Glossary
Aberrations
Imperfections or undesired variations in a signal. For example, aberrations in a
TDR’s excitation signal are the result of the finite switching speed of the instrument’s
electronics and cause it to deviate from a perfect step signal.
AC
Alternating current, a method of delivering electrical energy by periodically changing the
direction of the electric field in a conductor.
Accuracy
The difference between a measured or estimated value of a quantity and its true
value. Accuracy and precision are both important factors to consider when assessing the
reliability of a measurement.
Analog
Refers to a signal that is continuous with respect to both time and value. Analog
circuitry produces and/or measures analog signals. This is in contradistinction to digital
signals, which are discontinuous in both time and value. The sampling circuitry of a TDR
converts the analog voltage signal detected on a cable to a digital value for representation
on the display and in the instrument’s memory.
Cable
Conductors of electricity that are usually shielded and insulated. Cables typically contain
at least two conductors, one to deliver the electrical signal and one to act as the return
path. The conductor acting as the return path may be referred to as the “shield,”
“ground,” or “ground wire”. A cable with such a ground return path is known as an
unbalanced cable, an example of which is typical coaxial cable. Another general type of
cable is called balanced cable, an example of which is twisted pair Ethernet cable. In
balanced cable, two signal wires carrying differential signals of opposite polarity are both
separated from ground by an equal impedance.
Cable Attenuation
A quantity describing the energy in a signal that is absorbed, reflected, or
otherwise lost during propagation through a cable. Higher frequencies are typically the
most susceptible to attenuation. Cable attenuation can distort some TDR measurements.
Attenuation is often expressed in decibels (dB) at one or more frequencies. See also dB
and Return Loss.
Cable Fault
A defect in a cable or other condition that makes a cable less able to deliver
electrical energy than was designed. Damage to the shield, conductor, or insulation, bad
splices, and poorly mated connectors are frequently encountered cable faults.
Capacitance
See Reactance
Characteristic Impedance
The ratio of the amplitude of voltage and current in an electrical
signal propagating in a cable. In a coaxial cable, this value (usually written
Z
0
) is, in large
part, related to the geometrical relationship of conductor to return path conductor. Cables
are usually designed to match the impedance of the source and load to which they are
attached in order to maximize power transfer.
Conductor
A substance that allows electricity to flow through it with minimal resistance. Most
conductors are metals. However, there are many non-metallic conductors, including salt
solutions, graphite, and any element in its plasma state.
dB
dB is the abbreviation for decibel. Decibels are a method of expressing power or voltage
ratios as logarithms. When used for voltage ratios, as in TDR, the formula for decibels is
dB
= 20
·
log
10
(
V
i
/V
r
) where
V
i
is the voltage of the incident pulse and
V
r
is the voltage
CT100B TDR Cable Analyzers Operator’s Manual
111
