Glossary
Resolution
For a given parameter, the smallest increment that can be measured or displayed.
In the setting of TDR, resolution may refer to time base 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
)
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 considered a complete characterization of a linear device for the frequency
range of the scattering parameters.
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.
Smith Chart
A method for displaying complex valued electrical measurements, such as an S
11
,
on a polar plot, graphically showing both magnitude and phase angle.
Smoothing
Any technique for reducing noise in a signal. The CT100B uses time averaging to
smooth traces.
SP232
A serial protocol used by the Tektronix 1502B and 1502C. This protocol allowed
transmission of TDR trace data across a serial port. Using a USB to serial port adapter, a
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CT100B TDR Cable Analyzers Operator’s Manual
