Glossary
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
reflected back by the load. The dB vertical scale on the CT100B refers to the amount of
voltage gain the instrument applies to the signal before displaying it. For example, when the
instrument is amplifying the voltage by a factor of 100, this indicator would read:
verticalscale = 20 · log
10
(100) = 40 dB.
DC Direct current is unidirectional flow of electrical current. The term DC is also synonymous
with constant; for example, a perfect DC voltage source does not vary from a set value.
Batteries are an example of a DC voltage source.
DHCP Dynamic Host Configuration Protocol. Used in computer networks to automatically assign
network settings to computers.
Dielectric A nonconducting substance or insulator. May also refer to the dielectric strength or
relative permittivity of a medium, which is a measure of the electrical energy stored by a
medium when an electrical potential of a given frequency is applied across it. See
also Insulation.
Digital Refers to signals in which information is represented by variables that are discrete or
discontinuous both in time and in value. This is in contradiction to an analog signal, which is
continuous both in time and value.
Domain A mathematical term that refers to the set of values for which a function is defined. A
time-domain instrument such as a TDR performs its function by recording measurements as
a function of time.
DUT Device Under Test. The DUT is the electrical cable, device, or network that is attached to
the port of the CT100B. It is the object to be measured.
FFT The Fast Fourier Transform algorithm is a fast method of performing a discrete Fourier
transform on a time history. A Fourier transform converts time-domain data into the
frequency domain. Since the CT100B always uses the FFT algorithm, the term is used in this
manual to describe both the algorithm and its outputs. An “FFT Trace” is a
frequency-domain trace created by running the FFT algorithm on a TDR trace.
Impedance The ratio of voltage to electrical current in a cable or circuit. Impedance is a
frequency-dependent value that is influenced both by resistive and reactive components.
Impedance is expressed in terms of ohms but should not be confused with resistance, which is
a frequency-independent quantity. Most cables have impedances that vary little over the
range of frequencies with which they are used. 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.
See also Reactance and Resistance.
Impedance Mismatch A point in a cable or device under test in which the characteristic
impedance changes, causing a partial reflection of the energy in a test signal. See
also Reflection Coefficient.
Incident Pulse The excitation pulse produced by a TDR and injected into the cable under test.
The trace produced by the TDR is the temporally resolved reflections produced by the cable
in response to the incident pulse. The incident pulse in the CT100B is a step-rise signal with
a finite rise time. See also Rise Time.
Inductance See Reactance.
Insulation A coating on an electrical conductor that inhibits the flow of electricity. Insulation
serves as both a means of providing electrical safety and ensuring signal integrity.
142 CT100B TDR Cable Analyzers Operator’s Manual
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