Glossary
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 risetime 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
∣
∣
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.
Risetime
With respect to the incident pulse, the time required for the signal to change from 10% to 90% (or
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 risetime of a pulser-sampler system is proportional to the root sum of squares of the pulse
and sampler risetimes.
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
=
x
1
2
x
2
2
x
3
2
...
x
n
2
/
n
where x represents discrete samples and n is the total number of
samples.
Sampling Efficiency
The CT100 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.
Short Circuit
The condition in which the conductor in a cable or circuit comes into direct contact with the return
path conductor or earth ground. The electrical length of the cable measured by TDR is shortened
to the point of the short circuit.
Mohr CT100 / CT100HF Operator's Manual
113
