Glossary
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
demonstrates zero electrical resistance.
Resolution For a given parameter, the smallest increment that can be measured or displayed. In
the setting of TDR, resolution may refer to timebase resolution, which describes the smallest
increment of time used by the pulser-sampler system to produce signals and measure
reflections, or spatial resolution, which is dependent on the system rise time and determines
the ability of the TDR to separate two closely spaced cable faults.
Return Loss A measure of the power reflected by impedance changes in a cable. Return loss is
typically expressed as a logarithm of the reflection coefficient, ρ: RL(dB) = −20 · log
10
|ρ|
where RL is return loss. Cable faults such as shorts and opens, which return all of the
incident energy in the TDR signal, have return losses of 0 dB.
Rho (ρ) See Reflection Coefficient
Rise Time With respect to the incident pulse, the time required for the signal to change from 10%
to 90%, or alternatively 20% to 80%, of its final value. With respect to the sampling
electronics, the time required for the sampled value to change from 10% to 90% of the final
value when a perfect step signal is applied. The rise time of a pulser-sampler system is
approximately equal to the root sum of squares of the pulse and sampler rise times.
RMS An acronym for Root Mean Square, also abbreviated rms and known as the quadratic mean.
This is a useful statistical technique when considering time-varying electrical quantities for
which simple DC definitions are not accurate, such as when determining the power dissipated
by an AC source. The formula for calculating an rms value of a given time-varying signal is:
x
rms
=
s
x
2
1
+ x
2
2
+ x
2
3
+ ··· + x
2
n
n
where x
i
represent discrete samples and n is the total number of samples.
S
11
A scattering parameter that measures complex return loss on port 1 of a linear electronic
device. See Scattering Parameters.
S
21
A scattering parameter that measures complex transmission loss from port 1 to port 2 on a
linear electronic device. See Scattering Parameters.
Sampling Efficiency The CT100B makes measurements through a process known as sequential
sampling. In sequential sampling, a succession of incident pulses followed by discrete samples
progressively builds up a given TDR trace. Sampling efficiency describes the ability of the
sampling circuitry to adjust to rapid changes in impedance within a TDR trace. Low
sampling efficiency leads to a trace that appears too smooth.
Scattering Parameters Scattering parameters are the complex ratios of output signal to input
signal for a linear electronic device. For a device with two ports, four scattering parameters
may be generated: S
11
, S
21
, S
12
, and S
22
. A one-port device has only one parameter, S
11
,
while a four-port device has 16. S
11
is the complex ratio of output at port 1 to an input at
port 1. S
21
is the complex ratio of an output on port 2 to an input on port 1, and so on. The
assignment of port 1 and port 2 is arbitrary. A full set of scattering parameters are
144 CT100B TDR Cable Analyzers Operator’s Manual
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