisions high means that each division is 0.5 rho (500 millirho).
A pulse set to be four divisions high would make each division 0.25 rho (250
millirho).
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 will appear on the waveform, constantly constantly moving up and down on the
display. The NOISE FILTER control sets how many waveforms will be averaged
together to make the waveform displayed. Noisy waveforms appear to fluctuate
around the real signal. Because it is random, noise will sometimes add to the real
signal and sometimes subtract energy from the real signal. By adding several noisy
waveforms together, the noise can be “averaged” out of the signal because the
average amount of noise adding to the signal will be nearly the same as the average
amount of noise subtracting from the signal. More waveforms in an average are
more likely to approach the real signal (although it takes longer to acquire and add
together more waveforms).
Insulation
Jitter
LCD
Millirho
Noise
Summary of Contents for 1503C
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