Glossary
Layer Peeling
A mathematical technique for reducing multiple reflections in a TDR trace.
TDRs measure reflections from changes in impedance, but if there are multiple impedance
changes, then the reflections from one will partially reflect back off of another. Reducing
the effect of these extra reflections improves the measurement of the impedance changes.
LCD
Liquid crystal display. The display used by this instrument is an active-matrix thin-film
transistor LCD, abbreviated TFT-LCD.
Millirho (
mρ
)
Noise
Any undesirable electrical energy that impairs the ability of an electronic system to
transmit or receive a signal or make an accurate measurement. In TDR, noise is typically
related to thermal and/or electrical noise that interferes with the time base or sampling
circuitry and is usually random, although a nearby strong electromagnetic radiator can
cause non-random synchronous noise to be measured on unshielded cable. In the case of
random noise, averaging is an effective means of noise suppression.
Open Circuit
Describes a non-terminated cable or cable with a broken conductor that reflects
all energy within the incident pulse.
OSL (Open Short Load)
A method of calibrating one port analyzers using precision open,
short and load terminations, where the load matches the output impedance of the
analyzer. Each termination contributes to reducing error in an
S
11
measurement.
Phase
A horizontal shift in a sinusoid. Phase is given as an angle. A sinusoid with phase of
π
radians (180 degrees) is precisely inverted compared to the same sinusoid with 0 phase. A
sinusoid with phase of 2
π
radians (360 degrees) is precisely equal to an the same sinusoid
with 0 phase, i.e. it has no phase shift.
Precision
The variation in the value of a variable measured multiple times. Precision and
accuracy are both important factors in determining the reliability of a given measurement.
Precision may also be used to describe the number of digits or the unit of the least
significant digit with which a particular quantity is expressed.
Reactance
The imaginary component of electrical impedance. Reactance describes the
opposition of a conductor to the flow of alternating current. Impedance is described by
Z
=
R
+
j
·
X
where
R
is resistance,
j
is the imaginary unit, and
X
is the reactance at a
given frequency. If
X >
0, the impedance is inductive, if
X
= 0 then the impedance is
purely resistive, and if
X <
0 the impedance is capacitive.
Reflection Coefficient
In TDR, a coefficient describing the amplitude of a reflected signal
produced by an impedance mismatch in relation to the incident pulse. The reflection
coefficient Gamma (Γ) is defined by the relation Γ = (
Z
t
−
Z
0
)
/
(
Z
t
+
Z
0
) where
Z
t
is the
impedance at time t and
Z
0
is the characteristic impedance of the cable, and is usually
described in parts per thousand with units of rho (
ρ
) or millirho (m
ρ
). The coefficient
ρ
ranges from -1 (short circuit) to +1 (open circuit). A reflection coefficient of zero indicates
that there is no impedance mismatch and no reflection of electrical energy.
Reflectometer
An instrument that measures reflections to determine the state of a system. The
CT100B measures the reflections of electrical energy.
Resistance
A conductor’s opposition to electrical current. The reciprocal of resistance is
conductance. Electrical resistance can often be considered a constant that does not vary
with respect to the voltage or current applied to an object. When considering the
impedance of a circuit element, resistance is also frequency invariant. Most materials,
including conductors, have some degree of electrical resistance. A special class of materials
called superconductors demonstrate zero electrical resistance.
CT100B TDR Cable Analyzers Operator’s Manual
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